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Social bonds are crucial to many animal species. To maintain these bonds, individuals must be able to differentiate the identity of conspecifics. Pair-bonding primates, in general, maintain close bonds with their selected mate. Little is known about visual preferences of pair-bonded primates. To characterize visual preference for images of familiar and unfamiliar faces, we assessed visual attention in coppery titi monkeys, Plecturocebus cupreus. Coppery titi monkeys, like many other pair-bonding species, show a behavioural partner preference when placed in a partner preference paradigm and maintain greater durations of physical proximity to their pair mate compared to an unfamiliar stranger. Using a previously validated noninvasive eye-tracking method, we investigated whether titi monkeys display visual partner preference. We presented adult titi monkeys with 10 static slides showing two conspecific faces side by side: either (1) their partner's face and a stranger's face, or (2) two strangers' faces. Face side was counterbalanced, and slide presentation order was randomized, across all subjects. We present five looking-behaviour outcome measures for a study of 40 titi monkeys. We found no evidence of a visual preference for still photographs of one's pair mate, but we did find that age, pair tenure and parenting experience predicted looking behaviour. Animals with longer pair tenures spent more time looking at facial images. Younger animals looked at the screen for the first time faster, spent less time looking and looked fewer times at the stimuli compared to older animals. Parenting status positively predicted fixation count, total visit duration and visit count, such that parents with more experience looked at the stimuli longer and more times than animals without parenting experience. This study is the first to characterize social looking in a pair-bonded monkey. • We investigated whether titi monkeys display visual partner preference. • We presented static slides of the pair mate vs a stranger or two strangers. • We found no evidence of a visual preference for one's pair mate. • Parents with more experience looked at stimuli longer and more times. • Age predicted shorter durations of looking. [ABSTRACT FROM AUTHOR]

Coppery titi monkeys maintain pair bonds through affiliative behaviours (e.g. tail twining, maintaining social proximity). Findings from previous studies suggest that the amount of time pairs spend in affiliative contact is not stable over time within pairs and differs across pairs. The present study investigates several predictors (temperament, age at pairing, pregnancy status and presence of infant) that may help explain observed changes in affiliation over time and between pairs. We hypothesized that affiliation varies for individual dyads and that time-varying and dyad level predictors explain variability in affiliation. We analysed pair affiliation using scan samples collected five times per day for 1 year for 29 pairs. We used Bayesian nonlinear mixed-effects effects modelling to test the effects of predictors. Pregnancy predicted higher affiliation levels (ϒ 02 = 0.40, 95% highest posterior density interval (HPDI): [0.21, 0.58]). The presence of an infant and aggression did not predict variability in affiliation. Models would not converge when we included age at pairing, so we cannot draw conclusions about the role of age in affiliation. Findings from this study suggest that time-varying predictors like pregnancy status help explain the large degree of variability observed in affiliative interactions between partners in this species. It is possible that hormonal changes associated with pregnancy are driving changes in affiliative behaviours in pairs. An alternative explanation is that pairs that are more affiliative to begin with are more likely to get pregnant. Future studies will assess whether these patterns persist in subsequent pairings. • Titi monkey dyads varied in their levels of affiliation. • Pregnancy predicted higher levels of affiliation. • Aggression and infant presence did not significantly explain affiliation patterns. • Effect of age at pairing could not be evaluated using our models. [ABSTRACT FROM AUTHOR]

Mating systems, and individual mating tactics therein, are regularly defined by patterns of space use and are inherent reflections of spatial cognition. Despite this, spatial learning is rarely considered in the context of mating tactics. The goal of this study was to determine the relationship between spatial learning and the mating tactics that males adopt. Specifically, we asked whether spatial learning predicts the adoption of a particular tactic, whether the adoption of a particular tactic results in differences in spatial learning and/or whether individual variance in spatial learning is flexible or persistent across time and mating tactic experiences. To this end, we assessed spatial learning of male prairie voles, Microtus ochrogaster , under laboratory conditions before and after allowing them to live freely in seminatural field enclosures. Male prairie voles adopt two types of mating tactics, each with distinct patterns of space use. Monogamous 'resident' males have relatively small home ranges and exhibit high degrees of territory overlap with one female (partner) and very little with others. Wanderer males, by contrast, have large territories that overlap with multiple females and remain single. Our data revealed that spatial learning might be more associated with wandering behaviour overall when comparing between tactics. However, we also found that the significance of space use and mating behaviour was contextually dependent, and that spatial learning impacted space use uniquely within each tactic. The relationship with pre-test spatial learning explained the variance for some patterns of space use in the field for residents, and the relationship with post-test spatial learning explained the variance for some patterns of space use in the field for wanderers. Taken together, we conclude that differences in spatial learning ability and how they relate to mating tactics are contextual and best interpreted by examining within- and between-tactic variation. • We tested spatial learning in the lab and alternative mating tactics in the field. • Spatial learning relates to individual variation in space use among mating tactics. • Good spatial learning predicted space use most characteristic of monogamous males. • Good spatial learning related to uncharacteristic space use for nonmonogamous males. • Spatial cognition contextualizes reproductive decision making in prairie voles. [ABSTRACT FROM AUTHOR]

We investigated factors leading to variation in social complexity or 'social systems' among plateau pika family groups within a contiguous local population across 2 years. Plateau pikas are small, diurnal, nonhibernating, sexually monomorphic lagomorphs that occupy family home ranges on open alpine meadows on the Qinghai-Tibetan Plateau. Expression of the social organization, social structure, mating system and parental care system in plateau pikas did not follow expectations from traditional ecological or evolutionary explanations. Variability in plateau pika family group size and the transitions of group size between years allowed us to investigate potential advantages and disadvantages of group living. Evidence that group living served to protect pikas against predation was weak. Although social huddling could have minimized thermoregulatory costs during the extremely cold Tibetan winters, there was no correlation of overwinter survivorship among pika families of different sizes. There was no apparent group-living benefit with regard to foraging, and the occurrence of cohesive social families on the flat, continuous meadow contradicts the hypothesis that sociality is related to patchiness of critical resources. Cost of maintaining burrows appeared unrelated to group size. Most interactions between pikas occurred within family groups and were affiliative (99% of adult interactions; 97% of adult–juvenile interactions), and most interactions between adult males of different family groups were aggressive (96% of interactions). Matings were primarily within families (88% of copulations). Pikas also possess a complex vocal repertoire that enhanced interactions within social families. Demographic constraints associated with variable overwinter survivorship appeared to be the dominant precondition that produced a given family size and mating system type, coupled with selective dispersal by some pikas before the start of the breeding season. Paternal care enhanced juvenile survival, and thus led to an equalization of reproductive success among adults in families with different mating combinations. • Plateau pikas live in families occupying exclusive burrow systems and home ranges. • Pika family group size varied from one to five adults within and between years. • Pikas failed to adhere to conventional explanations for establishment of group size. • Pika families exhibited multiple social mating system types. • Pika families did not adhere to explanations for establishment of mating system types. [ABSTRACT FROM AUTHOR]

Individual-specific mate preferences are thought to be widespread, but they are still poorly understood in terms of mechanisms and function. Earlier work on a songbird (the zebra finch, Taeniopygia guttata) showed predominantly individual-specific mate preferences and signs of behavioural incompatibility with certain partners. However, the phenotypic target of preference or aversion remains unclear. A previous study suggested that female preferences may be related to variation in male song. Male zebra finches have individually distinct songs and together with individual-specific female song preferences they could function according to a 'key–lock principle'. Here we report on a preregistered study in which we tested the hypothesis that individual females respond to males with highly similar songs in a similar way. We selected 18 duos of males with nearly identical songs and released them with females in communal breeding aviaries. We scored female preference as the female's responsiveness to male courtship and confirmed that females indeed had strong individual-specific mate preferences. However, female responsiveness scores were not positively correlated across males closely matched for their song, thereby rejecting the idea of a song-based 'key–lock principle' as an explanation for the observed individual-specific preferences. We suggest researchers should focus on details of the interactions between potential partners to elucidate the cause of behavioural incompatibility. • Individual-specific mate preferences are common but poorly understood. • Mate preferences differ strongly between individual female zebra finches. • Individual males differ strikingly in their songs. • Females respond differently to males with exceptionally similar songs. • Song traits are hence not the target of individual-specific preferences. [ABSTRACT FROM AUTHOR]

Animals create diverse structures, both individually and cooperatively, using materials from their environment. One striking example is the nests birds build for reproduction, which protect the offspring from external stressors such as predators and temperature, promoting reproductive success. To construct a nest successfully, birds need to make various decisions, for example regarding the nest material and their time budgets. Research has focused mainly on species where one sex is primarily responsible for building the nest. In contrast, the cooperative strategies of monogamous species in which both sexes contribute to nest building are poorly understood. Here we investigated the role of both sexes in nest building and fitness correlates of behaviour in wild, monogamous jackdaw pairs, Corvus monedula. We show that both partners contributed to nest building and behaved similarly, with females and males present in the nestbox for a comparable duration and transporting material to the nest equally often. However, while females spent more time constructing the nest, males tended to invest more time in vigilance, potentially as a means of coping with competition for nest cavities. These findings suggest a moderate degree of division of labour, which may facilitate cooperation. Moreover, some aspects of behaviour were related to proxies of reproductive success (lay date and egg volume). Females that contributed relatively more to bringing material laid earlier clutches and pairs that spent less time together in the nestbox had larger eggs. Thus, selection pressures may act on how nest-building pairs spend their time and cooperatively divide the labour. We conclude that cooperative nest building in birds could be associated with monogamy and obligate biparental care and provides a vital but relatively untapped context through which to study the evolution of cooperation. • In monogamous jackdaws, mates behaved similarly and cooperated to build their nest. • Females built more and called more frequently; males tended to be more vigilant. • Females that contributed relatively more to bringing material had earlier clutches. • Pairs that spent more time together in the nestbox laid smaller eggs. • Cooperation may be crucial in light of genetic monogamy and nest site competition. [ABSTRACT FROM AUTHOR]

Social interactions among individuals play a central role in behavioural ecology and studying cohesion of monogamous pairs is important to understand their social relationships. Here, we tested five hypotheses to investigate patterns of pair cohesion in an obligate monogamous mammal, the Eurasian beaver, Castor fiber. We hypothesized that pair cohesion is related to the spatial structure of the territory, territory defence, resource competition, offspring provisioning and the age difference between pair members. We analysed GPS data of 14 beaver pairs and calculated the distance between pair members, and the proportion of territory and core area overlap. While overall territory size was similar between partners and overlap was high, the distance between pair members was large, averaging 500 m, and increased with increasing territory size. Moreover, the proportion of core area overlap was comparatively low and decreased with territory size, also indicating low pair cohesion. Pair members were closest at the beginning and end of their nightly activity periods, suggesting that variation in pair cohesion was caused by the spatial structure of territories with the lodge serving as a central place. We found no support for the other hypotheses, but the overall low pair cohesion might nevertheless serve to increase territory defensibility and reduce intrapair competition for resources. Ultimately, low pair cohesion might be caused by the obligate monogamous mating system of beavers, leading to the defence of spatially structured territories rather than the defence of a partner. • Beavers exhibit low pair cohesion, with pair members being on average 500 m apart. • This may increase territory defence efficiency via enhanced patrolling effort. • Low pair cohesion may also reduce predation risk and competition for food. • Low pair cohesion might be caused by the beavers' monogamous mating system. [ABSTRACT FROM AUTHOR]

In monogamous mating systems, pair mates frequently share similar characteristics, such as size or ornamentation, a pattern suggesting mutual mate choice. When pairs persist across multiple seasons, preferences for familiar pair mates can either reinforce or disrupt patterns of assortative pairing. Snapping shrimp (Alpheus angulosus) are socially monogamous shrimp that can be found in size-assortative pairs year-round; both sexes could benefit from pairing with larger individuals. To determine the role of pair mate preferences for either size or established pair mates in size-assortative pairing, we performed simultaneous choice trials to test whether (1) either or both sexes prefer larger pair mates, (2) females prefer males with larger claws, or (3) either or both sexes prefer current pair mates over size-matched unfamiliar potential mates. Males, but not females, preferred larger potential pair mates in both the reproductive and nonreproductive seasons; males also showed more mate-searching behaviour, switching more frequently between choice options than females. Females, but not males, preferred their current pair mate over a novel potential pair mate; females also engaged in more frequent switching between pair mates and novel males, but only during the nonreproductive season. Thus, while both males and females exhibit preferences in pairing, size-assortative pairing in snapping shrimp does not result from mutual mate choice for size, but rather preferences for size in one sex, with size-assortative pairing reinforced by preferences for established pair mates in the other. Furthermore, pairing behaviour may be dynamic, shifting across seasons even in a year-round socially monogamous species. • Male snapping shrimp preferred larger females, even during the nonreproductive season. • Females preferred their current pair mate over size-matched novel males. • Females increased mate sampling during the nonreproductive season. • Males showed no preference for current mates; females showed no size preferences. • Sex differences in preferences led to year-round size-assortative monogamy. [ABSTRACT FROM AUTHOR]

Division of labour allows group-living species to efficiently complete tasks while minimizing resource expenditure. This generally involves task allocation between individuals. In territorial defence and parental care specifically, division of labour can be completed using different strategies, often involving one or more individuals defending the territory while others care for offspring. Little is known, however, about division of labour by monogamous and biparental mammals. We investigated labour division strategies across territorial defence and pup retrievals by monogamous, biparental, territorial California mice along with the role of vocal communication in coordinating these behaviours. Male and female pair-bonded California mice display aggression towards same-sex intruders and retrieve pups while alone, but how pairs complete these tasks while together remains unknown. We found that California mouse pairs used one of two strategies during territorial defence: (1) joint defence or (2) divided defence. Overall, these strategies were not altered by the intruder sex or the birth of pups. However, postpartum individuals spent more time alone in the nest and pairs spent less time together investigating intruders. Pup retrievals, conversely, followed a sex-specific strategy where mothers retrieved pups in 89% of pairs while males retrieved pups in 11% of pairs. This study shows for the first time that a monogamous and biparental rodent uses different strategies to divide labour during vital tasks of territorial defence and pup retrieval. Moreover, we found that vocal communication via sustained vocalizations were predictive of aggressive behaviour but not retrieval behaviour, indicating that vocalizations may only play a role in coordinating specific behaviours. Importantly, these strategies were task dependent and robust across intruder sex and parental status, providing a framework for better understanding division of labour in mammals. • Pairs defended territories across contexts using a joint or divided defence strategy. • There was a sex difference in pup retrievals such that females predominantly retrieved. • Male sustained vocalizations predicted defence behaviour but not retrieval behaviour. [ABSTRACT FROM AUTHOR]

The prairie vole has proven a valuable animal model for the neurobiological study of social monogamy and pair bonding. Previous research has focused almost exclusively on virgin prairie voles forming pair-bonds for the first time – a paradigm with limited relevance to human social behavior. In the present study, we used stud males to assess the impact of repeated pair-bond formation and dissolution on the behaviors and neurobiology relevant to subsequent pair-bond formation. Stud males were tested for behavioral and neurobiological effects of repeated pair-bonding after the 1st, 5th, and 10th pairing. Aged breeder males that experienced minimal pair-bond dissolution were included to control for the effects of aging. Results showed that male prairie voles readily form new pair-bonds after repeated pair-bond dissolution. In terms of social monogamy, old age was associated with males spending less time in close social contact with unfamiliar females. There were no effects of age nor number of lifetime pairings on depressive-like behavior or paternal behavior toward pups. Within the brain, the patterns of oxytocin (OTR) and vasopressin type 1a (V1aR) receptors were largely unaffected, with the following exceptions: 1) males with only a single pairing had higher OTR densities in the paraventricular thalamus and bed nucleus of the stria terminalis; 2) there was an age-related increase in the density of OTR in the caudate putamen and an age-related decline in the density of V1aR in the cortical amygdala. The present findings have translational relevance to human social behavior in the context of aging and social experience. • Male prairie voles readily form new pair-bonds at least ten times. • Repeated pair-bond dissolution did not negatively impact affect. • Male voles did not show experience-related changes in paternal behavior. • Older males associated less with strange females. [ABSTRACT FROM AUTHOR]

Animal interactions such as competition are mediated by complex social strategies, which consist of behaviours and cognitive mechanisms that guide their production. As a result, behaviour is highly flexible. This poses a challenge to understanding how competition plays out in natural systems, because the course of a contest can essentially be rewritten by prior experience and/or changes in social context. Here we addressed this gap by studying how both of these factors interact to reconfigure competitive strategies used in territorial defence by a wild bird. After experimentally inducing the winner effect, a cognitive-behavioural phenomenon in which winning a contest increases the probability of winning again in the future, we found that male red-bellied woodpeckers, Melanerpes carolinus , adopted a new social strategy marked by more flexible transitions between different aggressive displays, as well as increasing overall aggressive output. However, this effect was mitigated by the arrival of the female social mate; in response to this momentary shift in social context, males decreased their use of territorial drum displays and became less likely to move around the territory or switch display modes during competition. In other words, the winner effect increased spatiotemporal diversity of territorial strategies, such that males frequently changed their location and display output. A female's arrival, however, reversed this effect. More specifically, males tended to revert to advertisement and social vocalizations on female arrival, which suggests that the need to attend to the social mate may supersede the threat of territorial intrusion. This is consistent with a model in which competitive outcomes are impacted by the interactive effects of an individual's past experiences and shifts in present-day social context, which may allow monogamous animals to effectively manage the competing demands of driving off intruders and attending to the social mate. Highlights • We show that free-living male woodpeckers experience a winner effect. • Males are more aggressive after winning, but not if their (female) mate is present. • With a female present, males favour multifunctional calls over aggressive drums. • The winner effect is 'reversible' to optimize territorial strategy across contexts. [ABSTRACT FROM AUTHOR]

Abstract The mating distribution caused by mate choice can be expressed as a gain in information with respect to random mating. In that view, the population phenotype frequencies had been considered constant during the breeding season. Here, such restriction was relaxed to consider encounter–mating processes in which first, the encounter between partners depends on the phenotype distribution of the population, and second, the mating after the encounter depends on the mutual mating propensities. Under this setting, the population phenotype frequencies are no longer constant because the process of pair formation occurs in discrete intervals of time, called mating rounds where at least one mating pair is formed, and the frequency of phenotypes available for the next mating, changes. Provided that there are more than one mating round per breeding season, the population phenotype frequencies are no longer constant. Similarly to the constant case, we describe the change in the mating phenotypes by the flow of information with respect to random mating. This information can be partitioned into sexual selection, assortative mating (sexual isolation) and their mixed effect. Likewise, the pairwise statistics for total change, sexual selection and assortative mating are generalized for variable population phenotype frequencies. The new tests had more power to detect the effects of non-random mating when the phenotype frequencies varied during the breeding season. The differences in power were high for sexual selection but minor for assortative mating scenarios. However, the application of the new formulas requires the estimation of phenotype frequencies at each mating round. Therefore, choosing one or another type of statistics would depend on the biological scenario as well as the availability and easiness to split the sampling in more than one mating round. [ABSTRACT FROM AUTHOR]

Highlights • ERα-immunoreactivity (-ir) was quantified in Peromyscus californicus mice isolated or pair-housed for 4 or 24 days. • Isolated males had significantly greater ERα-ir in the VMH, and similar patterns in the BST and MPOA. • Males housed for 24 days had significantly greater ERα-ir in the BST, VMH, and MPOA compared to 4 days. • Females housed for 24 days had significantly greater ERα-ir in the DG than females housed for 4 days. • Results demonstrate that social environment affects ERα-ir in specific limbic system areas. Abstract Social environment has well-established effects on an animal's social behavior and associated neuroendocrine responses. The presence of estrogen receptor alpha (ERα) in limbic system brain regions is related to the expression of a variety of social, reproductive and aggressive behaviors. We hypothesized that alterations to the social environment, specifically social isolation, would cause changes in ERα throughout the limbic system. The number of ERα immunoreactive (ERα-ir) cells within specific limbic system brain regions was quantified in male and female California mice (Peromyscus californicus) , isolated or same sex pair-housed for 4 or 24 days. Peromyscus californicus is a highly social rodent species (monogamous and bi-parental) and therefore, may be particularly sensitive to manipulations of its social environment. Isolated males had a significantly greater number of ERα-ir cells in the ventromedial nucleus of the hypothalamus (VMH) and similar patterns within the bed nucleus of the stria terminalis (BST) and medial preoptic area (MPOA). Males housed for 24 days had a significantly greater number of ERα-ir cells in the BST, VMH, MPOA when compared with males housed for 4 days. Females housed for 24 days had significantly greater ERα-ir in the dentate gyrus of the hippocampus (DG) when compared with females housed for 4 days. No differences were found in the medial amygdala (MeA). These data demonstrate that social environment has region and sex specific effects on ERα-ir cells in this species. These results add to the comparative evidence regarding ERα, demonstrating a consistent role for ERα in species specific responsiveness to changes in the social environment. [ABSTRACT FROM AUTHOR]

Social relationships, affiliative social attachments, are important for many species. The best studied types of relationships are monogamous pair bonds. However, it remains unclear how generalizable models of pair bonding are across types of social attachments. Zebra finches are a fascinating system to explore the neurobiology of social relationships because they form various adult bonds with both same- and opposite-sex partners. To test whether different bonds are supported by a single brain network, we quantified individuals' neuroendocrine state after either 24 h or 2 weeks of co-housing with a novel same- or opposite-sex partner. We defined neuroendocrine state by the expression of 22 genes related to 4 major signaling pathways (dopamine, steroid, nonapeptide, and opioid) in six brain regions associated with affiliation or communication [nucleus accumbens (NAc), nucleus taeniae of the amygdala (TnA), medial preoptic area (POM), and periaqueductal gray (PAG), ventral tegmental area, and auditory cortex]. Overall, we found dissociable effects of social contexts (same- or opposite-sex partnerships) and duration of co-housing. Social bonding impacted the neuroendocrine state of four regions in males (NAc, TnA, POM, and PAG) and three regions in females (NAc, TnA, and POM). Monogamous pair bonding specifically appeared to impact male NAc. However, the patterns of gene expression in zebra finches were different than has previously been reported in mammals. Together, our results support the view that there are numerous mechanisms regulating social relationships and highlight the need to further our understanding of how social interactions shape social bonds. • Expression patterns of 22 neuroendocrine genes differ across brain regions. • Social relationships impact neuroendocrine state of NAc, TnA, POM and PAG. • Social relationships may have sex-specific effects in POM and PAG. • Opposite-sex pair bonds may have distinct neuroendocrine states in the NAc. • Different social relationships may utilize distinct brain networks. [ABSTRACT FROM AUTHOR]

Abstract In socially-monogamous species, intolerance of interactions between a pairmate and a sexual rival (i.e., mate-guarding) promotes the preservation of long-lasting partnerships. One promising neurobiological candidate for the regulation of mate-guarding behavior in monogamous primates is the oxytocin (OT) system, given its established role in both the development of monogamous bonds and the behavioral processes that facilitate the preservation of those bonds. In this study, male and female marmosets were exposed to a same-sex intruder in their home environment during conditions when their pairmate was present and absent, and across three treatment conditions (OT receptor agonist; saline control; OT receptor antagonist). Saline-treated marmosets spent significantly more time in proximity to the intruder, relative to the empty pairmate enclosure, when their pairmate was absent. However, when marmosets received OT they spent less time in proximity to the intruder, indicating that OT may reduce interest in a same-sex stranger in a territorial context. When their pairmate was present, saline-treated marmosets spent equal time in proximity to both intruder and pairmate; yet when they received OT they spent significantly more time in proximity to the intruder, indicating that OT may increase interest in a same-sex stranger in a mate-guarding context. While OT treatment did not directly influence the expression of aggression, OT system manipulations impacted the expression of selective social interest during an intruder challenge, suggesting that OT may enhance adaptive responses to social challenges. Moreover, these findings add to the converging evidence that the OT system regulates behavioral processes that underlie the preservation of established relationships. Highlights • Mate-guarding behavior promotes the preservation of established relationships. • Marmosets were quicker to aggress toward a same-sex intruder when their mate was present. • OT reduced interest in a same-sex stranger in a territorial context. • OT increased interest in a same-sex stranger in a mate-guarding context. • OT system regulates adaptive responses to challenges to established bonds. [ABSTRACT FROM AUTHOR]

Acoustic communication is vital to complex social behaviours such as territorial defence. The use of ultrasonic vocalizations, particularly in territorial defence by monogamous species and females, remains understudied. We studied ultrasonic vocalization production and associated aggression in the monogamous, biparental and territorial California mouse, Peromyscus californicus , in which both males and females were found to display similar levels of physical aggression against same-sex intruders. We identified specific ultrasonic vocalization calls that are modulated based on social context: (1) sustained vocalizations, which are long, low-bandwidth calls ranging from 22 to 25 kHz, and (2) barks, which are short, high-intensity calls beginning and ending in the audible range. Despite similarities in physical aggression, sex differences emerged in vocal communication. Only resident males, and not females, produced sustained vocalizations prior to the onset of physical aggression, and were found to shorten the duration of individual sustained vocalization calls over both the course of the pre-encounter phase and from the pre-encounter to encounter phase. In addition, the degree of sustained vocalization shortening in males predicted offensive aggression of the resident. Males exhibited shorter sustained vocalization calls during encounters than females. Barks occurred more frequently during female–female physical aggression than in male–male encounters, and correlated highly with defensive aggression by intruders. Finally, a newly identified highly complex call, sweep phrases, was recorded in a subset of both sexes in the pre- and post-encounter phases. The overall results indicate that ultrasonic vocalizations may play an important role in territorial defence during both territorial advertisement and aggression in a monogamous rodent. Overall, this monogamous species showed sex similarities in physical aggression but sex differences in vocal communication and a more sophisticated function for sustained vocalizations than previously recognized. [ABSTRACT FROM AUTHOR]

Pair bonds can provide social benefits to long-term monogamous species alongside their benefits for reproduction. However, little is known about when these bonds form, in particular how long they are present before breeding. Previous studies of pair formation in long-term monogamous birds have been rather data-limited, but for many migratory birds they report pair formation on the wintering grounds. We provide the first systematic investigation of prebreeding association patterns of long-term monogamous pairs by examining entire life histories based on tracking data of migratory whooping cranes, Grus americana . We found that a substantial portion (62%) of breeding pairs started associating at least 12 months before first breeding, with 16 of 58 breeding pairs beginning to associate over 2 years before first breeding. For most pairs, these associations with future breeding partners also became unique and distinguishable from association patterns with nonpartner individuals 12 months before first breeding. In addition, 60% of pair associations began before at least one partner had reached nominal sexual maturity. Most pairs began associating in the late spring upon arrival at the summer grounds, while associations beginning at other times of the year were rare. Patterns in the associations of pairs prior to breeding can point to the potential benefits of prebreeding relationships, for instance providing support in competitive interactions or increasing partner familiarity. [ABSTRACT FROM AUTHOR]

Most studies examining rodent ultrasonic vocalizations (USVs) have investigated pericopulatory vocal behaviour in polygynous rodents, while vocalizations related to pair bond maintenance in monogamous rodents remain unexplored. In the monogamous California mouse, Peromyscus californicus , we used ultrasonic playbacks and post-playback social interactions to assess possible functions of USVs. We found that females responded with approach towards USVs of an unfamiliar male (bonded male from another pair) compared to a noise control, but displayed no difference in response to calls of their partner versus noise. Responsiveness to unfamiliar males does not appear to reflect an interest in extrapair copulations because during post-playback social interactions, females displayed more agonistic behaviours and fewer affiliative behaviours towards unfamiliar, sexually naïve ‘stranger’ males than towards their partners. We speculate that approach to unfamiliar male USVs instead may be related to territorial defence. We further explored associations within the data set. Interestingly, female affiliation with her partner was predicted by USV output, particularly a higher number and proportion of complex call types, produced during the male partner USV elicitation phase. Female approach towards USVs was related to syllable duration of one call type in partner USVs (not in unfamiliar USVs) but no other features, and sufficient variation exists in syllable duration to allow females to theoretically distinguish between individuals based on this measure. Similarly, while pregnancy state did not influence female social behaviour, it decreased approach to playback of partner USVs but not to that of unfamiliar USVs. Overall, our results illuminate concepts about vocal communication in monogamous rodent species with strong pair bonds and suggest that functions of USVs in rodents can extend beyond mate choice. [ABSTRACT FROM AUTHOR]

As dispersal is a dangerous part of an individual's life, its timing is important to increase the chances of survival and successful establishment of a territory. We investigated factors affecting the timing of natal dispersal in the Eurasian beaver, Castor fiber , a territorial, monogamous, long-lived mammal, using data from an 18-year individual-based study (1998–2015). We tested hypotheses about the causes of dispersal onset, namely competitive ability, kin competition (sibling competition and offspring–parent competition), population density and intolerance by an incoming, unrelated dominant individual. Only 9% of individuals remained philopatric and became dominant after both of their parents disappeared. Average age at dispersal was 3.5 years, with some individuals delaying dispersal up to age 7 years. Beavers dispersed more frequently with increasing age (i.e. with increasing competitive ability and possibly experience) and when population density was lower. Further, both females and males delayed dispersal with increasing same-sex parental age. Older parents were either more tolerant towards philopatric subordinates, or subordinates awaited the disappearance of their senescing parents to take over the natal territory. From comparisons with other populations, we conclude that the high population density in our area was possibly the ultimate driver of dispersal with individuals delaying dispersal to increase their competitive ability. [ABSTRACT FROM AUTHOR]

Cesarean delivery is associated with diminished plasma levels of several 'birth-signaling' hormones, such as oxytocin and vasopressin. These same hormones have been previously shown to exert organizational effects when acting in early life. For example, our previous work found a broadly gregarious phenotype in prairie voles exposed to oxytocin at birth. Meanwhile, cesarean delivery has been previously associated with changes in social behavior and metabolic processes related to oxytocin and vasopressin. In the present study, we investigated the long-term neurodevelopmental consequences of cesarean delivery in prairie voles. After cross-fostering, vole pups delivered either via cesarean or vaginal delivery were studied throughout development. Cesarean-delivered pups responded to isolation differently in terms of their vocalizations (albeit in opposite directions in the two experiments), huddled in less cohesive groups under warmed conditions, and shed less heat. As young adults, we observed no differences in anxiety-like or alloparental behavior. However, in adulthood, cesarean-delivered voles of both sexes failed to form partner preferences with opposite sex conspecifics. In a follow-up study, we replicated this deficit in partner-preference formation among cesarean-delivered voles and were able to normalize pair-bonding behavior by treating cesarean-delivered vole pups with oxytocin (0.25 mg/kg) at delivery. Finally, we detected minor differences in regional oxytocin receptor expression within the brains of cesarean-delivered voles, as well as microbial composition of the gut. Gene expression changes in the gut epithelium indicated that cesarean-delivered male voles have altered gut development. These results speak to the possibility of unintended developmental consequences of cesarean delivery, which currently accounts for 32.9 % of deliveries in the U.S. and suggest that further research should be directed at whether hormone replacement at delivery influences behavioral outcomes in later life. • Prairie voles delivered by cesarean section behaved differently as pups, indicating altered thermoregulation in early life. • Prairie voles delivered by cesarean section fail to form partner preferences as adults. • Oxytocin administration directly to neonates immediately following cesarean delivery normalizes adult partner preference formation. • Birth mode minimally affected oxytocin and vasopressin receptor expression in the brain and microbial composition of the gut. • Male prairie voles delivered by cesarean section did have lasting changes in gut epithelium gene expression in adulthood. [ABSTRACT FROM AUTHOR]

Pair bonds are the cornerstone of a monogamous relationship. When individuals of the same species engage in monogamy and promiscuity (i.e. alternative reproductive tactics) it can be difficult to determine which tactic confers greater fitness, as measures of fitness can be difficult to ascertain. However, in these circumstances, whether animals preferentially establish pair bonds can reveal decisions that presumably reflect the animals' assessment of how to best maximize reproductive success. In nature, the majority of prairie voles, Microtus ochrogaster , establishes pair bonds and engages in social monogamy while a minority of individuals remains single and presumably mates promiscuously. The existence of these two tactics raises the interesting question: do bonded male prairie voles choose to ‘settle’ (for just one partner) or are they preferentially ‘settling down’? To determine which of these two tactics is preferred, we provided single male prairie voles simultaneous access to two sexually receptive females for 24 h and then subsequently tested males in ‘partner preference tests’ with each female independently contrasted with a novel female. We aimed to determine whether males would form a pair bond with one, both or none of the original females. We found that males formed pair bonds with one of the two females. We also investigated male- and female-initiated aggression and found that during the bonding process males were more aggressive with females that they did not ultimately form a bond with. In the partner preference tests, males showed more aggression towards unfamiliar females than towards familiar females. Mismatches in male- and female-initiated aggression suggest that aggressive interactions may be perpetuated more by males than by females. Taken together, our results demonstrate that under conditions that are ideal for forgoing bonding and engaging in multiple matings, males choose to establish a pair bond, suggesting that selective pressures may have facilitated bonding by males. [ABSTRACT FROM AUTHOR]

• Haldane's arithmetic is introduced. • Conditions for evolutionary stability of sib altruism are given. • Evolutionary stability is equivalent to Haldane's arithmetic in the studied model. • Generalized Hamilton's rules are formulated. The moral rule "Risk your life to save your family members" is, at the same time, a biological phenomenon. The prominent population geneticist, J.B.S. Haldane told his friends that he would risk his life to save two drowning brothers, but not one – so the story goes. In biological terms, Haldane's arithmetic claims that sib altruism is evolutionarily rational, whenever by "self-sacrifice" an altruistic gene "rescues", on average, more than one copy of itself in its lineage. Here, we derive conditions for evolutionary stability of sib altruism, using population genetic models for three mating systems (monogamy, promiscuity and polygyny) with linear and non-linear group effect on the siblings' survival rate. We show that for all considered selection situations, the condition of evolutionary stability is equivalent to Haldane's arithmetic. The condition for evolutionary stability is formulated in terms of genetic relatedness and the group effect on the survival probability, similarly to the classical Hamilton's rule. We can set up a "scale of mating systems", since in pairwise interactions the chance of evolutionary stability of sib altruism decreases in this order: monogamy, polygyny and promiscuity. Practice of marrying and siblings' solidarity are moral rules in a secular world and in various religious traditions. These moral rules are not evolutionarily independent, in the sense that the subsistence of sib altruism is more likely in a monogamous population. [ABSTRACT FROM AUTHOR]

Relatively little is known about serotonergic involvement in pair-bonding despite its putative role in regulating social behavior. Here we sought to determine if pharmacological elevation of serotonin 1A (5-HT 1A ) receptor activity would lead to changes in social behavior in pair-bonded male titi monkeys ( Callicebus cupreus ). Adult males in established heterosexual pairs were injected daily with the selective 5-HT 1A agonist 8-OH-DPAT or saline for 15 days using a within-subjects design. Social behavior with the female pair-mate was quantified, and plasma concentrations of oxytocin, vasopressin, and cortisol were measured. When treated with saline, subjects showed reduced plasma oxytocin concentrations, while 8-OH-DPAT treatment buffered this decrease. Treatment with 8-OH-DPAT also led to decreased plasma cortisol 15 minutes post-injection and decreased social behavior directed toward the pair-mate including approaching, initiating contact, lipsmacking, and grooming. The reduction in affiliative behavior seen with increased activity at 5-HT 1A receptors indicates a substantial role of serotonin activity in the expression of social behavior. In addition, results indicate that the effects of 5-HT 1A agonism on social behavior in adulthood differ between rodents and primates. [ABSTRACT FROM AUTHOR]

Oxytocin (OT) and vasopressin (AVP) are important hypothalamic neuropeptides that regulate peripheral physiology, and have emerged as important modulators of brain function, particularly in the social realm. OT structure and the genes that ultimately determine structure are highly conserved among diverse eutherian mammals, but recent discoveries have identified surprising variability in OT and peptide structure in New World monkeys (NWM), with five new OT variants identified to date. This review explores these new findings in light of comparative OT/AVP ligand evolution, documents coevolutionary changes in the oxytocin and vasopressin receptors (OTR and V1aR), and highlights the distribution of neuropeptidergic neurons and receptors in the primate brain. Finally, the behavioral consequences of OT and AVP in regulating NWM sociality are summarized, demonstrating important neuromodulatory effects of these compounds and OT ligand-specific influences in certain social domains. [ABSTRACT FROM AUTHOR]

The coordination of behaviours between mates is a central aspect of the biology of the monogamous pair bonding in birds. This coordination may rely on intrapair acoustic communication, which is surprisingly poorly understood. Here we examined the impact of an increased level of background noise on intrapair acoustic communication at the nest in the zebra finch, Taeniopygia guttata . We monitored how partners adapted their acoustic interactions in response to a playback of wind noise inside the nestbox during incubation. Both zebra finch parents incubate and use coordinated call duets when they meet at the nest. The incubating parent can vocalize to its partner either outside the nestbox (sentinel duets) or inside the nestbox (relief and visit duets), depending on the context of the meeting. Pairs use these duets to communicate on predation threats (sentinel duets), incubation duties (relief) and other nesting activities (visit duets). Each of these duets probably represents a critical component of pair coordination. In response to the noise playback, partners called less and more rapidly during visit and relief duets. Female and male calls were more regularly and precisely alternated during relief duets. Mates increased the number of visit duets and their spatial proximity during sentinel duets. Furthermore, both females and males produced louder, higher-frequency and less broadband calls. Taken together our results show that birds use several strategies to adjust to noise during incubation, underlining the importance of effective intrapair communication for breeding pairs. [ABSTRACT FROM AUTHOR]

Monogamous pair bonds can be transient or long-lasting, which varies across species. The neuroendocrine mechanisms regulating pair maintenance behaviours are largely unknown, yet fundamental to our understanding of monogamy. Furthermore, the expression and regulation of pair maintenance behaviour is likely to be greatly influenced by social and environmental contexts. Our previous research suggested that androgens might regulate long-term pair maintenance behaviour in the monogamous zebra finch, Taeniopygia guttata . Here, we tested the hypothesis that testosterone treatment to males affects long-term pair maintenance behaviour in zebra finches. Established pairs were randomly assigned to one of two groups: control ( N = 8) or testosterone ( N = 7). Males were given either an empty or a testosterone-filled Silastic implant. Physical and acoustic affiliative behaviours and plasma testosterone levels were examined at three time points: pre-implantation, 30 days post-implant and 60 days post-implant. Importantly, we examined affiliative behaviours under two contexts: in the home cage (baseline) and following a brief chase (post stressor). Male testosterone treatment had no effects on behaviour during the baseline period, but significantly affected behaviours during the post stressor period. Specifically, testosterone-treated males spent less time in close proximity to their partner and sang more. To our knowledge, this is the first report of a context-dependent effect in the neuroendocrine regulation of pair maintenance behaviour, as well as the first report of an inhibitory effect of testosterone on zebra finch pairing behaviours. These results raise interesting questions about the function of affiliative behaviours in established pair bonds. [ABSTRACT FROM AUTHOR]

Oxytocin (OT) is a deeply conserved nonapeptide that acts both peripherally and centrally to modulate reproductive physiology and sociosexual behavior across divergent taxa, including humans. In vertebrates, the distribution of the oxytocin receptor (OTR) in the brain is variable within and across species, and OTR signaling is critical for a variety of species-typical social and reproductive behaviors, including affiliative and pair bonding behaviors in multiple socially monogamous lineages of fishes, birds, and mammals. Early work in prairie voles suggested that the endogenous OT system modulates mating-induced partner preference formation in females but not males; however, there is significant evidence that central OTRs may modulate pair bonding behavior in both sexes. In addition, it remains unclear how transient windows of central OTR signaling during sociosexual interaction modulate neural activity to produce enduring shifts in sociobehavioral phenotypes, including the formation of selective social bonds. Here we re-examine the role of the central OT system in partner preference formation in male prairie voles using a selective OTR antagonist delivered intracranially. We then use the same antagonist to examine how central OTRs modulate behavior and immediate early gene (Fos) expression, a metric of neuronal activation, in males during brief sociosexual interaction with a female. Our results suggest that, as in females, OTR signaling is critical for partner preference formation in males and enhances correlated activation across sensory and reward processing brain areas during sociosexual interaction. These results are consistent with the hypothesis that central OTR signaling facilitates social bond formation by coordinating activity across a pair bonding neural network. [ABSTRACT FROM AUTHOR]

This article is part of a Special Issue “Parental Care”. Mother–infant bonding is a characteristic of virtually all mammals. The maternal neural system may have provided the scaffold upon which other types of social bonds in mammals have been built. For example, most mammals exhibit a polygamous mating system, but monogamy and pair bonding between mating partners occur in ~ 5% of mammalian species. In mammals, it is plausible that the neural mechanisms that promote mother–infant bonding have been modified by natural selection to establish the capacity to develop a selective bond with a mate during the evolution of monogamous mating strategies. Here we compare the details of the neural mechanisms that promote mother–infant bonding in rats and other mammals with those that underpin pair bond formation in the monogamous prairie vole. Although details remain to be resolved, remarkable similarities and a few differences between the mechanisms underlying these two types of bond formation are revealed. For example, amygdala and nucleus accumbens–ventral pallidum (NA–VP) circuits are involved in both types of bond formation, and dopamine and oxytocin actions within NA appear to promote the synaptic plasticity that allows either infant or mating partner stimuli to persistently activate NA–VP attraction circuits, leading to an enduring social attraction and bonding. Further, although the medial preoptic area is essential for maternal behavior, its role in pair bonding remains to be determined. Our review concludes by examining the broader implications of this comparative analysis, and evidence is provided that the maternal care system may have also provided the basic neural foundation for other types of strong social relationships, beyond pair bonding, in mammals, including humans. [ABSTRACT FROM AUTHOR]

In this paper, we study the α th power monogamy properties related to the entanglement measure in bipartite states. The monogamy relations related to the α th power of negativity and the Convex-Roof Extended Negativity are obtained for N -qubit states. We also give a tighter bound of hierarchical monogamy inequality for the entanglement of formation. We find that the GHZ state and W state can be used to distinguish both the α th power of the concurrence for 0 < α < 2 and the α th power of the entanglement of formation for 0 < α ≤ 1 2 . Furthermore, we compare concurrence with negativity in terms of monogamy property and investigate the difference between them. [ABSTRACT FROM AUTHOR]

Social recognition is an integral component of behaviour that underlies many much larger behavioural suites. For example, monogamous pair bonding is relatively meaningless if an individual cannot recall with whom the bond was with. The prairie vole, Microtus ochrogaster , is a socially monogamous rodent, well known for its long-term pair bonds between males and females. Although previous work has shown that bonded males reliably spend more time with their pair-mate over an unfamiliar female, recent work has demonstrated that single male prairie voles do not discriminate between females. This discrepancy raises the important question: do paired males distinguish between nonmate females? We asked whether pair bonding alters the expression of social recognition in male voles by comparing social recognition of single and pair-bonded males using the habituation/dishabituation paradigm. We found that pair-bonded, but not single male prairie voles showed social recognition of (nonmate) females, suggesting a shift in cognitive behaviour after pair bond formation. This difference was not due to differences in motivation to engage in social exchanges, as males attempted to contact unfamiliar females at similar levels. Based on these data, we speculate that the stage of life (single or bonded) influences the relevance of attending to social information of same- and opposite-sex conspecifics. [ABSTRACT FROM AUTHOR]

The utility of monogamy (in practice) as a strategy for preventing sexually transmitted infections (STIs) was investigated. By reviewing recent literature surrounding monogamous relationships and sexual behaviors, the authors determined that monogamy might not prevent against STIs as expected. First, the authors elucidate the ways in which public health officials and the general public define and interpret monogamy and discuss how this contributes to monogamy as an ineffectual STI prevention strategy. Second, the authors provide evidence that individuals' compliance with monogamy is likely to be low, similar to rates of compliance with other medical advice. Lastly, the authors draw upon recent research findings suggesting that people who label themselves as monogamous are less likely to engage in safer sex behaviors than people who have an explicit agreement with their partner to be non-monogamous. Future research and clinical directions to promote sexual health and destigmatize sexual behaviors are considered. [ABSTRACT FROM AUTHOR]

Physiological resonance – where the physiological state of a subject generates the same state in a perceiver – has been proposed as a proximate mechanism facilitating pro-social behaviours. While mainly described in mammals, state matching in physiology and behaviour could be a phylogenetically shared trait among social vertebrates. Birds show complex social lives and cognitive abilities, and their monogamous pair-bond is a highly coordinated partnership, therefore we hypothesised that birds express state matching between mates. We show that calls of male zebra finches Taeniopygia guttata produced during corticosterone treatment (after oral administration of exogenous corticosterone and during visual separation from the partner) provoke both an increase in corticosterone concentrations and behavioural changes in their female partner compared to control calls (regular calls emitted by the same male during visual separation from the partner only), whereas calls produced during corticosterone treatment by unfamiliar males have no such effect. Irrespective of the caller status (mate/non-mate), calls' acoustic properties were predictive of female corticosterone concentration after playback, but the identity of mate calls was necessary to fully explain female responses. Female responses were unlikely due to a failure of the call-based mate recognition system: in a discrimination task, females perceive calls produced during corticosterone treatment as being more similar to the control calls of the same male than to control calls of other males, even after taking acoustical differences into account. These results constitute the first evidence of physiological resonance solely on acoustic cues in birds, and support the presence of empathic processes. [ABSTRACT FROM AUTHOR]

The steroid hormone testosterone (T) is a well-known mediator of male sexual behavior in vertebrates. However, less is known about T's rapid effects on sexual behavior, particularly those involving ultrasonic vocalizations (USVs), a mode of communication that can influence mate acquisition in rodents. Using the monogamous California mouse, Peromyscus californicus , we tested whether T rapidly alters male USV production by giving T or saline injections to non-paired (sexually naïve) males and paired (paternally experienced and pair-bonded) males immediately prior to a brief exposure to an unrelated, novel female. Among non-paired males, no differences in the total number of USVs were observed; however, T increased the proportion of simple sweeps produced. Among paired males, T decreased the number of USVs produced, and this change was driven by a reduction in simple sweeps. These results suggest a differential rapid effect of T pulses between non-paired and paired males upon exposure to a novel female. Additionally, we observed a positive correlation in the production of USVs made between males and novel females, and this relationship was altered by T. Given the importance of USVs in sexual communication, our study supports an essential concept of monogamy in that mate fidelity is reinforced by decreased responsiveness to prospective mates outside of the pair bond. The central mechanism in pair bonded males that decreases their responsiveness to novel females appears to be one that T can trigger. This is among the first studies to demonstrate that T can inhibit sexually related behaviors and do so rapidly. [ABSTRACT FROM AUTHOR]

The Neotropical cotingas (Cotingidae: Aves) are a group of passerine birds that are characterized by extreme diversity in morphology, ecology, breeding system, and behavior. Here, we present a comprehensive phylogeny of the Neotropical cotingas based on six nuclear and mitochondrial loci (∼7500 bp) for a sample of 61 cotinga species in all 25 genera, and 22 species of suboscine outgroups. Our taxon sample more than doubles the number of cotinga species studied in previous analyses, and allows us to test the monophyly of the cotingas as well as their intrageneric relationships with high resolution. We analyze our genetic data using a Bayesian species tree method, and concatenated Bayesian and maximum likelihood methods, and present a highly supported phylogenetic hypothesis. We confirm the monophyly of the cotingas, and present the first phylogenetic evidence for the relationships of Phibalura flavirostris as the sister group to Ampelion and Doliornis , and the paraphyly of Lipaugus with respect to Tijuca. In addition, we resolve the diverse radiations within the Cotinga , Lipaugus , Pipreola , and Procnias genera. We find no support for Darwin’s (1871) hypothesis that the increase in sexual selection associated with polygynous breeding systems drives the evolution of color dimorphism in the cotingas, at least when analyzed at a broad categorical scale. Finally, we present a new comprehensive phylogenetic classification of all cotinga species. [ABSTRACT FROM AUTHOR]

Monogamy of quantum correlation measures puts restrictions on the sharability of quantum correlations in multiparty quantum states. Multiparty quantum states can satisfy or violate monogamy relations with respect to given quantum correlations. We show that all multiparty quantum states can be made monogamous with respect to all measures. More precisely, given any quantum correlation measure that is non-monogamic for a multiparty quantum state, it is always possible to find a monotonically increasing function of the measure that is monogamous for the same state. The statement holds for all quantum states, whether pure or mixed, in all finite dimensions and for an arbitrary number of parties. The monotonically increasing function of the quantum correlation measure satisfies all the properties that are expected for quantum correlations to follow. We illustrate the concepts by considering a thermodynamic measure of quantum correlation, called the quantum work deficit. [ABSTRACT FROM AUTHOR]

An important component of mate choice is the sex of the potential partner, yet little is known about the factors that cause an individual to pair with the opposite sex. In the current study, we addressed the role of exposure to adult females during development in adult mate choice using a socially monogamous and biparental avian species, the zebra finch, Taeniopygia guttata. In our experimental set-up, zebra finch offspring were either raised in the presence of adult males and females in their aviaries (control) or in the absence of all adult females including their mothers (female-deprived). The female-deprived subjects were raised successfully by the male parents and showed no growth deficits compared to controls. When we tested partner preference of control and female-deprived subjects as adults, 57% of males that had been raised without females paired with each other. In contrast, 76% of male offspring that were raised with adults of both sexes paired with females. The most likely interpretation of this dramatic effect on pairing outcome is imprinting on adult males in the aviary in the absence of all adult females. These results thus suggest that sexual imprinting or some other form of social learning is a key developmental process for sexual partner preference in addition to preference for species and individual characteristics. [Copyright &y& Elsevier]

Repeated formation and subsequent dissolution of romantic relationships is common in humans across a lifetime. The socially monogamous prairie vole (Microtus ochrogaster) is used to study mechanisms of these bonds. At least in the laboratory, male prairie voles form bonds with a new female partner after loss of a previous partner. Initial bond formation depends on activation of dopamine D2-like receptors in the nucleus accumbens. Blocking activity of this receptor subtype disrupts formation of an animal's first pair bond. It is not known if these same D2-like receptors facilitate pair bonding with a subsequent partner after previous partner loss. This study examined the effects of D2-like receptor blockade on repeated pair bonding in male prairie voles. Males were paired with an initial female and allowed to mate before being separated. After a 5-day separation, males were then treated with either saline or eticlopride, a selective D2-receptor antagonist, prior to being paired with a second female and being allowed to mate. After a second separation, males were tested to determine if they developed a preference for spending time with their first or second mate. Eticlopride-treated males spent more time in a cage containing one of their previous partners compared to time in an empty cage but did not form a selective preference for either partner. Saline-treated males preferred their second, more recent partner. D2 receptor antagonism, then, disrupts bond formation in a second pairing but does not help to maintain a bond with the initial partner. • Males prefer their most recent partner over a previously mated partner. • D2 receptor antagonism blocks preference for a second partner after loss of the first. • D2 receptor antagonism does not preserve preference for an initial partner. [ABSTRACT FROM AUTHOR]

Abstract: Neuroendocrine pathways that regulate social behavior are remarkably conserved across divergent taxa. The neuropeptides arginine vasotocin/vasopressin (AVT/AVP) and their receptor V1a mediate aggression, space use, and mating behavior in male vertebrates. The hormone prolactin (PRL) also regulates social behavior across species, most notably paternal behavior. Both hormone systems may be involved in the evolution of monogamous mating systems. We compared AVT, AVT receptor V1a2, PRL, and PRL receptor PRLR1 gene expression in the brains as well as circulating androgen concentrations of free-living reproductively active males of two closely related North American cichlid species, the monogamous Herichthys cyanoguttatus and the polygynous Herichthys minckleyi. We found that H. cyanoguttatus males bond with a single female and together they cooperatively defend a small territory in which they reproduce. In H. minckleyi, a small number of large males defend large territories in which they mate with several females. Levels of V1a2 mRNA were higher in the hypothalamus of H. minckleyi, and PRLR1 expression was higher in the hypothalamus and telencephalon of H. minckleyi. 11-ketotestosterone levels were higher in H. minckleyi, while testosterone levels were higher in H. cyanoguttatus. Our results indicate that a highly active AVT/V1a2 circuit(s) in the brain is associated with space use and social dominance and that pair bonding is mediated either by a different, less active AVT/V1a2 circuit or by another neuroendocrine system. [Copyright &y& Elsevier]

Abstract: Wild zebra finches form long-term monogamous pair-bonds that are actively maintained year-round, even when not in breeding condition. These desert finches are opportunistic breeders, and breeding is highly influenced by unpredictable rainfall. Their high levels of affiliation and complex breeding patterns make zebra finches an excellent model in which to study the endocrine regulation of affiliation. Here, we compared zebra finch pairs that were provided with water ad libitum (control) or water restricted. We examined (1) reproductive physiology, (2) pair-maintenance behaviors in several contexts, and (3) circulating and brain steroid levels. In females, water restriction profoundly reduced largest ovarian follicle size, ovary size, oviduct size, and egg laying. In males, water restriction had no effect on testes size but decreased systemic testosterone levels. However, in the hypothalamus, local testosterone and estradiol levels were unaffected by water restriction in both sexes. Systemic and local levels of the androgen precursor dehydroepiandrosterone (DHEA) were also unaffected by water restriction. Lastly, in three different behavioral paradigms, we examined a variety of pair-maintenance behaviors, and none were reduced by water restriction. Taken together, these correlational data are consistent with the hypothesis that local production of sex steroids in the brain promotes the expression of pair-maintenance behaviors in non-breeding zebra finches. [Copyright &y& Elsevier]

Abstract: The majority of human societies practice polygynous marriage, in line with the typical mating pattern found in mammals. Polygyny in humans is often associated with the transfer of wealth to a male’s sister’s offspring, and it has been suggested that this “mother’s brother phenomenon” is adaptive when paternity confidence is low. Polyandry, on the other hand, while virtually unknown in mammals, is practiced by a few human societies, and it has been suggested that this is adaptive if the co-husbands are genetically related. The evolution of human marriage strategies, therefore, can be studied in the framework of kin selection and game theory, as strategic transmission of wealth by males and strategic paternity allocation by females can evolve to maximize inclusive fitness. Here I analyse the stability of polygynous and polyandrous marriage using a game theoretical model previously developed to study monogamy. I show that the “mother’s brother phenomenon” depends on the degree of resource depletion through division, whereas the paternity threshold commonly discussed in the anthropological literature is largely irrelevant. Resource depletion through division is also the major determinant of the stability of polyandry, whereas relatedness between co-husbands is not essential. Finally, I show that when females control the transfer of their own resources, monogamy is stable under more general conditions than previously believed. [Copyright &y& Elsevier]

Inbreeding is generally rare in large, natural populations yet mate choice often appears to be random with respect to kinship. This suggests that the risks of inbreeding may be small because passive mechanisms of inbreeding avoidance, for example dispersal, are effective at lowering inbreeding risk. Previous theoretical and empirical studies have assumed that the risks of inbreeding are constant over an individual''s life span, but in the lek-breeding black grouse, inbreeding increases with female age. To determine whether inbreeding avoidance mechanisms are also age dependent, we generated four null models of random mate choice ranging from complete randomness to more biologically realistic mate choice criteria and compared these to 8years of data on inbreeding levels at four different female age classes. We additionally tested whether mate fidelity decreased inbreeding risk. Observed inbreeding in female age classes 1, 2 or ≥3 were not significantly different from random, but was approximately 3.5 times higher in female age class ≥4. Alternative models using mate choice criteria showed no significant differences between observed and expected levels for any age class. Our results are in line with previous studies on noncooperatively breeding passerine birds, which indicate that mating is random with respect to kinship and that increased inbreeding in older females can be explained by reduced male availability caused by restrictive mate choice criteria. We also found that individuals that switched mates significantly increased relatedness to partners suggesting that mate fidelity may have evolved as an important passive inbreeding avoidance mechanism. [Copyright &y& Elsevier]

Many monogamous animals engage in a period of pair bonding prior to their ultimate mate choice (reproduction). One hypothesis about the function of this behaviour is that it allows individuals to continuously assess the quality of their partner as well as other potential mates. I tested this hypothesis in the convict cichlid, Amatitlania nigrofasciata, by providing males and females with a choice between their partner and a novel individual of the opposite sex at various time points after formation of a pair bond. Sex differences were observed in patterns of behaviour, with females significantly more likely to spend time with their partner than with another male, while males generally spent more time with a novel female than with their partner. The time spent in a pair bond affected this pattern, however, with both sexes showing a decreased time-based partner preference as the time in a pair bond increased. After forming a pair bond, males were largely aggressive towards novel females, treating them as territorial intruders, while pair-bonded females generally ignored novel males. Neither sex appeared to actively court potential mates apart from their partner. In a second experiment, I attempted to alter this pattern by presenting pair-bonded individuals with novel potential mates of relatively higher quality (larger size) than their partner. When given a ‘better option’, neither females nor males showed a preference for their partner over a large novel mate. All males continued to treat novel females as territorial intruders, while a subset of females shifted their preference from their partner to a larger novel male. These results support the hypothesis that the role of pair bonding may differ between the sexes in this species, functioning as a prolonged period of mate assessment in females, but not in males. [ABSTRACT FROM AUTHOR]

Abstract: Despite its well-described role in female affiliation, the influence of oxytocin on male pairbonding is largely unknown. However, recent human studies indicate that this nonapeptide has a potent influence on male behaviors commonly associated with monogamy. Here we investigated the distribution of oxytocin receptors (OTR) throughout the forebrain of the socially monogamous male prairie vole (Microtus ochrogaster). Because males vary in both sexual and spatial fidelity, we explored the extent to which OTR predicted monogamous or non-monogamous patterns of space use, mating success and sexual fidelity in free-living males. We found that monogamous males expressed higher OTR density in the nucleus accumbens than non-monogamous males, a result that mirrors species differences in voles with different mating systems. OTR density in the posterior portion of the insula predicted mating success. Finally, OTR in the hippocampus and septohippocampal nucleus, which are nuclei associated with spatial memory, predicted patterns of space use and reproductive success within mating tactics. Our data highlight the importance of oxytocin receptor in neural structures associated with pairbonding and socio-spatial memory in male mating tactics. The role of memory in mating systems is often neglected, despite the fact that mating tactics impose an inherently spatial challenge for animals. Identifying mechanisms responsible for relating information about the social world with mechanisms mediating pairbonding and mating tactics is crucial to fully appreciate the suite of factors driving mating systems. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior. [Copyright &y& Elsevier]

Abstract: Female assessment of male attractiveness and how preferred qualities impact reproductive success is central to the study of mate choice. Male attractiveness may depend on traits beneficial to the reproductive success (RS) of any female, termed ‘universal quality’, and/or on behavioral and biological interactions between potential mates that reflect ‘compatibility’. The steroid hormone testosterone (T) often underlies male attractiveness in rodents and is associated with enhanced paternal care in the monogamous and biparental California mouse (Peromyscus californicus). We hypothesized that (1) T-characteristics are universally attractive to female California mice and that (2) if reproductive success is higher for females mated with preferred males, then females mated with males preferred by other females will also have higher reproductive success. Alternatively, we speculated that pair compatibility, based on emergent pair qualities, is important for a species with coordinated offspring care. We assessed individual T-characteristics in three ways: (1) T-response to GnRH challenges (2) baseline T-level and (3) T-response to a female. Testosterone-response did not predict female preference, but females spent more time investigating males with higher baseline T (accounting for only 9.6% of the variation in investigation time). None of the T-measures was associated with RS. Females paired with males they preferred produced litters more quickly and had higher RS than females paired with their non-preferred males. Naïve females who did not undergo preference tests had equivalent RS regardless of whether their mate was preferred or non-preferred by another female. These data suggest that higher male T elicits investigation, but female preference in the California mouse is more strongly linked with compatibility because individual preference was a better predictor of RS than any T measure. [Copyright &y& Elsevier]

Social monogamy in vertebrates often involves the joint defence of an all-purpose territory by pair members. Although both sexes presumably benefit from holding a territory, sex-related variation is often observed in territory defence at the interspecific level, particularly in birds. Most studies of territorial defence in monogamous birds, however, have been conducted on songbirds or on bird species living in temperate areas. In contrast, data on nonoscine tropical bird species remain scarce. We experimentally studied territorial defence in 20 pairs of Zenaida doves, a tropical and territorial monogamous bird species that maintains year-round pair bonds. Overall, males responded to intrusions more frequently than females. However, sex of the intruder had no effect on the reactivity of male or female residents. The use of aggressive behaviours of various intensities did not differ between males and females, irrespective of the sex of intruder. Although the percentage of male responses was correlated significantly and negatively with that of the females, the observed percentage of joint defence was significantly lower than would have been expected if pair members were reacting independently of each other. Finally, variation in reactivity of females to intruders was positively related to variation in intrusion pressure between trials, whereas no such trend was observed in males. We discuss our results in relation to the mating system of Zenaida doves and previous studies of territorial defence in birds. [Copyright &y& Elsevier]

Pair bond duration and age are suggested to be important determinants for reproductive success in long-term monogamous pair bonding. These two effects are generally difficult to separate, however, and experimental approaches are scarce. We experimentally examined the role of the extended pair bond period, controlling for age and experience, in the bearded reedling, Panurus biarmicus. In this species pairs form in juvenile flocks and usually remain stable for life. We experimentally manipulated the pair-formation period in aviary pairs prior to their first breeding season and observed the effect of varying pair bond duration on breeding success, male parental effort and breeding coordination. This was done for pairs with long (6.5months), medium (4months) and short (2weeks) formation periods. Our results suggest that the length of the bonding period does not influence male parental effort, as males with medium and short formation periods did not reduce their parental effort in nest building, during incubation or in nestling provisioning. Nevertheless, coordination of pair partners (in terms of simultaneous start of nest building) occurred more often when the pair bond period was long, and these pairs also started breeding earlier. Pairs with long bonds succeeded more often in breeding attempts and had higher hatching and fledging success than pairs with medium and short formation periods. Thus, our results support the hypothesis that the duration of the pair bond may improve coordination within a pair before the start of breeding and thereby may result in enhanced reproductive success. [ABSTRACT FROM AUTHOR]

Abstract: Oxytocin receptors (OXTR) in the nucleus accumbens (NAcc) promote alloparental behavior and partner preference formation in female prairie voles. Within the NAcc there is significant individual variation in OXTR binding and virgin juvenile and adult females with a high density of OXTR in the NAcc display an elevated propensity to engage in alloparental behavior toward novel pups. Over-expression of OXTR in the NAcc of adult female prairie voles using viral vector gene transfer facilitates partner preference formation, but has no effect on alloparental behavior, even though OXTR antagonists infused into the NAcc blocks both behaviors. We therefore hypothesized that long-term increases in OXTR signaling during development may underlie the relationship between adult OXTR density in the NAcc and alloparental behavior. To test this hypothesis, we used viral vector gene transfer to increase OXTR density in the NAcc of prepubertal, 21day old female prairie voles and tested for both alloparental behavior and partner preference formation as adults. Consistent with a developmental impact of OXTR signaling, adults over-expressing OXTR from weaning display both increased alloparental behavior and partner preference formation. Thus, the relatively acute impact of elevated OXTR signaling in the NAcc on partner preference formation previously reported appears to be dissociable from the effects of longer term, developmentally relevant OXTR signaling necessary for modulating alloparental behavior. These results are consistent with the hypothesis that oxytocin can have both long-term “organizational” effects as well as acute “activational” effects on affiliative behaviors. [Copyright &y& Elsevier]

In socially monogamous mammals, male behaviour can have important consequences on production and survival of offspring. Therefore, females in these species could enhance their reproductive success by discriminating among potential male social partners and mates on the basis of phenotypic cues correlated with male fidelity and paternal behaviour. Prairie voles, Microtus ochrogaster, are socially monogamous rodents that typically form pair bonds and show extensive biparental care. In male prairie voles, variation in the length of microsatellite DNA in the regulatory region of the avpr1a gene encoding the vasopressin 1a receptor (V1aR) underlies differences in V1aR neural expression and is correlated with significant differences in partner preference and paternal behaviour. Previous laboratory studies demonstrated that males possessing longer avpr1a microsatellite alleles spend more time with their female social partner, sire offspring with fewer females and provide more paternal care relative to males with shorter avpr1a microsatellite alleles. Our results from laboratory preference trials showed that female voles from populations in Illinois and Kansas displayed significant social and sexual preferences for males that possessed longer avpr1a microsatellite alleles. These findings are consistent with the hypothesis that females are using phenotypic cues whose expression is correlated with a male’s avpr1a microsatellite genotype to discriminate among potential mates. These social and sexual preferences for males with longer avpr1a microsatellite alleles may result in increased female reproductive success. [ABSTRACT FROM AUTHOR]

Abstract: In several mammalian species, lactating females show blunted neural, hormonal, and behavioral responses to stressors. It is not known whether new fathers also show stress hyporesponsiveness in species in which males provide infant care. To test this possibility, we determined the effects of male and female reproductive status on stress responsiveness in the biparental, monogamous California mouse (Peromyscus californicus). Breeding (N=8 females, 8 males), nonbreeding (N=10 females, 10 males) and virgin mice (N=12 females, 9 males) were exposed to a 5-min predator-urine stressor at two time points, corresponding to the early postpartum (5–7days postpartum) and mid/late postpartum (19–21days postpartum) phases, and blood samples were collected immediately afterwards. Baseline blood samples were obtained 2days prior to each stress test. Baseline plasma corticosterone (CORT) concentrations did not differ among male or female groups. CORT responses to the stressor did not differ among female reproductive groups, and all three groups showed distinct behavioral responses to predator urine. Virgin males tended to increase their CORT response from the first to the second stress test, while breeding and nonbreeding males did not. Moreover, virgin and nonbreeding males showed significant behavioral changes in response to predator urine, whereas breeding males did not. These results suggest that adrenocortical responses to a repeated stressor in male California mice may be modulated by cohabitation with a female, whereas behavioral responses to stress may be blunted by parental status. [Copyright &y& Elsevier]

Abstract: The formation of enduring relationships between adult mates (i.e., pair bonds) is an integral aspect of human social behavior and has been implicated in both physical and psychological health. However, due to the inherent complexity of these bonds and the relative rarity with which they are formed in other mammalian species, we know surprisingly little about their underlying neurobiology. Over the past few decades, the prairie vole (Microtus ochrogaster) has emerged as an animal model of pair bonding. Research in this socially monogamous rodent has provided valuable insight into the neurobiological mechanisms that regulate pair bonding behaviors. Here, we review these studies and discuss the neural regulation of three behaviors inherent to pair bonding: the formation of partner preferences, the subsequent development of selective aggression toward unfamiliar conspecifics, and the bi-parental care of young. We focus on the role of vasopressin, oxytocin, and dopamine in the regulation of these behaviors, but also discuss the involvement of other neuropeptides, neurotransmitters, and hormones. These studies may not only contribute to the understanding of pair bonding in our own species, but may also offer insight into the underlying causes of social deficits noted in several mental health disorders. [ABSTRACT FROM AUTHOR]