High-ranking Females Research Articles

Factors affecting aggression among females in captive groups of rhesus macaques (Macaca mulatta)

Captive groups of primates often exhibit higher rates of aggression than wild, free-ranging groups. It is important to determine which factors influence aggression in captivity because aggression, particularly intense aggression, can be harmful to animal health and well-being. In this study, we investigated the effect of ground substrate as well as season, rank, age, and group size on rates of agonistic interactions per female in seven captive groups of rhesus macaques (n = 70 females, 1,723 focal samples) at the California National Primate Research Center. Agonistic interactions were divided into three categories: displacements, mild aggression, and intense aggression. Females living in enclosures with gravel substrate were 1.7 times more likely to be involved in intense aggression (e.g. chases and physical contact) than females living in enclosures with grass (Poisson regression model: P < 0.001). High-ranking females were at least 1.3 times more likely to be involved in mild (e.g. threats and lunges) aggression than lower-ranking females (low rank: P = 0.03; mid rank: P = 0.001). Females of all ranks were 1.5-1.9 times more likely to be involved in both intense and mild aggression during the breeding season than other seasons. Age and group size did not affect rates of mild or intense aggression. These findings indicate that although some aggression appears to be natural and unavoidable, i.e. aggression during the breeding season, the well-being of captive macaques can be improved by developing grass substrate in outdoor enclosures.

Social behavior, foraging strategies, and fecal glucocorticoids in female blue monkeys (Cercopithecus mitis): potential fitness benefits of high rank in a forest guenon

Socioecological theory predicts that aggressive feeding competition is associated with linear dominance hierarchies and reproductive advantages for high-ranking females. Female blue monkeys contest fruits and have a linear dominance hierarchy, yet previous research has shown no evidence that high-ranking females benefit from greater feeding success or fertility. Here, we assess whether individuals differ in fecal glucocorticoid (fGC) excretion and examine proximate determinants of such differences to infer potential fitness correlates of rank, using data collected from two study groups in the Kakamega Forest, Kenya. We found that higher ranking females had preferential access to fruits in both groups, although the behavioral mechanisms leading to this effect varied between groups. Despite a consistent rank difference in feeding on fruits, an overall rank effect on fGCs emerged in only one group; females of this group spent comparatively more time feeding on fruits, fruits accounted for a greater proportion of the diet, and females engaged in more frequent food-related agonism. In addition, more females in this group were lactating during a period of low fruit availability, when rank effects on fGCs were particularly strong. Regardless of fruit availability, among lactating females of both groups higher rank was associated with lower fGC levels, indicating lower energetic stress in higher ranking females when energy demands were particularly high. Individual rates of agonism, a potential psychological stressor, were unrelated to fGCs at all times. After we accounted for rates of agonism and feeding on fruits, females of one group who groomed others more had lower fGCs, suggesting that variable social coping behavior can contribute to fGC variation in some groups. This study provides the first empirical evidence that high-ranking female blue monkeys may obtain fitness benefits from their social status, by gaining priority of access to fruits during critical times in the reproductive cycle.

Energetics during reproduction: A doubly labeled water study of lactating baboons

Understanding the costs and regulation of reproduction in primates requires understanding the separate but linked effects of energy availability and total energy expenditure (TEE). We compared variation in TEE and energy intake (EI) between two periods, early lactation and after the resumption of sexual cycling, for eight females from two groups of normally reproducing colony-living baboons (Papio h. anubis). Total energy expenditure was assessed using the doubly labeled water method. TEE was correlated with maternal mass both during early lactation and after the resumption of cycling. TEE after the resumption of cycling was positively related to infant growth rates; mothers with rapidly growing infants had higher energy expenditure. TEE was however unrelated to maternal rank and only weakly associated with reproductive parameters such as delay to conception. EI in early lactation was related to infant mass and interbirth intervals, but unrelated to infant growth or reproductive parameters once cycling had resumed. Energy availability (EA; the difference between intake and expenditure) differed significantly between subordinate and dominant females during early lactation, was highly variable among individuals as a function of body composition, and is suggested to follow a nonlinear relationship as a complex function of social status, lactation stage, infant growth, and female fertility. Thus, as a consequence of reduced energy availability, subordinate females in this captive context may experience reproductive delays even though the total energy expenditure after the return of cycling was similar between high and low ranking females.

The Effects of the Social Hierarchy Destabilization on the Foraging Activity of Eusocial WaspMischocyttarus cerberus styxRichards, 1940 (Hymenoptera: Vespidae: Polistinae)

The genusMischocyttaruscomprises 245 species of neotropical basal eusocial wasps. They form small colonies (rarely more than few tens of individuals); castes are morphologically undifferentiated and determined behaviorally by agonistic interactions. The aim of this study was to verify the effects of the experimental disruption of social hierarchy on foraging activity ofMischocyttarus cerberus styx. We observed six colonies in postemergence phase and recorded data on the foraging activity under two experimental conditions: (1) removal of lower-ranked females and (2) removal of higher ranked females, except the queen. Our results showed that the removal of higher-ranked females had higher effect on the number of foraging trips ofM. cerberus styxthan the removal of lower-ranked females (the number of foraging trips/hour decreased by 66.4% and 32.7%, resp.). Such results are likely due to the social organization of this species and the presence of a distinct class of females, which in this study were regarded as intermediates. Our data also showed that, irrespective of the hierarchical status of the females, the removal of two or three individuals affected significantly the number of foraging trips in this species.

Maternal Infant Monitoring in a Free-ranging Group of Japanese Macaques (Macaca fuscata)

In nonhuman primates, females with infants visually monitor their infants from a distance to detect and consequently avoid potential threats to their infants. We recorded maternal visual monitoring of infants (infant monitoring) ages 7–18 wk in a free-ranging, provisioned group of Japanese macaques (Macaca fuscata). The infant monitoring rate declined as a function of infants’ ages in weeks and increased when the infants were beyond their mother’s reach, indicating that infant monitoring reflects the vulnerability of infants. Females with infants increased infant monitoring when their infants were handled by other group members but not when their infants moved alone. This suggests that intragroup threats (harassment/mishandling or kidnapping) have a relatively stronger influence on infant monitoring than external threats (predation or infanticide) under the condition of this study. Infant monitoring of middle-/low-ranking females was more frequent than that of high-ranking females when their infants were handled by other individuals. This may reflect greater intragroup threats to infants of middle-/low-ranking females; however, further study is needed to confirm this. During important activities (feeding or grooming), the infant monitoring rate was lower than that during other activities (resting or self-directed behavior). However, even during important activities, females with infants increased infant monitoring when infants were handled. This indicates that females with infants face a trade-off between infant monitoring and other important activities, and even if females have to reduce the time spent on important activities, they increase infant monitoring when their infants face greater potential intragroup threats.

How feeding competition determines female chimpanzee gregariousness and ranging in the Taï National Park, Côte d'Ivoire

Socioecological theory suggests that feeding competition shapes female social relationships. Chimpanzees (Pan troglodytes) live in fission-fusion societies that allow them to react flexibly to increased feeding competition by forming smaller foraging parties when food is scarce. In chimpanzees at Gombe and Kibale, female dominance rank can crucially influence feeding competition and reproductive success as high-ranking females monopolize core areas of relatively high quality, are more gregarious, and have higher body mass and reproductive success than low-ranking females. Chimpanzee females in Taï National Park do not monopolize core areas; they use the entire territory as do the males of their community and are highly gregarious. Although female chimpanzees in Taï generally exhibit a linear dominance hierarchy benefits of high rank are currently not well understood. We used a multivariate analysis of long-term data from two Taï chimpanzee communities to test whether high-ranking females (1) increase gregariousness and (2) minimize their travel costs. We found that high-ranking females were more gregarious than low-rankers only when food was scarce. During periods of food scarcity, high rank allowed females to enjoy benefits of gregariousness, while low-ranking females strongly decreased their gregariousness. High-ranking females traveled more than low-ranking females, suggesting that low-rankers might follow a strategy to minimize energy expenditure. Our results suggest that, in contrast to other chimpanzee populations and depending on the prevailing ecological conditions, female chimpanzees at Taï respond differently to varying levels of feeding competition. Care needs to be taken before generalizing results found in any one chimpanzee population to the species level.

Social capital and physiological stress levels in free-ranging adult female rhesus macaques

Social animals with greater access to social support, i.e. higher levels of social capital, may be able to cope better with the challenges they face in their day-to-day lives, and this may be reflected in lower physiological stress levels. Here, we examine the relationship between social capital and fecal glucocorticoid (GC) levels in pregnant free-ranging adult female rhesus macaques. In addition to social capital measures based on direct connections between social partners, which have been examined previously, we use social network analysis to generate measures of social capital based on indirect connections (i.e. connections between pairs of individuals which result from their mutual direct connection to a third party). We consider social capital based on three different types of affiliative association: grooming, the exchange of affiliative vocalizations and proximity. After controlling for variables known to affect GC output in primates (e.g. month of pregnancy), GC levels of females were significantly predicted by a social network measure of indirect connectedness in the proximity network, proximity reach, in interaction with dominance rank. High ranking females had significantly lower GC levels in months in which they had low levels of proximity reach (i.e. in months in which their proximity networks were smaller and therefore more focused). The results of our study add to a growing body of evidence which suggests that social capital may be an important means by which gregarious animals cope with day-to-day challenges. Our study also joins a small body of recent research which has demonstrated that indirect connections may be important factors in the lives of social animals.

Strong and Consistent Social Bonds Enhance the Longevity of Female Baboons

Longevity is a major component of variation in fitness in long-lived iteroparous species [1-4]. Among female baboons, variation in breeding lifespan accounts for approximately 50% of the variation in lifetime fitness [5, 6]. However, we know little about the causes of variation in longevity in primates or other long-lived mammals. Savannah baboons form strong, equitable, and enduring relationships with specific female partners, particularly with close relatives and agemates [7-10]. The quality of females' social relationships influences their ability to cope with stressful events [11-13] and is associated with variation in female reproductive success [9, 14]. Here we show that dominance rank and the quality of close social bonds have independent effects on the longevity of female chacma baboons (Papio hamadryas ursinus). High-ranking females live longer than lower-ranking females. In addition, females who form stronger and more stable social bonds with other females live significantly longer than females who form weaker and less stable relationships. These data extend our understanding of the adaptive value of social bonds in baboons and complement a growing body of evidence that indicates that social bonds have adaptive value in a range of taxa, from mice to humans [9, 14-19].

Fecal glucocorticoids as indicators of metabolic stress in female Sykes' monkeys (C ercopithecus mitis albogularis)

Because of their mediating role in the stress response and potential effects on fitness, glucocorticoid (GC) hormones are increasingly used to assess the physiological costs of environmental and behavioral variation among wild vertebrates. Identifying the proximate causes of GC variation, however, is complicated by simultaneous exposure to multiple potentially stressful stimuli. Here, we use data from a partially provisioned social group of Sykes' monkeys to evaluate the effects of potential psychological and metabolic stressors on temporal and individual variation in fecal GC (fGC) excretion among 11 adult females. Despite high rates of agonism over provisioned foods fGCs declined during periods of high provisioning frequency when fruit availability was dominated by neem ( Azadirachta indica), an item requiring great feeding effort. Provisioned foods did not prevent fGC increases when availability of the most preferred main fruit item, tamarind ( Tamarindus indica), declined drastically. Although rank-related differences in access to provisioned foods and rates of agonism did not lead to an overall effect of rank on fGCs, low-ranking females excreted more fGCs than high-ranking females during a period of high provisioning intensity and low fruit availability. The emergence of this rank effect was associated with elevated feeding effort in all females, a greater access to provisioned items by high-ranking females, and a higher proportion of time spent moving in low-ranking females. Our findings suggest that metabolic stressors were the primary determinants of both temporal and individual variation in fGCs, indicating potential fitness benefits for high-ranking females when food availability is limited.

Digit ratio (2D:4D) and dominance rank in female rhesus macaques (Macaca mulatta)

Female rhesus macaques exhibit matrilineal dominance structures, and high dominance rank confers fitness benefits across a lifetime and across generations. Rank effects are “inherited” through social processes that are well understood; however, biological mechanisms that might impact these processes are not well known. Recently, it has been shown that prenatal androgens appear to be implicated in supporting dominance rank hierarchies in some mammals. In humans, interindividual differences in the second (index) to fourth (ring) digit ratio (2D:4D) have been linked indirectly to variation in prenatal androgens, with low 2D:4D in both sexes associated with higher inferred prenatal androgen effects. 2D:4D has also been related to dominant social behavior and has been shown to co-vary with social systems across nonhuman primate species. Here, we investigate how 2D:4D co-varies with socially inherited dominance rank in female rhesus macaques. Low 2D:4D was associated with higher-ranking females, while higher 2D:4D was associated with lower-ranking females. Similar relationships were also shown between ranked families within matrilines. This is the first study to show such a relationship between 2D:4D and dominance rank in a nonhuman primate and suggests that prenatal androgen effects could be involved in the maintenance of dominance rank in female cercopithecine primates.

The strategic use of sex in wild female western gorillas

Human females, unlike most mammals, are sexually active outside of fertile periods. This decoupling of sexual behavior from its conceptive function has had an enormous impact on human social relationships, and yet we know little about why there was selection for nonconceptive mating. Here we examine one form of nonconceptive mating, the mating that occurs during pregnancy or post-conceptive (PC) mating, in wild western gorillas (Gorilla gorilla). Using a near complete mating record for five females during gestation, we show that pregnant females varied in the timing and frequency of mating, and used PC mating conditionally, synchronizing copulations to occur on days when other females mated, and refraining from mating for lengthy periods when no other females mated. As pregnant females mated exclusively with the same male before and after conception, and mated in response to group female (and not male) behavior, we conclude that western gorillas used PC mating as a form of female competition, and not to confuse paternity or to obtain immediate benefits from the male, as suggested earlier. The male initiated copulations preferentially with females of high rank, rather than distinguishing between pregnant and cycling females. Therefore, PC mating appears to be a strategy by which high-ranking pregnant females attempt to minimize male interest in other females, while reinforcing their own status and potentially delaying conception in others. These findings indicate that female-mating competition is more important than considered earlier, and may be a factor in the evolution of nonconceptive mating in humans.

O brother, where art thou? The varying influence of older siblings in rank acquisition by female baboons

Males in sexually dimorphic species like baboons appear to have surprisingly little influence on the reproduction and dominance ranks of their female kin, even though they could potentially increase their fitness by helping their relatives improve their ranks. Male baboons are able to dominate females several years before they emigrate, but their presence has no effect on relatives’ dominance ranks, at least when female kin are present. As a result, females usually acquire ranks within their matriline, above their older sisters. We describe the process of rank acquisition among orphaned and non-orphaned juvenile and adolescent females in a group of free-ranging baboons. Orphaned females were significantly more likely than non-orphaned females to acquire unexpected ranks. Orphaned females with older sisters often acquired ranks within the matriline, but below their older sisters’. Orphaned females with older brothers were likely to rise in rank above their matriline. Females’ interventions on behalf of younger sisters always supported the existing female dominance hierarchy, while males’ interventions tended to act against it. Similarly, in playback experiments, females appeared to be willing to support their younger sister only in disputes with lower-ranking females. In contrast, males appeared to be willing to support their sister even in disputes with higher-ranking females. Fraternal support enables females to improve their dominance ranks, but only if their mothers have died. It remains a puzzle why males have so little influence on their female relatives’ ranks when female kin are present, and so much when they are absent.

Food site residence time and female competitive relationships in wild gray-cheeked mangabeys (Lophocebus albigena).

Authors of socioecological models propose that food distribution affects female social relationships in that clumped food resources, such as fruit, result in strong dominance hierarchies and favor coalition formation with female relatives. A number of Old World monkey species have been used to test predictions of the socioecological models. However, arboreal forest-living Old World monkeys have been understudied in this regard, and it is legitimate to ask whether predominantly arboreal primates living in tropical forests exhibit similar or different patterns of behavior. Therefore, the goal of our study was to investigate female dominance relationships in relation to food in gray-cheeked mangabeys (Lophocebus albigena). Since gray-cheeked mangabeys are largely frugivorous, we predicted that females would have linear dominance hierarchies and form coalitions. In addition, recent studies suggest that long food site residence time is another important factor in eliciting competitive interactions. Therefore, we also predicted that when foods had long site residence times, higher-ranking females would be able to spend longer at the resource than lower-ranking females. Analyses showed that coalitions were rare relative to some other Old World primate species, but females had linear dominance hierarchies. We found that, contrary to expectation, fruit was not associated with more agonism and did not involve long site residence times. However, bark, a food with a long site residence time and potentially high resource value, was associated with more agonism, and higher-ranking females were able to spend more time feeding on it than lower-ranking females. These results suggest that higher-ranking females may benefit from higher food and energy intake rates when food site residence times are long. These findings also add to accumulating evidence that food site residence time is a behavioral contributor to female dominance hierarchies in group-living species.

Factors influencing hair loss among female captive rhesus macaques ( Macaca mulatta)

Although rare among wild animals, hair loss is common among captive animals, which suggests that some aspect of the captive environment contributes to abnormal hair loss. Female rhesus macaques ( Macaca mulatta) at the California National Primate Research Center (CNPRC) housed in outdoor enclosures exhibited hair loss that varied significantly by pregnancy, season, ground substrate, rank, and age as well as by several pair-wise interactions. Pregnant females were 2.4 times more likely to have worse coat condition than non-pregnant females ( P < 0.001). Among pregnant females, pronounced hair loss was apparent 1–2 months into gestation, as well as during the month following parturition. Females in general exhibited the lowest degree of hair loss in Fall (Fall vs. Winter: P < 0.001; Fall vs. Spring: P < 0.001; Fall vs. Summer: P < 0.001). Independent of reproductive condition and seasonality, macaques housed in enclosures with gravel substrate were 3.7 times more likely to have greater hair loss than those in enclosures with grass substrate ( P < 0.001), and low-ranking females exhibited significantly greater hair loss than high-ranking females ( P < 0.001). Older females had significantly greater hair loss than younger females ( P < 0.001). Finally, females in larger groups tended to have greater hair loss than those in smaller groups. These results indicate that multiple factors influence the degree of hair loss among female captive rhesus macaques, some of which include housing conditions that can be modified to improve coat condition, such as planting grass in outdoor enclosures and adopting management procedures that reduce levels of social stress experienced by lower-ranking animals and animals living at higher densities.

Does anal gland scent signal identity in the spotted hyaena?

Recognition requires the existence of distinct cues and the ability to detect and discriminate between these signals. Using gas chromatography–mass spectrometry, we investigated the chemical composition of 143 anal scent gland secretions from 84 individually known spotted hyaenas, Crocuta crocuta, in three clans in the Serengeti National Park, Tanzania. In total, 252 volatile compounds were detected. Comparison of the chemical composition of individual scent profiles by calculating the Bray–Curtis similarity index and using an analysis of similarities revealed that scent profiles varied significantly between individuals but also changed in composition over time. There was no difference in complexity, in terms of the number of different compounds, between male and female scent or between scent from high- and low-ranking females. Scent profiles of closely related females were as similar as profiles of unrelated females. Although there was no evidence of rank-related changes in profiles across the entire female social hierarchy, the profiles of high-ranking females differed significantly from those of low-ranking females, suggesting the presence of an olfactory badge of status. Behaviourally, adult females responded to scent deposited on vegetation by other females significantly faster and more often, and were more likely to anoint themselves with that scent, as the social rank of the female who had deposited the scent increased. These results suggest that spotted hyaena scent carries both an olfactory badge of status and individually distinct cues that are likely to be useful in the maintenance of a social network within their fission–fusion society.

The expression of affect in African elephant (Loxodonta africana) rumble vocalizations.

Affective states are thought to be expressed in the mammalian voice, but such investigations are most common in primates. Source and filter features of rumbles were analyzed from 6 adult female African elephants (Loxodonta africana) at Disney's Animal Kingdom. Rumbles produced during periods of minimal social interaction ("low affect") were compared to those produced during dominance interactions ("high affect"). Low-ranking females produced rumbles with increased and more variable fundamental frequencies, and increased durations and amplitudes during dominance interactions with superiors, compared to the low affect context. This acoustic response is consistent with the expression of affect in mammals and may signal submission to superiors. The 2 highest ranking females were codominant and competed for alpha status. They produced rumbles with decreased and less variable fundamental frequencies, increased durations and amplitudes, and a decrease in formant dispersion during dominance interactions with each other, compared to the low affect context. This response is not generally consistent with the expression of affect, but may signal large body size to competitors. These results suggest that affect can be expressed in the voiced sounds of elephants.

Microhabitat Variation and Its Effects on Dietary Composition and Intragroup Feeding Interactions Between Adult Female Lemur catta During the Dry Season at Beza Mahafaly Special Reserve, Southwestern Madagascar

We compared diets and female feeding agonism in 2 groups of ring-tailed lemurs living in markedly diverse microhabitats in and near the Beza Mahafaly Reserve, Madagascar during mid-to-late gestation periods and height of the dry season to test predictions concerning differences in resource availability, preferred foods, female rank, and degree and frequency of feeding agonism in relation to usurpability and monopolization of food resources. Quadrat sampling in disturbed forest habitat revealed a greater number of plant species than in the gallery forest home range area, but females in both groups consumed nearly equal numbers of actual food plant species. Higher-ranking females in disturbed forest consumed human food scraps from the researchers camp significantly more often than low-ranking females did, while there was no rank effect for consumption of any food type between females in the gallery forest group. Higher rates of female feeding agonism in both groups occurred around usurpable fruit and leaves, as well as over monopolizable human food scraps in the disturbed forest group. There is no association between degree of agonism and food type in either group, and rate of feeding agonism is similar for both groups. The most highly contested food items came in large packages and were high in nutrients: beneficial foods for gestating females in the height of dry season in southwestern Madagascar.

Abdominal odours of young, low-ranking European rabbit mothers are less attractive to pups: an experiment with animals living under natural breeding conditions

Lactating rabbit females (Oryctolagus cuniculus) emit abdominal, ventral odour cues that are involved in guiding the pups to the nipples. We tested the impact of the females’ social rank and age on the emission of this odour signal; both factors are highly collinear in wild rabbits and we did not aim to disentangle them in our study. We predicted that the abdominal odours of young, low-ranking females, which are usually more stressed and have a comparatively lower body condition, should be less attractive to pups than those of older, high-ranking females. For the experiment, we caught females from a population of wild-type European rabbits living in a large field enclosure. Pairs of adult females were placed in boxes with wire-mesh floor fixed over an arena with three rabbit pups aged 4–7 days old. First, we evaluated the effectiveness of our two-choice design by testing lactating females (LF) against non-lactating (NLF) controls (n = 17 pairs). In accordance with recent studies, the pups showed a clear preference for lactating females by means of a higher exploration time and more local searching under the ventral region of these females. In a second experiment, the pups chose between lactating 1-year-old, low-ranking females (Y/LRF) and older, high-ranking females (O/HRF) (n = 10 pairs). These latter assays suggested a comparatively lower attractiveness of Y/LRF females to pups searching for milk. Although the pups showed no significant preference in exploration behaviour, they preferably directed local searching events to the abdominal region of O/HRF females. In conclusion, our study suggests that mother–offspring chemosensory communication is modified by the mother’s age and social rank, i.e., by a complex of factors that are strongly linked in rabbit females in the wild.

Factors affecting fecal glucocorticoid levels in semi‐free‐ranging female mandrills (Mandrillus sphinx)

Subordinate female cercopithecine primates often experience decreased reproductive success in comparison with high-ranking females, with a later age at sexual maturity and first reproduction and/or longer interbirth intervals. One explanation that has traditionally been advanced to explain this is high levels of chronic social stress in subordinates, resulting from agonistic and aggressive interactions and leading to higher basal levels of glucocorticoids. We assessed the relationships among fecal cortisol levels and reproductive condition, dominance rank, degree of social support, and fertility in female mandrills (Mandrillus sphinx) living in a semi-free-ranging colony in Franceville, Gabon. Lower-ranking females in this colony have a reproductive disadvantage relative to higher-ranking females, and we were interested in determining whether this relationship between dominance rank and reproductive success is mediated through stress hormones. We analyzed 340 fecal samples from 19 females, collected over a 14-month period. We found that pregnant females experienced higher fecal cortisol levels than cycling or lactating females. This is similar to results for other primate species and is likely owing to increased metabolic demands and interactions between the hypothalamus-pituitary-adrenal axis, estrogen, and placental production of corticotrophin releasing hormones during pregnancy. There was no influence of dominance rank on fecal cortisol levels, suggesting that subordinate females do not suffer chronic stress. This may be because female mandrills have a stable social hierarchy, with low levels of aggression and high social support. However, we found no relationship between matriline size, as a measure of social support, and fecal cortisol levels. Subordinates may be able to avoid aggression from dominants in the large enclosure or may react only transiently to specific aggressive events, rather than continuously expecting them. Finally, we found no relationship between fecal cortisol levels and fertility. There was no difference in fecal cortisol levels between conceptive and nonconceptive cycles, and no significant relationship between fecal cortisol level and either the length of postpartum amenorrhea or the number of cycles before conception. This suggests that the influence of dominance rank on female reproductive success in this population is not mediated through chronic stress in subordinate females, and that alternative explanations of the relationship between social rank and reproduction should be sought.

Female Chimpanzees Use Copulation Calls Flexibly to Prevent Social Competition

The adaptive function of copulation calls in female primates has been debated for years. One influential idea is that copulation calls are a sexually selected trait, which enables females to advertise their receptive state to males. Male-male competition ensues and females benefit by getting better mating partners and higher quality offspring. We analysed the copulation calling behaviour of wild female chimpanzees (Pan troglodytes schweinfurthii) at Budongo Forest, Uganda, but found no support for the male-male competition hypothesis. Hormone analysis showed that the calling behaviour of copulating females was unrelated to their fertile period and likelihood of conception. Instead, females called significantly more while with high-ranking males, but suppressed their calls if high-ranking females were nearby. Copulation calling may therefore be one potential strategy employed by female chimpanzees to advertise receptivity to high-ranked males, confuse paternity and secure future support from these socially important individuals. Competition between females can be dangerously high in wild chimpanzees, and our results indicate that females use their copulation calls strategically to minimise the risks associated with such competition.