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Male-male competition is a component of Darwin's theory of precopulatory intrasexual selection, where males compete for access to resources important for reproduction and successful males pass on traits that endow greater competitive ability to their offspring, thereby exaggerating the trait(s) over evolutionary time. Metals, such as lead (Pb) often occur in urban mangrove forests, where shore crabs reside, and being neurotoxic may cause sub-lethal effects on behaviour. In this study, we asked whether exposure to Pb influences the outcomes of male-male competition in the Semaphore crab, Heloecius cordiformis, thereby altering outcomes of intrasexual selection. Crabs were exposed to an environmentally relevant, and a behaviourally relevant, concentration of lead (i.e. Pb, 10 µg/L and 100 µg/L respectively) under laboratory conditions for 96 h and subsequently allowed to compete for burrow ownership in experimental arenas. Exposed crabs (100 µg/L) were less successful than control crabs in chela size-matched paired competitive interactions, spent less time in burrows and predominantly lost burrow ownership. Further, exposure to Pb contamination (both 10 µg/L and 100 µg/L) negated the chela size advantage in size-asymmetric competitive interactions. This contaminant-induced loss of size-related competitive advantage may result in relaxation in selection pressure on body size and smaller male carapace widths and especially male chela lengths in contaminated locations. Assessment of a range of locations in the wild found a consistent pattern of metal-associated size declines in more contaminated locations for males, but not for females. [ABSTRACT FROM AUTHOR]

According to sexual selection theory, the sexes are faced with opposing evolutionary goals. Male fitness benefits from access to females, whereas female fitness is constrained by food resources and safety for themselves and their offspring. Particularly in large solitary carnivores, such as polar bears (Ursus maritimus), these divergent goals can potentially lead to conflict between the sexes. Outside the mating season, when polar bears are on the move across vast distances, the consequences of such conflict can become apparent when individuals arrive at the same food source. To investigate interrelationships between the sexes, we observed successive polar bears visiting a bird breeding colony to feed on clutches of eggs. We found that males succeeded females more frequently and more closely than expected by chance. Moreover, when males were closer to conspecifics, they walked faster, spent less time in the colony and ingested less food. In contrast, female foraging performance was not associated with proximity to other bears. Irrespective of proximity, females generally spent short periods in the colony and ingested fewer clutches than males. Our results suggest that in polar bears, there is a trade-off between the benefits of food intake and the opportunities (in males) and risks (in females) posed by encountering conspecifics. [ABSTRACT FROM AUTHOR]

Abstract Male-male competition is a component of Darwin’s theory of precopulatory intrasexual selection, where males compete for access to resources important for reproduction and successful males pass on traits that endow greater competitive ability to their offspring, thereby exaggerating the trait(s) over evolutionary time. Metals, such as lead (Pb) often occur in urban mangrove forests, where shore crabs reside, and being neurotoxic may cause sub-lethal effects on behaviour. In this study, we asked whether exposure to Pb influences the outcomes of male-male competition in the Semaphore crab, Heloecius cordiformis, thereby altering outcomes of intrasexual selection. Crabs were exposed to an environmentally relevant, and a behaviourally relevant, concentration of lead (i.e. Pb, 10 µg/L and 100 µg/L respectively) under laboratory conditions for 96 h and subsequently allowed to compete for burrow ownership in experimental arenas. Exposed crabs (100 µg/L) were less successful than control crabs in chela size-matched paired competitive interactions, spent less time in burrows and predominantly lost burrow ownership. Further, exposure to Pb contamination (both 10 µg/L and 100 µg/L) negated the chela size advantage in size-asymmetric competitive interactions. This contaminant-induced loss of size-related competitive advantage may result in relaxation in selection pressure on body size and smaller male carapace widths and especially male chela lengths in contaminated locations. Assessment of a range of locations in the wild found a consistent pattern of metal-associated size declines in more contaminated locations for males, but not for females.

Abstract According to sexual selection theory, the sexes are faced with opposing evolutionary goals. Male fitness benefits from access to females, whereas female fitness is constrained by food resources and safety for themselves and their offspring. Particularly in large solitary carnivores, such as polar bears (Ursus maritimus), these divergent goals can potentially lead to conflict between the sexes. Outside the mating season, when polar bears are on the move across vast distances, the consequences of such conflict can become apparent when individuals arrive at the same food source. To investigate interrelationships between the sexes, we observed successive polar bears visiting a bird breeding colony to feed on clutches of eggs. We found that males succeeded females more frequently and more closely than expected by chance. Moreover, when males were closer to conspecifics, they walked faster, spent less time in the colony and ingested less food. In contrast, female foraging performance was not associated with proximity to other bears. Irrespective of proximity, females generally spent short periods in the colony and ingested fewer clutches than males. Our results suggest that in polar bears, there is a trade-off between the benefits of food intake and the opportunities (in males) and risks (in females) posed by encountering conspecifics.

Sexual selection has been suggested to influence the expression of male behavioral consistency. However, despite predictions, direct experimental support for this hypothesis has been lacking. Here, we investigated whether sexual selection altered male behavioral consistency in Drosophila melanogaster —a species with both pre- and postcopulatory sexual selection. We took 1,144 measures of locomotor activity (a fitness-related trait in D. melanogaster) from 286 flies derived from replicated populations that have experimentally evolved under either high or low levels of sexual selection for >320 generations. We found that high sexual selection males were more consistent (decreased within-individual variance) in their locomotor activity than male conspecifics from low sexual selection populations. There were no differences in behavioral consistency between females from the high and low sexual selection populations. Furthermore, while females were more behaviorally consistent than males in the low sexual selection populations, there were no sex differences in behavioral consistency in high sexual selection populations. Our results demonstrate that behavioral plasticity is reduced in males from populations exposed to high levels of sexual selection. Disentangling whether these effects represent an evolved response to changes in the intensity of selection or are manifested through nongenetic parental effects represents a challenge for future research. [ABSTRACT FROM AUTHOR]

Humans have undergone a long evolutionary history of violent agonistic exchanges, which would have placed selective pressures on greater body size and the psychophysical systems that detect them. The present work showed that greater body size in humans predicted increased knockout power during combative contests (Study 1a-1b: total N = 5,866; Study 2: N = 44 openweight fights). In agonistic exchanges reflective of ancestral size asymmetries, heavier combatants were 200% more likely to win against their lighter counterparts because they were 200% more likely to knock them out (Study 2). Human dominance judgments (total N = 500 MTurkers) accurately tracked the frequency with which men (N = 516) knocked out similar-sized adversaries (Study 3). Humans were able to directly perceive a man's knockout power because they were attending to cues of a man's body size. Human dominance judgments—which are important across numerous psychological domains, including attractiveness, leadership, and legal decision-making—accurately predict the likelihood with which a potential mate, ally, or rival can incapacitate their adversaries. [ABSTRACT FROM AUTHOR]

Exaggerated ornaments provide opportunities to understand how selection can operate at different levels to shape the evolution of a trait. While these features aid their bearer in attracting mates or fending off competitors, they can also be costly and influenced by the environment and genetic variation. The eyestalks of the stalk-eyed fly, Teleopsis dalmanni, are of interest because eyestalk length is the target of both intra- and intersexual selection and is also reduced by loci on a highly-divergent sex ratio X chromosome (XSR), a meiotic driver accounting for up to 30% of wild X chromosomes. Male stalk-eyed flies fight to control access to females and over food using a combination of low-intensity displays and high-intensity physical fights. We staged, filmed, and scored contests between pairs of eyespan-matched males to evaluate whether X chromosome type impacts the behavior and outcome of aggressive interactions. While our results broadly match expectations from previous studies, we found that XSR males used more high-intensity behaviors than males carrying a non-driving, standard X chromosome (XST), particularly when their eyestalks were of similar size or smaller than their opponents. Additionally, we found that when XSR males use high-intensity behaviors, they win more bouts than when they use low-intensity behaviors. Taken together, these results suggest that XSR impacts male aggressive behavior to compensate for the shorter eyestalks of XSR males and may help to explain how this selfish chromosome is maintained.

Heteranthery, the presence of two or more anther types in the same flower, is taxonomically widespread among bee-pollinated angiosperms, yet has puzzled botanists since Darwin. We test two competing hypotheses for its evolution: the long-standing 'division of labour' hypothesis, which posits that some anthers are specialized as food rewards for bees whereas others are specialized for surreptitious pollination, and our new hypothesis that heteranthery is a way to gradually release pollen that maximizes pollen delivery. We examine the evolution of heteranthery and associated traits across the genus Clarkia (Onagraceae) and study plant-pollinator interactions in two heterantherous Clarkia species. Across species, heteranthery is associated with bee pollination, delayed dehiscence and colour crypsis of one anther whorl, and movement of that anther whorl upon dehiscence. Our mechanistic studies in heterantherous species show that bees notice, forage on and export pollen from each anther whorl when it is dehiscing, and that heteranthery promotes pollen export. We find no support for division of labour, but multifarious evidence that heteranthery is a mechanism for gradual pollen presentation that probably evolved through indirect male-male competition for siring success.

Many sexual weapons used in sexual combat and/or as visual displays of dominance (e.g., antlers, horns) show positive allometry with body size, but allometry in species with more than one sexual weapon is not well studied. We examined the static and evolutionary allometric relationships between body size and tusks (pure combat weapons) and/or antlers (both a visual signal and combat weapon) from thirty-three artiodactyl species including the muntjaks and tufted deer (Muntiacinae), which uniquely have both antlers and tusks. In Muntiacinae species, we found a positive static allometric relationship between antler length and skull length, whereas tusk size scaled isometrically, suggesting greater energy investment in antlers as signals over tusks as combative weapons when both are present. Evolutionarily, we found that in species with only one weapon (either solely tusked or solely antlered), their weapon scaled with positive allometry with body mass, and the relationship for antlers levels off at larger body sizes. When we included the dual-weaponed Muntiacinae species, the positive allometric trend for tusks was not conserved, resulting in an isometric relationship, suggesting the possession of antlers negatively affects the energy investment in tusks as weapons. Overall, our findings show that species that possess dual weapons invest less energy in each weapon relative to single-weaponed species. [ABSTRACT FROM AUTHOR]

Background: Evidence of male-male courtship display is widespread across the animal kingdom. Yet, its function and evolutionary origin remain unclear. Here, we hypothesise that male-male courtship display evolved in response to selection pressure exerted by intrasexual competition during male-female courtship interactions. Intrasexual competition can be caused by bystander male pressure through eavesdropping and exploiting on displayer male's courtship interactions with females. This bystander pressure can lead to an audience effect by the displayer, who will change their courtship behaviour in the presence of bystanders and display directly towards them, even in the absence of females, as an intimidation strategy. In species where this selection pressure has taken place, we predict that the male courtship display will have a dual function: attract females and deter competitors. Therefore, we expected to find more evidence of bystander-related behaviours in species for which male-male courtship display is linked to intrasexual competition compared to species for which other explanatory hypotheses are more plausible (e.g., mistaken identity or courtship practice). Methodology: We conducted two systematic reviews to test this hypothesis. First, we conducted a search for studies of species with courtship display between males and of the hypotheses provided to explain this behaviour. Our goal was to identify the species with male-male courtship display and evidence of intrasexual competition. Second, among the species with male-male courtship display, we searched for evidence of bystander-related behaviours, i.e., articles referring to eavesdropping, exploitation, and audience effect during male-female courtship interactions. Our goal was to test whether species with intrasexual competition are also more likely to show bystander-related behaviours. Results: Although most studies reporting male courtship display towards other males do not suggest any explanatory hypothesis for this behaviour, the intrasexual competition hypothesis was largely mentioned and supported by some studies reviewed. Additionally, there is more evidence of eavesdropping and of all three bystander-related behaviours combined in species for which the intrasexual competition hypothesis was suggested. Conclusions: Overall, our review supports the hypothesis that intrasexual competition can play a key role in male courtship display evolution, namely that male-male courtship display may have evolved as a secondary function of male-female courtship interactions via bystander male pressure. However, our review also shows that despite the increasing interest in same-sex sexual behaviours, and male-male courtship display in particular, most studies were found to be merely descriptive, and the hypotheses they suggested to explain courtship display between males mostly speculative. This highlights an important gap in the literature. To clarify both the evolution and the function of male-male courtship display, this behaviour needs to be empirically studied more often. Our review can help advancing this research area, as it makes the 20 species with male-male courtship display for which the intrasexual competition hypothesis was suggested excellent candidates for empirical research. [ABSTRACT FROM AUTHOR]

Background Evidence of male-male courtship display is widespread across the animal kingdom. Yet, its function and evolutionary origin remain unclear. Here, we hypothesise that male-male courtship display evolved in response to selection pressure exerted by intrasexual competition during male-female courtship interactions. Intrasexual competition can be caused by bystander male pressure through eavesdropping and exploiting on displayer male’s courtship interactions with females. This bystander pressure can lead to an audience effect by the displayer, who will change their courtship behaviour in the presence of bystanders and display directly towards them, even in the absence of females, as an intimidation strategy. In species where this selection pressure has taken place, we predict that the male courtship display will have a dual function: attract females and deter competitors. Therefore, we expected to find more evidence of bystander-related behaviours in species for which male-male courtship display is linked to intrasexual competition compared to species for which other explanatory hypotheses are more plausible (e.g., mistaken identity or courtship practice). Methodology We conducted two systematic reviews to test this hypothesis. First, we conducted a search for studies of species with courtship display between males and of the hypotheses provided to explain this behaviour. Our goal was to identify the species with male-male courtship display and evidence of intrasexual competition. Second, among the species with male-male courtship display, we searched for evidence of bystander-related behaviours, i.e., articles referring to eavesdropping, exploitation, and audience effect during male-female courtship interactions. Our goal was to test whether species with intrasexual competition are also more likely to show bystander-related behaviours. Results Although most studies reporting male courtship display towards other males do not suggest any explanatory hypothesis for this behaviour, the intrasexual competition hypothesis was largely mentioned and supported by some studies reviewed. Additionally, there is more evidence of eavesdropping and of all three bystander-related behaviours combined in species for which the intrasexual competition hypothesis was suggested. Conclusions Overall, our review supports the hypothesis that intrasexual competition can play a key role in male courtship display evolution, namely that male-male courtship display may have evolved as a secondary function of male-female courtship interactions via bystander male pressure. However, our review also shows that despite the increasing interest in same-sex sexual behaviours, and male-male courtship display in particular, most studies were found to be merely descriptive, and the hypotheses they suggested to explain courtship display between males mostly speculative. This highlights an important gap in the literature. To clarify both the evolution and the function of male-male courtship display, this behaviour needs to be empirically studied more often. Our review can help advancing this research area, as it makes the 20 species with male-male courtship display for which the intrasexual competition hypothesis was suggested excellent candidates for empirical research.

The author proposes that evolutionary, primatological, and Darwinian feminist perspectives would enhance the understanding of the myriad factors which influence patterns of female-to-female cooperation and competition across species. Topics discussed include Darwin's sexual selection theory, evolutionary perspectives on male-female relationships, and thoughts on intervention.

Sexual selection is a powerful diversifier of phenotype, behavior and cognition. Here we compare cognitive-behavioral traits across four reproductive phenotypes (females and three alternative males) of wild-caught ocellated wrasse (Symphodus ocellatus). Both sex and alternative male phenotypes are environmentally determined with sex determination occuring within the first year, and males transition between alternative phenotypes across 2 years (sneaker to satellite or satellite to nesting). We captured 151 ocellated wrasse and tested them on different behavior and cognition assays (scototaxis, shoaling, and two detour-reaching tasks). We found greater divergence across alternative male reproductive phenotypes than differences between the sexes in behavior, problem-solving, and relationships between these traits. Nesting males were significantly less bold than others, while sneaker males were faster problem-solvers and the only phenotype to display a cognitive-behavioral syndrome (significant correlation between boldness and problem-solving speed). Combining these results with prior measurements of sex steroid and stress hormone across males, suggests that nesting and sneaker males represent different coping styles. Our data suggests that transitioning between alternative male phenotypes requires more than changes in physiology (size and ornamentation) and mating tactic (sneaking vs. cooperation), but also involves significant shifts in cognitive-behavioral and coping style plasticity.

In this study, the author evaluated two adult age groups of the Mexican rivulus Millerichthys robustus with body size asymmetries to determine the strategies used by an annual killifish during agonistic interactions of different ontogenetic stages. To achieve this goal, the author first characterized the ethogram of agonistic interactions of M. robustus composed of seven behavioural units in males and five behavioural units in females. The author then analysed agonistic interaction strategies used by males and females with body size asymmetries in two groups of different adult ages that represent different ontogenetic stages: (a) just after sexual maturity was reached, at 5 weeks of age, and (b) near natural death, at 24 weeks of age. The agonistic behaviour patterns of M. robustus were compatible with the logic of mutual assessment. Large males had an advantage during their interactions in both age groups, winning all of the encounters. Nonetheless, there was more aggression in 5‐week‐old fish encounters. In addition, small 24‐week‐old fish were more aggressive than small 5‐week‐old fish. These changing strategies may be because of the cost–benefits required during a fight at each ontogenetic stage. In the female encounters, size did not predict winners, as both small and large fish won a similar number of encounters, and some contests remained unresolved regardless of age group. There was a tendency for small females of any age to risk more than males in fights to maintain reproductive fitness. [ABSTRACT FROM AUTHOR]

Mammals kill both conspecific infants and adults. Whereas infanticide has been profusely studied, the killing of non-infants (adulticide) has seldom attracted the attention of researchers. Mammals kill conspecific adults by at least four, non-exclusive reasons: during intrasexual aggression for mating opportunities, to defend valuable resources, to protect their progeny and to prey upon conspecifics. In this study, we test which reason is most likely to explain male and female adulticide in mammals. For this, we recorded the presence of adulticide, the ecological and behavioural traits, and the phylogenetic relationship for more than 1000 species. Adulticide has been recorded in over 350 species from the most important Mammalian clades. Male adulticide was phylogenetically correlated with the presence of size dimorphism and intrasexually selected weapons. Female adulticide was phylogenetically associated with the occurrence of infanticide. These results indicate that the evolutionary pathways underlying the evolution of adulticide differ between sexes in mammals. Whereas males commit adulticide to increase breeding opportunities and to compete with other males for mating, females commit adulticide mainly to defend offspring from infanticidal conspecifics. [ABSTRACT FROM AUTHOR]

For mammalian females, which bear the energetic costs of gestation and lactation, competition over food resources is generally expected to have a more important impact on reproductive success than competition over mates. However, there are some situations in which mammalian females compete over access to mates, limited supplies of sperm or caretaking for their offspring. Competition over both access to food and access to mates have been documented in female baboons (Papio spp.). Here, we examined the relative importance of competition over food and mates for wild female olive baboons, Papio anubis , in shaping the patterns of aggression among females. Lactating and pregnant females were more aggressive than females in other reproductive states, and sexually receptive females initiated and received relatively little aggression. There was no evidence that females competed over caretakers for their offspring or reduced future competition through reproductive suppression. Our data suggest that competition over food plays a more important role than competition over mates for female olive baboons. The body of current evidence suggests that evolution has finely tuned female baboons' responses to the competitive pressures that they face in their local environments. • Female olive baboons' condition declined as group size increased. • Among lactating females, condition declined as rank declined. • Energetic costs of lactation seem to drive patterns of female–female aggression. • We found little evidence of competition over mates or reproductive suppression. • Intraspecific comparisons highlight females' ability to adapt to local challenges. [ABSTRACT FROM AUTHOR]

Abstract Background Comparing sexual size dimorphism (SSD) in the light of the phylogenetic hypothesis may help to understand the phenotypic evolution associated with sexual selection (size of whole body and of reproduction-related body parts). Within a macroevolutionary framework, we evaluated the association between the evolution of SSD and the evolution of reproduction-related phenotypic traits, and whether this association has favored female fecundity, considering also variations according to reproductive modes. We focused on the lizard species that inhabit the Chaco Domain since this is a natural unit with a high diversity of species. Results The residual SSD was related positively with the residuals of the reproduction-related phenotypic traits that estimate intrasexual selection and with the residuals of inter-limb length and, according to fecundity selection, those residuals were related positively with the residuals of clutch size in oviparous species. Lizards of the Chaco Domain present a high diversity of SSD patterns, probably related to the evolution of reproductive strategies. Conclusions Our findings highlight that the sexual selection may have acted on the whole-body size as well as on the size of body parts related to reproduction. Male and female phenotypes evolutionarily respond to variations in SSD, and an understanding of these patterns is essential for elucidating the processes shaping sexual phenotype diversity from a macroevolutionary perspective.

Heteranthery, the presence of two or more anther types in the same flower, is taxonomically widespread among bee-pollinated angiosperms, yet has puzzled botanists since Darwin. We test two competing hypotheses for its evolution: the long-standing 'division of labour' hypothesis, which posits that some anthers are specialized as food rewards for bees whereas others are specialized for surreptitious pollination, and our new hypothesis that heteranthery is a way to gradually release pollen that maximizes pollen delivery. We examine the evolution of heteranthery and associated traits across the genus Clarkia (Onagraceae) and study plant–pollinator interactions in two heterantherous Clarkia species. Across species, heteranthery is associated with bee pollination, delayed dehiscence and colour crypsis of one anther whorl, and movement of that anther whorl upon dehiscence. Our mechanistic studies in heterantherous species show that bees notice, forage on and export pollen from each anther whorl when it is dehiscing, and that heteranthery promotes pollen export. We find no support for division of labour, but multifarious evidence that heteranthery is a mechanism for gradual pollen presentation that probably evolved through indirect male–male competition for siring success. [ABSTRACT FROM AUTHOR]

Rapid colour change is widespread in animals and allows them to respond dynamically to the physical and social environment. However, few studies have examined the information conveyed by dynamic colour signals in different social contexts, such as courtship and rivalry contests. Furthermore, dynamic colour change on different body regions may be subject to different selection pressures and therefore serve different functions. We tested whether male colour or colour change predicts contest outcome in male–male interactions or the likelihood of copulation attempts in male–female interactions in the central bearded dragon, Pogona vitticeps. The extent of beard darkening strongly predicted contest outcome, but neither colour (beard, dorsal) nor colour change predicted copulation attempts. Dorsal colour showed little consistent change and did not predict contest outcome or copulation attempts. Previous work shows that dorsal but not beard colour change occurs in response to both background colour and temperature in the laboratory and in the wild. Taken together, these results indicate that dynamic colour change in bearded dragons varies in relation to social context and can serve different functions depending on the body region. Our results add to the growing appreciation of how rapid colour change enables animals to accommodate the multiple functions of colour, including communication to different receivers, camouflage and thermoregulation. • Male bearded dragon lizards change colour during interactions with other lizards. • Winners had darker beards, and their beard changed more than those of losers. • Residency was the strongest predictor of contest outcome. • Neither colour nor the extent of colour change predicted copulation attempts. [ABSTRACT FROM AUTHOR]

Sexual selection is widespread in animals, but quite naturally studied in adults. Juvenile males in most animals are not differentiated from females and coloration is usually drab. However, there is no reason to suspect that sexual differences cannot develop before puberty, influence social interactions, and then have fitness pay-offs later in life. Juvenile collared lizards (Crotaphytus collaris (Say, 1822)) show marked dichromatism: males develop bright dorsolateral orange bars whereas females do not. These juvenile orange bars (JOB) disappear at sexual maturity, when males develop different colour traits maintained by sexual selection. We conducted field experiments with juvenile males on their developing territories in which we utilized staged intruders of juvenile males (with JOB) and juvenile females (lacking JOB) and also juvenile male intruders whose JOB were manipulated. Residents reacted significantly more aggressively toward males vs. females, and also toward males whose JOB were emphasized with paint than those whose JOB were masked by paint. These JOB are used in signalling among juveniles and we suggest the social relations established then are retained until sexual maturation the next spring (after the JOB are lost) to benefit males that previously displayed strong JOB by increased matings in the spring as sexually mature yearlings as per a phenomenon we call precocial sexual selection. [ABSTRACT FROM AUTHOR]

Traditionally it has been suggested that sexual selection can cause sexual size dimorphism (SSD). However, a recent review in gastropods shows that SSD itself can also cause sexual selection (Ng et al., 2019. Animal Behaviour , 148, 53–62). This may be the case if mate choice exists, with males preferring to mate with females similar in body size but somewhat larger than themselves (female-biased preference). We formally investigated this verbal explanation by computer simulations using a Gaussian mating preference function. Parameters of that function were also estimated from empirical data. Our results suggest that sexual selection (estimated as selection differential) is strong when mate choice is high and exerted by only one of the sexes, being influenced by SSD and the magnitude of the female-biased preference. All these factors cause a negative relationship between SSD and the (sexual) selection differential, similar to that observed in Ng et al.'s review on gastropods. Empirical estimates of male mate choice from wild-captured mating pairs of different gastropod species confirm that mate choice by males is biased towards females slightly larger than themselves. Our results also illustrate that if mate choice is truly involved in determining SSD, present-day sexual selection cannot be used to estimate the past magnitude of mate choice, as SSD influences present-day patterns of sexual selection. • Dimorphism is negatively correlated with sexual selection differential in gastropods. • Dimorphism plus similarity-based mate choice can cause strong sexual selection. • Preference bias can have the opposite effect than dimorphism on sexual selection. • Past mate choice causing dimorphism cannot be estimated from present sexual selection. [ABSTRACT FROM AUTHOR]

Female body size is more sensitive to the environmental conditions during development than male body size in many insect species with female-biased sexual size dimorphism (SSD) (i.e., females are larger than males). However, the sexual difference in body size plasticity is largely unknown in species with male-biased SSD. Here, I conducted a laboratory experiment using low- and high-quality diets to examine sexual differences in body size plasticity in the male-larger rhinoceros beetle Trypoxylus dichotomus Linnaeus, 1771 (Coleoptera: Scarabaeidae). I found that male body size was much more greatly affected by nutritional status than female body size. The stronger condition dependency in male body size is likely associated with sexual selection for male body size. Furthermore, in wild-caught beetles, males showed larger body size differences between years within populations, while the body size of females was more constant. Within populations and years, male body size also showed greater variation than female body size. In addition, SSD of field-caught beetles was usually much smaller than that of lab-reared beetles of corresponding populations. Thus, the stronger condition dependency of male body size in this species probably has profound effects on the variation of observed body size and SSD among and within populations. [ABSTRACT FROM AUTHOR]

Sexually dimorphic weaponry often results from intrasexual selection, and weapon size can vary seasonally when costs of bearing the weapon exceed the benefits outside of the reproductive season. Weapons can also be favored in competition over nonreproductive resources such as food or shelter, and if such nonreproductive competition occurs year‐round, weapons may be less likely to vary seasonally. In snapping shrimp (Alpheus angulosus), both sexes have an enlarged snapping claw (a potentially deadly weapon), and males of many species have larger claws than females, although females are more aggressive. This contrasting sexual dimorphism (larger weaponry in males, higher aggression in females) raises the question of whether weaponry and aggression are favored by the same mechanisms in males and females. We used field data to determine whether either sex shows seasonal variation in claw size such as described above. We found sexual dimorphism increased during the reproductive season due to opposing changes in both male and female claw size. Males had larger claws during the reproductive season than during the nonreproductive season, a pattern consistent with sexual selection. Females, however, had larger claws during the nonreproductive season than during the reproductive season—a previously unknown pattern of variation in weapon size. The observed changes in female weapon size suggest a trade‐off between claw growth and reproduction in the reproductive season, with investment in claw growth primarily in the nonreproductive season. Sexually dimorphic weaponry in snapping shrimp, then, varies seasonally due to sex differences in seasonal patterns of investment in claw growth, suggesting claws may be advantageous for both sexes but in different contexts. Thus, understanding sexual dimorphisms through the lens of one sex yields an incomplete understanding of the factors favoring their evolution. [ABSTRACT FROM AUTHOR]

Income and capital breeding describe two dichotomous breeding strategies that characterise the allocation of resources to reproduction. Capital breeders utilise stored endogenous resources (typically lipids) to finance reproduction, whereas income breeders use exogenous resources (typically carbohydrates). The basis for such characterisation has mainly come from studying females, yet for many species, male and female reproductive success may be determined by substantially different factors. Females allocate resources to offspring production, whereas males typically allocate resources to accessing mating opportunities, e.g. from contests or displays. The primary metabolic fuel (lipids or carbohydrates) in males appears to be dependent on the type of activity being performed (i.e. high versus low intensity or long versus short duration), rather than capital or income breeding strategy per se. Males performing sustained, long-duration effort typically utilise lipids, whereas those undergoing intense activity more often utilise carbohydrates. As a result, either fuel type can be used in either strategy. Breeding season duration can constrain strategy choice; lipids and carbohydrates can be used in short breeding season species, but only lipids provide a viable fuel source for long breeding season capital breeders. Both capital- and income-breeding males must manage their resource use during the breeding season, but capital breeders must also copewith physiological stressors associated with extended fasting. Overall, the capital-income breeding concept applies equally to male reproduction, but compared with females, there are different physical and physiological constraints that shape choice of strategy. This Commentary also highlights some key future areas that need to be investigated to further understand how capital-income breeding strategies shape male mating strategies. [ABSTRACT FROM AUTHOR]

Agonistic character displacement (ACD) occurs when selection to avoid maladaptive interspecific aggression leads to the evolution of agonistic signals and/or associated behavioural biases in sympatry. Here, we test for a pattern consistent with ACD in male colour pattern in darters (Percidae: Etheostoma). Male colour pattern has been shown to function in male–male competition rather than female mating preferences in several darter species. Additionally, males bias their aggression towards conspecific over heterospecific males in sympatry but not in allopatry, consistent with divergent ACD in male behavioural biases. We use a common garden approach to show that differences in male colour pattern among four closely related darter species are genetically based. Additionally, we demonstrate that some aspects of male colour pattern exhibit enhanced differences in sympatric compared to allopatric populations of two darter species, consistent with ACD. However, other male colour traits are more similar between species in sympatry compared with allopatry, indicating that not all signal components are under strong divergent selection in sympatry. This study provides evidence that interspecific male–male aggressive interactions alone can promote elaborate male signal evolution both between and within species. We discuss the implications this has for male-driven ACD and cascade ACD. [ABSTRACT FROM AUTHOR]

Sensory drive predicts that the conditions under which signaling takes place have large effects on signals, sensory systems, and behavior. The coupling of an ecological genetics approach with sensory drive has been fruitful. An ecological genetics approach compares populations that experience different environments and asks whether population differences are adaptive and are the result of genetic and/or environmental variation. The multi-faceted effects of signaling environments are well-exemplified by the bluefin killifish. In this system, males with blue anal fins are abundant in tannin-stained swamps that lack UV/blue light but are absent in clear springs where UV/blue light is abundant. Past work indicates that lighting environments shape genetic and environmental variation in color patterns, visual systems, and behavior. Less is known about the selective forces creating the across population correlations between UV/blue light and the abundance of blue males. Here, we present three new experiments that investigate the roles of lighting environments on male competition, female mate choice, and predation. We found strong effects of lighting environments on male competition where blue males were more likely to emerge as dominant in tea-stained water than in clear water. Our preliminary study on predation indicated that blue males may be less susceptible to predation in tea-stained water than in clear water. However, there was little evidence for female preferences favoring blue males. The resulting pattern is one where the effects of lighting environments on genetic variation and phenotypic plasticity match the direction of selection and favor the expression of blue males in swamps. [ABSTRACT FROM AUTHOR]

Recent research reveals that female reproductive competition is common and may shape the social and reproductive strategies of female mammals. This study explores the determinants and intensity of female intrasexual conflicts in a wild promiscuous primate, the chacma baboon, Papio ursinus . We tested a suite of hypotheses to assess whether female–female aggression was primarily driven by instantaneous competition for food, mates or paternal care, or aimed at reducing future competition among offspring via reproductive suppression. Behavioural data were gathered from 53 females in two groups over two contrasting 2-year periods (2005–2006, 2013–2014): the first characterized by stability in the male dominance hierarchies, the second by instability induced by several immigration events and male take-overs. In both periods, we found that sexually receptive females received high levels of aggression from other sexually receptive females, consistent with competition over mating opportunities. In the unstable period, females exchanged higher rates of aggression than in the stable period, regardless of reproductive state, but lactating females received most aggression, consistent with higher competition over social access to male partners in response to increased infanticide risk. There was no evidence that aggression between females reflected either competition over food or reproductive suppression. These findings indicate that patterns of aggression between females fluctuate with sociodemographic factors affecting sexual and social access to males and reflect reproductive competition more closely than resource competition in this promiscuous primate society. [ABSTRACT FROM AUTHOR]

Males of many insect species exhibit multiple sexually selected weapons, but the functional roles and evolution of them are poorly understood. The males of the flower beetle, Dicronocephalus wallichii bourgoini are equipped with exaggerated horns and elongated forelegs. To explore the selective forces acting on these two weapons, we investigated their functions by examining male-male competition and allometric relationships between the weapons and body size. We found that competition between mate-guarding (i.e. owners) and unpaired males (i.e. intruders) frequently occurred in the field. In the early phase of the contests, males mainly used their forelegs, likely, to assess the body size of their opponents. If the foreleg length of the owner was shorter than that of the intruder, the intruder approached to take over the owner's mate. Escalated contests occurred, in which both opponents tried to drag each other away from the females or the substrate using their horns. This suggests that the multiple weapons in this species are specialized for specific phases of contests. The males with larger bodies and weapons were more successful in defending their ownership of mates or taking over guarded females. The allometric slope of horns was positive in small males, but it decreased in large males. In contrast, male forelegs exhibited isometry without a switch point, and the slope was significantly steeper than that of female forelegs. Our findings suggest that sexual selection acts on both male weapon traits in D. w. bourgoini but that antagonistic natural selection constrains the further exaggeration of these traits in different ways. [ABSTRACT FROM AUTHOR]

Sexual size dimorphism results when female and male body size is influenced differently by natural and sexual selection. Typically, in polygynous species larger male body size is thought to be favored in competition for mates and constraints on maximal body size are due to countervailing natural selection on either sex; however, it has been postulated that sexual selection itself may result in stabilizing selection at an optimal mass. Here we test this hypothesis by retrospectively assessing the influence of body mass, one metric of body size, on the fitness of 113 wild-derived house mice (Mus musculus) residing within ten replicate semi-natural enclosures from previous studies conducted by our laboratory. Enclosures possess similar levels of sexual selection, but relaxed natural selection, relative to natural systems. Heavier females produced more offspring, while males of intermediate mass had the highest fitness. Female results suggest that some aspect of natural selection, absent from enclosures, acts to decrease their body mass, while the upper and lower boundaries of male mass are constrained by sexual selection. [ABSTRACT FROM AUTHOR]

Bright colouration appearing in one sex only can be driven by components of sexual selection including female choice, male competition or mate recognition. Male Litoria wilcoxii undergo rapid colour change from brown to yellow during amplexus, however, the function, if any, is unknown. We tested possible behavioural functions by observing breeding aggregations and behavioural responses (colour change, movement, call and amplexus duration) to varying stimuli (including model male and female frogs). We also examined whether colour change was a by-product of hormone release by comparing spermatic urine of frogs injected with epinephrine (colour change hormone) and hCG (triggers spermiation). Finally, the predation cost of being bright yellow was examined by placing frog models (yellow and brown) in the field and measuring predator attack rate. The behavioural responses of males to model females, brown/brown models (female with amplexing brown male), and brown/yellow models (female with amplexing yellow male), were similar to reactions towards real females, with the important exception that males did not attempt amplexus with brown/yellow models. Epinephrine injections triggered colour change but not sperm release in male frogs, while hCG induced sperm release but not colour change. Attack rates were low in predation trials with no difference in attack rates between yellow and brown models observed. Our study presents a novel function for rapid dynamic colour change as an intrasexual signal during amplexus that could avert sperm competition and displacement by other males. Significance statement: Colour displays during breeding are believed to have evolved through mechanisms of sexual selection (female choice, male-male competition or sexual recognition). Stony creek frogs ( Litoria wilcoxii) have been observed to rapidly change colour from brown (similar to female colouration) to bright yellow during amplexus, which is unusual as the colour change occurs after mate selection. Behavioural experiments were used to test hypotheses on the evolutionary function of colour change in this species. In L.wilcoxii, colour functions as an intrasexual signal during amplexus, we hypothesise that this could avert sperm competition and/or displacement by other males during amplexus. The function presented here is novel among amphibians, however as data on dynamic colour change in amphibians is lacking, this trait may be more common. [ABSTRACT FROM AUTHOR]

Alternative mating tactics are often expressed differentially based on a variety of factors associated with each mating context in order to maximize a male's reproductive success. In particular, males of many species attempt to reduce competition with males in the surrounding environment by altering their mating behaviors. In the wolf spider Rabidosa punctulata (Hentz, 1844) , males exhibit two distinct mating tactics: 1) courtship-comprised of visual and seismic signals or 2) direct mount-involving males grappling with females for copulation. In natural environments, these spiders are relatively dense. Competing males are often close by and could potentially intercept courtship displays, locate the nearby female and steal the copulation. Here we investigate whether males adjust their mating tactic expression in response to indirect and directly competing males, and whether these decisions affect their likelihood to copulate. In both experiments, the actions of a competing male did not affect the expression of any male mating behaviors in the other, suggesting a lack of an effect of scramble competition on tactic expression. Evidence from our triad mating experiment suggests a mating advantage for males that adopt the direct mount tactic when in direct competition with other males. In particular, direct mounts were most successful when multiple males were actively pursuing a female and when adopted first among the competitors. Additionally, we observed direct male mating interference whereby the copulating pair was broken up. Following these breakups, females were observed to mate multiply, often as a result of a direct mount. This new observation may provide a context in which males benefit through additional copulations by adopting the direct mount tactic. [ABSTRACT FROM AUTHOR]

To examine the effect of wing colors of Favonius taxila males on other males, an intact bright butterfly model or a dull butterfly model whose wing scales had been dislodged was placed in a potential territory, and the behavior of invading males was recorded. The invader showed decreased probability of landing there with a bright model in place compared with a dull model, and exhibited complex flights with frequent turning for a bright model, in contrast to simple flights with relatively linear paths for a dull model. These results support the idea of intrasexual selection in which bright colors of males' wings have evolved through male-male interactions. [ABSTRACT FROM AUTHOR]

Theory suggests that behaviours previously thought only to influence intrasexual selection, such as aggression between males, can also be co-opted to serve in intersexual selection as honest indicators of male quality. Our previous research revealed an audience effect: male Jamaican field crickets, Gryllus assimilis , fight more aggressively in front of a female audience. One way for this to evolve is if females are more attracted to more aggressive males or fight winners. Given the potential interplay between intra- and intersexual selection, we investigated how observing a fight influences subsequent female mate choice and mating behaviour. We utilized two female treatments: Observers watched the two males fight, whereas Non-observers watched two noninteracting males. We then immediately tested Observer and Non-observer females for their mating preferences towards male fight participants using dichotomous and no-choice tests. Our approach enabled us to assess whether females preferred either dominant (winner) or subordinate (loser) males and whether females gathered information about males while watching them fight that informed their mating preferences. While females did not prefer fight winners over losers in dichotomous choice tests, in the no-choice tests, females were more likely to mount winners and mount males that they had preferred in the dichotomous choice tests. Intra- and intersexual competition appears to be mutually reinforcing in this species, but eavesdropping is unlikely to explain the evolution of audience effects. [ABSTRACT FROM AUTHOR]

The traditional assumption within the research literature on human sexually dimorphic traits has been that many sex differences have arisen from intersexual selection. More recently, however, there has been a shift toward the idea that many male features, including male lower-pitched voices and male beard growth, might have arisen predominantly through intrasexual selection: that is, to serve the purpose of male-male competition instead of mate attraction. In this study, using a unique set of video stimuli, we measured people's perceptions of the dominance and attractiveness of men who differ both in terms of voice pitch (4 levels from lower to higher pitched) and beard growth (4 levels from clean shaven to a month's hair growth). We found a nonlinear relationship between lower pitch and increased attractiveness; men's vocal attractiveness peaked at around 96 Hz. Beard growth had equivocal effects on attractiveness judgments. In contrast, perceptions of men's dominance simply increased with increasing masculinity (i.e., with lower-pitched voices and greater beard growth). Together, these results suggest that the optimal level of physical masculinity might differ depending on whether the outcome is social dominance or mate attraction. These dual selection pressures might maintain some of the documented variability in male physical and behavioral masculinity that we see today. [ABSTRACT FROM AUTHOR]

Behavioral interactions between individuals of the same sex can affect the outcome of intersexual selection. For this reason, intrasexual selection is oftentimes examined independently from intersexual selection in studies aimed at understanding mating behaviors. However, a more complete understanding of sexual selection effects within a population can only come from exploring both intrasexual and intersexual selection and the potential interactions between the levels of selection. Association preferences in male and female Etheostoma fonticola do not reveal a size preference for same sex or opposite-sex individuals. However, male-male interactions may influence female choice in E. fonticola. We examined whether male E. fonticola that differ in body size exhibit differences in agonistic behaviors and mating success. Larger males do exhibit higher rates of aggressive behaviors and smaller males, in turn, exhibit more defensive behaviors. However, differences between larger and smaller males in male-male interactions did not translate into differences in spawning success. These results suggest that male size influences dominance relationships in E. fonticola but not mating success. There were also no differences between large and small males in mating attempts, which could be an outcome of the year-round breeding season found in this species, females laying eggs singly, or males fertilizing eggs individually. [ABSTRACT FROM AUTHOR]

Background: Intraspecific communication is of crucial importance throughout the animal kingdom and may involve a combination of visual, gustatory, olfactory and acoustic cues. Variation in male sex pheromone amount and composition may convey important information to female conspecifics, for instance on species identity or age. However, whether increased male pheromone titres are associated with fitness benefits for the female, thus indicating a role as an honest signal, is under debate. Results: Against this background, we tested in the butterfly Bicyclus anynana (1) whether young males being successful or unsuccessful in gaining a mating differed in sex pheromone titres and (2) for associations between male pheromone titres and spermatophore mass, eupyrene sperm number, and a variety of female and offspring life-history traits. Successful and unsuccessful males did not differ in pheromone titres, however eupyrene sperm number was much higher in successful males. Pheromone titres were not associated with any fitness-related female or offspring trait measured in our study, though correlation analyses yielded evidence for trade-offs among specific traits. Patterns did not differ among control and olfaction-blocked females. Conclusion: Therefore, we suggest that in young B. anynana pheromone titres do not indicate male quality. [ABSTRACT FROM AUTHOR]

A bright ultraviolet (UV) component in the coloration of males may signal individual quality and thus determine the outcome of male-male contests. Yet, the role of the UV component of coloration in resolving conflicts is still controversial relative to factors such as residency status and seasonality. Here, we investigated whether a reduction of UV reflectance of lateral blue spots in male wall lizards ( Podarcis muralis) interacts with residency status (resident vs. intruder) to influence agonistic behaviors, the outcome of contests, and basking time (a measure of spatial dominance). We performed this experiment during one breeding and one non-breeding season. The UV manipulation did not predict the outcome of contests. During the breeding season, the agonistic behaviors and basking time depended on the residency status of males but not on their UV treatment. During the non-breeding season, experimental factors affected basking time only. For a given male, the time spent basking depended in a complex manner on its residency status, its UV treatment, and those of its rival. UV reflectance of blue spots thus influences the processes of mutual assessment and spatial dominance, but is not a critical determinant of fighting success. Altogether, these results evidence context-dependent effects of the UV reflectance of blue spots on territorial behaviors according to residency status and, potentially, season. They also suggest that UV signaling may be more important than expected for male-male interactions during the non-breeding season. [ABSTRACT FROM AUTHOR]

Theory predicts that male mating success depends on resource holding potential ( RHP), which is reflected by proxies of condition, such as body mass, fat content, strength, or weaponry. In species lacking any physical means to inflict injuries upon combatants, such as butterflies, the factors determining mating success are less clear. Against this background, we explored the determinants of male mating success in the tropical butterfly Bicyclus anynana Butler ( Lepidoptera: Nymphalidae: Satyrinae), by comparing physiological, immunological, and morphological traits between winning and losing males. Our results showed that successful males are characterized by a better flight performance, evidenced by having longer wings, a heavier thorax, a lighter abdomen, a higher fat content, and higher phenoloxidase expression levels than their unsuccessful counterparts, when being compared after their first mating. Males that won three consecutive trials against the same combatant were also characterized by a better flight performance, having larger forewings, a higher body mass, and a higher fat content. Thus, successful males were larger and in better condition than unsuccessful ones. Strikingly, many differences found indicated an enhanced flight performance for the former, which we suggest ultimately plays the key role for male mating success in B. anynana. As fat is the main energy source for flying insects, being crucial to flight endurance and in turn presumably to male mating success, it may represent a key determinant at the proximate level. [ABSTRACT FROM AUTHOR]

Athletic contests constitute an important aspect of human affairs. However, the evolutionary origins of athletic behavior, that is, behavior which is associated with participating in and watching athletic contests, are rather obscure, as they do not seem to contribute directly to increasing survival or reproductive success. This article argues that athletic behavior has been shaped in ancestral times jointly by intrasexual selection forces, where men look for potential allies, and intersexual selection forces, where men look for spouses for their female relatives. On the basis of this evolutionary framework, a number of hypotheses are derived: First, men are more interested than women in taking part in and watching sports. In addition, men are more interested than women in taking part in and watching competitive sports than non-competitive sports, and are more interested in team sports than individual sports. Finally, men and women differ in their motives for participating in sports, while the sex differences in athletic behavior between men and women are consistent across different cultures. Evidence from a sample composed of 37 different countries provides support for all hypotheses. [ABSTRACT FROM PUBLISHER]