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Female Japanese macaques are seasonal breeders distinguished by their red-colored hindquarters, face, and nipple skin areas. Intensity of coloration seems to be associated with sexual attractiveness, behavior, and fluctuating sex steroids. Our aim was to investigate whether the color intensity of these regions differed between lactating (LA) and non-lactating (NLA) females during sexually inactive (SI) and active (SA) phases. Coloration scores of 19 adult females were classified using color tables. Estrogen and progesterone metabolites were determined in fecal samples. Weekly comparison between both groups revealed significantly increased coloration of the hindquarters area from week 13 (SI) until the end of the observation period, and for the nipple skin throughout the SI and SA periods. Face coloration differed marginally. Hormonally, NLA females showed significantly increased excretion rates of sex steroids at the end of the SI phase and throughout the whole SA period. Logistic regression analyses between elevated fecal steroids and nipple coloration disclosed a significant relationship for NLA females during the SI period. This connection persisted and included hindquarter coloration during the SA period. NLA females showed increased intromission with ejaculation, but no difference was found for intromission without ejaculation. In conclusion, results demonstrate increased endocrine excretion rates for NLA females during the whole observation period, paralleled by an enhanced, fertility-signaling sexual attractiveness.

Mate choice is a critical part of sexual selection. One constituent of mate choice is attractiveness, which serves as a projection surface for traits signalling quality and condition to prospective reproductive partners. In guinea pigs, females generally decide whom they will select as partner and when to switch to a new one. The aim of this study was to investigate for universality in male traits possibly associated with female preference. Characteristics of stress response were chosen, as they are relatively stable in their behavioural and physiological expression. Two consecutive experiments were done on isosexually kept males. In the first experiment, behavioural performance in a forced swim-test was noted, after basal endocrine status had been documented with three samples per individual. In a second experiment, the same males were presented to females in a round-robin choice paradigm. A discriminant analysis confirmed a categorization of males into groups that were preferred by different females either two times [more preferred (MP), n = 6] or less than that [less preferred (LP), n = 12]. Mean basal cortisol levels were comparable, but mean basal testosterone (T) was significantly higher in LP. This points at a scenario where higher T might be advantageous in male-male competition but perhaps less important for the formation of male-female bonds. Behaviourally, MP completed the swim-test significantly faster than LP, which may indicate greater goal directedness and motivation. We conclude that traits associated with stress responses may be components of male attractiveness, their stability perhaps reflecting adaptive qualities.

Taking blood samples is a common method in biomedical and biological research using guinea pigs. However, most blood sampling techniques are complicated and highly invasive and may therefore not be appropriate for certain research topics concerning stress and reproduction. Thus, a non-invasive method to measure steroid hormones is critically needed. The aim of this study was the biological validation of corresponding enzyme immunoassays for the measurement of fecal cortisol, progesterone, estrogen, and testosterone metabolites in guinea pigs. We examined the effect of subcutaneous injections of ACTH or saline on fecal cortisol metabolites to investigate the suitability of fecal samples to monitor adrenocortical activity. Furthermore, we investigated whether fecal sex steroid metabolites accurately reflected endocrine changes observed in plasma samples during female estrous cycles and male puberty, respectively. In addition, we compared fecal testosterone metabolites of intact males, castrated males, and females to investigate the reliability of fecal samples in discriminating gonadal status of males. Concentrations of fecal cortisol metabolites were significantly increased following ACTH challenge, indicating that adrenocortical activity can be monitored via fecal samples. Secondly, in females, plasma and fecal gonadal steroids were significantly correlated in most subjects. The assay for testosterone metabolites, on the other hand, could not clearly discriminate between test groups. From these findings we conclude that fecal samples can be used for the non-invasive assessment of adrenocortical and female reproductive status in guinea pigs. Testosterone metabolism seems to be more complex and further investigations are needed to establish a more suitable assay.

Polygynous mating systems can inflict considerable costs on males, often causing the emergence of alternative mating strategies to ensure reproductive success. These strategies can lead to different morphs of color, size, or behavior. The present work was done on guinea pigs, a polygynous rodent species. Some males can show a reduced readiness for conflict, perhaps employing an alternative mating strategy. To test this hypothesis, a two-stage experiment was set up, with males (N = 16) and females (N = 16) initially living in isosexual groups. Visual and olfactory contact was possible through a wire mesh. Male agonistic behavior was observed for 15 days, confirming the existence of less-conflict-involved (LCI, N = 7) and more-conflict-involved males (MCI, N = 9). Significant differences were found for a conditional parameter, body mass, and a morphometric one, testis width: LCI consistently surpassed MCI. Hormonally, cortisol was comparable, while testosterone was distinctly higher in MCI. Next, males and females were joined and observed for further 24 days. Males initially lost weight but reached original mass towards the end of the experiment. Testis width patterns were similar. Again, LCI had higher body mass. Cortisol was comparable, but testosterone release 3 days after merging was significantly higher in LCI. Behaviorally, LCI exhibited significantly less male–male aggression and more socio-sexual behavior than MCI. The former were more successful, with 57% capable of accessing females, in contrast to 11% of MCI. The existence of two distinct behavioral phenotypes in guinea pig males suggests that different reproductive strategies may exist in this species.

Couples sleeping in pairs is a modern phenomenon with potential side-effects on sleep structure and circadian rhythms. In order to examine its effects, 10 healthy heterosexual couples with stable relationships who regularly sleep apart or together in their home environments were recruited. The participants were asked to spend at least 10 nights alone and 10 nights together over the course of the study. Their sleep was monitored over 28 days in their homes with actigraphic techniques and sleep diaries. Group analyses were performed on their sleep efficiency and subjective sleep quality according to sleep condition. The daily variations in sleep fragmentation indices of the partners were used as a measure of their nocturnal activity synchronization. Sharing a sleeping space with a partner had negative effects on sleep in women, as documented in the actigraphic measurements of sleep efficiency and subjective assessments of sleep and awakening quality. Sexual contact mitigated the negative subjective report, without changing the objective results. Subjective assessments of sleep quality were lower in men than women when sleeping alone. They increased to the same level as women while sleeping in pairs with or without sexual contact. The sleep efficiency in men was not reduced by the presence of their partner unless sexual contact occurred. Analyses of sleep fragmentation showed that there was no synchronization of deviation from the norm levels between the partners on individual nights either when they slept alone or in couples. The sex differences in the nocturnal sleep reactions to partner presence may be attributed to sex-specific behavioral traits associated with cultural norms or parenting in women and the desire for group sleep in men.

Background Sex steroids affect many peripheral tissue sites in female mammals. Receptors for these hormones have been found in skin, fat, and bone. In women, these tissues can show morphological changes during the menstrual cycle that may be directly related to steroid secretion. Methods The present study was done on chimpanzees to document morphometric markers associated with these tissues (anogenital swelling volume, skin fold thickness as indicator of subcutaneous fat, bony diameters of mandible, wrist, and elbow) and to compare them with cyclic patterns of estradiol, progesterone, testosterone, gonadotropins, and prolactin. Results Swelling volume changed significantly over the menstrual cycle. All other morphometric parameters showed variation without statistical significance. Skin folds were thickest during the luteal phase. Bony diameters displayed similar but less distinctive changes. Testosterone correlated positively with diameter sites, inversely with subcutaneous fat. No relationships with either estradiol or progesterone were found. We assume that subcutaneous fat and morphometric bone parameters exhibit cycle-dependent changes that may be caused by changes in steroid secretion.

Numerous studies have demonstrated interactions between oxytocin (OT) secretion, adrenal activity, an animal's social environment, and stress responses. In the present study, we hypothesized that partner preference and pair bonding cause increased peripheral OT, which down-regulates the adrenal and behavioral response in stress. In addition, we tested whether these interactions depended on sex, the social environment of the individual, or the type of stressor. Experiments were carried out on guinea-pigs held in sexual-pairs or stressed by isolation. Among the paired individuals choice tests were carried out to document partner preference. Female preference for a male was expressed by spatial cohesion. Stress responses to abiotic stimuli were examined and compared between isolated and cohabited animals with or without preference. Results show that OT of cohabited animals was significantly higher than during isolation. OT levels were further increased in males preferred by females. Cortisol (CORT) levels were elevated in isolated animals. There were no significant differences between pairs with and without female preference. Behaviorally, partner preferences were expressed by high amounts of tactile contact. The stress-induced behavioral immobility response after exposure to a noise stressor was significantly reduced in preferred males and to a lesser extent in their female partners. Only females with preferences showed an endocrine stress response. Their levels of OT increased. There was no consistent post-stressor release of CORT. The data indicate that the social environment of an individual, here expressed as preference or isolation, influences peripheral OT secretion. Behavioral stress responses were similarly affected by social factors without a clear involvement of peripheral CORT or OT.

In socially-living animals, social enrichment enhances spatial learning and memory while separation from conspecifics can severely impair these abilities. In the present work, guinea pigs were kept in isolation or cohabitated in heterosexual pairs and then subjected to a labyrinth task. Latency-time to bait, error-rate, amount of movement and pre- and post-experimental cortisol (CORT) were registered. During a 5d-acquisition phase, single animals (N=19) showed a more efficient encoding of spatial information, with significantly decreased latency-time and error-rate over the time course. In contrast, cohabitated animals (N=19) did not show a significant improvement. Three days after acquisition, memory was tested in a retention test, under the same conditions. With regard to behavioral performance, there was no significant difference between cohabitated and single animals. Pre-experimental CORT was significantly higher in cohabitated animals when compared to single ones. Post-experimentally, CORT increased significantly in singles but not in cohabitated animals when compared to pre-experimental values. Thus, both groups did not differ from each other at that point. Social condition seemed to be an important modulator, in that learning and memory were more impaired in paired animals than in single ones. The failure of cohabitated animals to encode spatial memory more quickly may have been caused by a more chronically up-regulated HPA-axis. The post-experimental CORT increase of singles may be due to more efficient handling of short-term stress exposure.

Numerous studies point to central serotonin as an important modulator of maladaptive behaviors. In men, for instance, low concentrations of this neurotransmitter are related to hostile aggression. A key player in serotonin metabolism seems to be central cholesterol. It plays a fundamental role in maintaining the soundness of neuron membranes, especially in the exocytosis transport of serotonin vesicles into the synaptic cleft. In this review, we attempt an evolutionary approach to the neurobiological basis of human male violence. Hominid evolution was shaped by periods of starvation but also by energy demands of an increasingly complex brain. A lack of food resources reduces uptake of glucose and results in a decreased energy-supply for autonomous brain cholesterol synthesis. Consequently, concentrations of neuromembrane cholesterol decrease, which lead to a failure of the presynaptic re-uptake mechanism of serotonin and ultimately to low central serotonin. We propose that starvation might have affected the larger male brains earlier than those of females. Furthermore, this neurophysiological process diminished the threshold for hostile aggression, which in effect represented a prerequisite for being a successful hunter or scavenger. In a Darwinian sense, the odds to acquire reliable energetic resources made those males to attractive spouses in terms of paternal care and mate support. To underpin these mechanisms, a hypothetical four-stage model of synaptic membrane destabilization effected by a prolonged shortage of high-energy, cholesterol-containing food is illustrated.

Urinary metabolites of androgens and cortisol were measured in free-living male and female bonobos. Sex differences and correlations between adrenal and gonadal steroid excretion were investigated. The immunoreactive concentrations of androgens were measured with two different androgen assays. One assay used a testosterone (T) antibody raised with a 17beta-hydroxy group, and the other employed an antibody raised against a reduced form, 5alpha-androstane-17alpha-ol-3-one-CM (17alpha) with cross reactivity for epitestosterone and 5alpha-androstanedione. Both assays have been used in bonobo and chimpanzee studies where non-invasive techniques were employed. The levels of 17alpha-androgen metabolites were 1.7- and 3-fold higher than those of T-metabolites in males and females. The two androgen assay results correlated in males but not females. There was a sex difference in the T-metabolites measured. Male levels were significantly higher. Levels of 17alpha in the two sexes were similar. Cortisol metabolite levels (CORT) were similar between the sexes. The T-metabolites were significantly correlated with CORT in males but not in females. In females, the 17alpha-androgen metabolites correlated with CORT. This suggests that either androgen secretion or metabolism differs between the sexes. A parsimonious interpretation of the androgen assay cortisol/androgen correlation differences would be that larger components of dehydroepiandrosterone (DHEA), androstenedione or epitestosterone from the adrenal androgens were being excreted and measured in the females. The CORT/T metabolite interactions in males may reflect male-specific social or metabolic endocrine conditions.

The neuropeptide oxytocin (OXT) is a key factor in the initiation and regulation of sociosexual behavior. The present study analyzes the effects of cohabitation and social challenge on plasma OXT concentration rates in guinea pig pairs in relation to male sociosexual behavior. The cohabitation phase lasted 3 days. On day 4, the pair was socially challenged by introducing an unfamiliar male. Displayed male sexual behavior varied significantly during cohabitation, with peaks on day 1. Sociopositive behavior, i.e., side-by-side contact, was increased on days 3 and 4. Cohabitation per se led to elevated plasma OXT concentrations only in males. In contrast, both sexes reacted with increased plasma OXT concentrations to the social challenge (day 4). At that time, male OXT was significantly correlated with sexual behavior and female OXT with sociosexual behavior received from the partner. Additionally, pairs were synchronized in their OXT release during days 3 and 4. We conclude that cohabitation causes sexually dimorphic plasma OXT concentration patterns in guinea pigs. Secondly, the conformity of OXT release in both sexes may represent an endocrine marker for long-term cohabitation, which is reflected behaviorally by increased spatial proximity. Key terms: oxytocin, behavior, guinea pig, social challenge, cohabitation.

The socio-sexual environment of a female is known to affect ovarian function. Increased male contact can enhance menstrual cycle regularity. Conversely, social deprivation constitutes a form of stress that often alters cyclicity and the secretion of reproductive hormones. The present study was carried out on captive female chimpanzees to examine possible interactions among housing conditions, menstrual cycle length, morphological changes in secondary sexual character expression and endocrine release patterns related to follicular and luteal function. Animals were housed over a period of 2 years either with a male conspecific or singly. Blood samples were collected over three cycles, and anogenital swelling changes registered to define menstrual cycle phases. Fecal sampling techniques were used to monitor cortisol as a measure of stress-load. Male presence seemed to affect female cyclicity. Females housed with a male had shorter and more regular cycles than singly housed females. Prolactin, gonadotropins and estradiol levels were generally higher in paired females during specific cycle phases. Group variation was not always significant. No differences were found in progesterone. Sexually cohabited females tended to have lower fecal cortisol metabolites immediately before and after maximum tumescence. We suggest that the close behavioral, physical and olfactory contact with a male conspecific can act as a sort of zeitgeber to modulate ovarian function by stabilizing the female cycle and, perhaps, enhancing folliculogenesis and ovulation.