Sonya M. Kahlenberg

Male coercion and the costs of promiscuous mating for female chimpanzees

For reasons that are not yet clear, male aggression against females occurs frequently among primates with promiscuous mating systems. Here, we test the sexual coercion hypothesis that male aggression functions to constrain female mate choice. We use 10 years of behavioural and endocrine data from a community of wild chimpanzees (Pan troglodytes schweinfurthii) to show that sexual coercion is the probable primary function of male aggression against females. Specifically, we show that male aggression is targeted towards the most fecund females, is associated with high male mating success and is costly for the victims. Such aggression can be viewed as a counter-strategy to female attempts at paternity confusion, and a cost of multi-male mating.

Core area quality is associated with variance in reproductive success among female chimpanzees at Kibale National Park

Female East African chimpanzees, Pan troglodytes schweinfurthii, tend to range apart from each other in dispersed core areas, and they have dominance interactions with each other so rarely that it is difficult for observers to assess a dominance hierarchy. Nevertheless female chimpanzees can have high variance in fitness. Here, we test the hypothesis that female chimpanzee fitness variance is associated with variation in the foraging quality of their ranges. We studied range usage of 21 wild adult female chimpanzees within the Kanyawara community, Kibale National Park, Uganda. Core areas of individuals remained stable over a 9-year period and varied in their density of preferred foods. Females in neighbourhoods containing more preferred foods had elevated ovarian hormone production, shorter birth intervals and higher infant survivorship. Our results thus suggest that superior access to food may have enabled some community females to reproduce more successfully than others. Although dominance interactions are less frequent among females than among males of this species, we propose that the intensity of selection on intrasexual competition may be similar between the sexes. We discuss potential applications to other fissionefusion species.

Female Competition over Core Areas in Pan troglodytes schweinfurthii, Kibale National Park, Uganda

Aggression is rare among wild female chimpanzees. However, in the Kanyawara chimpanzee community in Kibale National Park, Uganda, stable use of food-rich core areas is linked to increased reproductive success, suggesting that contest competition might occur over access to the highest-quality ranges. To examine this hypothesis, we studied aggression and dominance relationships among Kanyawara females during a 10-yr period that included the immigration of 5 females into the community. We tested 2 predictions: 1) that female-female aggression should intensify when immigrants enter the community because this is when core area access is determined and 2) that the quality of core areas should reflect relative female dominance relationships. In support of the first prediction, female-female aggression increased 4-fold when new immigrants were in the community, with rates peaking when there were multiple immigrants. This pattern was due primarily to aggression by resident mothers toward immigrants and featured coalitionary aggression, a rare behavior among female chimpanzees. In support of the second prediction, females occupying core areas high in foraging quality ranked high overall and higher than expected for their ages, whereas females occupying low-quality core areas were lower-ranking and ranked lower than expected for their ages. Together, the data indicate that though female aggression does not regularly occur in chimpanzees, contest competition continues to play an important role in determining long-term access to resources, an important correlate of reproductive success.

Dynamics of social and energetic stress in wild female chimpanzees

Stress hormone measurements can reinforce and refine hypotheses about the costs of particular contexts or behaviors in wild animals. For social species, this is complicated because potential stressors may come from the physical environment, social environment, or some combination of both, while the stress response itself is generalized. Here, we present a multivariate examination of urinary cortisol dynamics over 6 years in the lives of wild female chimpanzees in the Kanyawara community of Kibale National Park, Uganda. We hypothesized that chimpanzee socioecology provides strong indications of both energetic and social stress to females, but that the salience of these stressors might vary over a females life history in accordance with their changing reproductive costs and social interactions. Using linear mixed models, we found that urinary cortisol levels increased significantly with age but were also elevated in young immigrants to the community. Across reproductive states, cycling, non-estrous females had relatively low cortisol compared to lactating, estrous, or pregnant females. Aggression from males led to higher cortisol levels among estrous females, frequent targets of aggressive sexual coercion. In contrast, energetic stress was most salient to lactating females, who experienced higher cortisol during months of low fruit consumption. Low dominance rank was associated with increased cortisol, particularly during the energetically demanding period of lactation. The effects of female conflict were felt widely, even among those who were the primary aggressors, providing further evidence that long-term resource competition, while apparently muted, exerts a far-reaching impact on the lives of chimpanzee females.

Males' Greater Tolerance of Same-Sex Peers

Three studies were conducted to examine the often-cited conclusion that human females are more sociable than males. Using perceptions of roommates, roommate changes at three collegiate institutions, and an experimental manipulation of friendship beliefs, the studies demonstrated unequivocally that males exhibit a higher threshold of tolerance for genetically unrelated same-sex individuals than females do. Tolerance was defined as acceptance of the stresses and strains within relationships. Results are discussed in terms of potential underlying mechanisms and ultimate explanations.

Sex differences in chimpanzees' use of sticks as play objects resemble those of children

Summary Sex differences in childrens toy play are robust and similar across cultures [1,2]. They include girls tending to play more with dolls and boys more with wheeled toys and pretend weaponry. This pattern is explained by socialization by elders and peers, male rejection of opposite-sex behavior and innate sex differences in activity preferences that are facilitated by specific toys [1]. Evidence for biological factors is controversial but mounting. For instance, girls who have been exposed to high fetal androgen levels are known to make relatively masculine toy choices [3]. Also, when presented with sex-stereotyped human toys, captive female monkeys play more with typically feminine toys, whereas male monkeys play more with masculine toys [1]. In human and nonhuman primates, juvenile females demonstrate a greater interest in infants, and males in rough-and-tumble play. This sex difference in activity preferences parallels adult behavior and may contribute to differences in toy play [1]. Here, we present the first evidence of sex differences in use of play objects in a wild primate, in chimpanzees ( Pan troglodytes ). We find that juveniles tend to carry sticks in a manner suggestive of rudimentary doll play and, as in children and captive monkeys, this behavior is more common in females than in males.

Female dominance in blue-eyed black lemurs(Eulemur macaco flavifrons).

Female dominance is unusual among mammals and has been described in detail for only a handful of species. Here we present data on the frequency and outcome of dominance interactions in seven semi-free ranging and captive groups of blue-eyed black lemurs (Eulemur macaco flavifrons) housed at the Duke University Primate Center. We collected over 260 hours of focal data during which all occurrences of dominant-subordinate interactions were recorded. We collected data outside the typical breeding and birthing seasons for this species, thus eliminating possible confounding factors and increased aggression associated with these periods. We found that females were dominant over males in all seven groups, with females winning 99% of all dominance interactions.E. m. flavifrons used aggressive dominance (e.g. chase, cuff, bite) in 81% of all interactions, with the remainder of interactions being decided using social dominance (e.g. deference in the form of supplants or cowers). Older females were dominant over younger females in two out of three multi-female groups (in each case, younger females were daughters), and younger males (sons of the dominant female) received less aggression from females than did older males (n = 2 groups). Caging and group size appear to play a minimal role in the expression of female dominance. While confirmation must await further observations on free-ranging groups ofE. m. flavifrons, our data strongly suggest that this subspecies can be characterized as female dominant.

Why male orangutans do not kill infants

Infanticide is widespread among mammals, is particularly common in primates, and has been shown to be an adaptive male strategy under certain conditions. Although no infanticides in wild orangutans have been reported to date, several authors have suggested that infanticide has been an important selection pressure influencing orangutan behavior and the evolution of orangutan social systems. In this paper, we critically assess this suggestion. We begin by investigating whether wild orangutans have been studied for a sufficiently long period that we might reasonably expect to have detected infanticide if it occurs. We consider whether orangutan females exhibit counterstrategies typically employed by other mammalian females. We also assess the hypothesis that orangutan females form special bonds with particular “protector males” to guard against infanticide. Lastly, we discuss socioecological reasons why orangutan males may not benefit from infanticide. We conclude that there is limited evidence for female counterstrategies and little support for the protector male hypothesis. Aspects of orangutan paternity certainty, lactational amenorrhea, and ranging behavior may explain why infanticide is not a strategy regularly employed by orangutan males on Sumatra or Borneo.

The Dynamics of Stress in Female Chimpanzees: Interactions of Social and Ecological Factors

Cortisol, as a biomarker of the generalized adaptive stress response, can provide critical information on the physiological effects of behavior. However, group-living animals face multiple interacting stressors from their social and ecological environments. While recent research has revealed the impact of particular social stressors in isolation, few studies have examined how diverse factors contribute to long-term stress hormone variation. We applied multivariate analyses to a 10-year dataset to investigate urinary cortisol variation in wild female chimpanzees (Pan troglodytes schweinfurthii) in Kibale National Park, Uganda. In interindividual contrasts, older females had higher cortisol levels, as did females that were low-ranking for their age. Over time, cortisol was significantly predicted by rates of aggression, particularly female-targeted aggression. The effect of male-female aggression was most pronounced for cycling, swollen females, who are the most frequent targets of sexual coercion. On the other hand, lactating females were strongly affected by group size, such that cortisol levels were elevated when parties contained more females and fewer males. Fruit consumption was an important covariate for lactating females, who experienced higher cortisol when the diet was poor. Our data indicate that both social and energetic factors contribute to stress variation in female chimpanzees, but that energetic stress increases in significance for females facing high reproductive costs. Our study also contributes to growing evidence that direct and indirect competition, while subtle in their expression, can have substantial impacts on female chimpanzees.

Male coercion and female choice in wild chimpanzees

Cortisol, as a biomarker of the generalized adaptive stress response, can provide critical information on the physiological effects of behavior. However, group-living animals face multiple interacting stressors from their social and ecological environments. While recent research has revealed the impact of particular social stressors in isolation, few studies have examined how diverse factors contribute to long-term stress hormone variation. We applied multivariate analyses to a 10-year dataset to investigate urinary cortisol variation in wild female chimpanzees (Pan troglodytes schweinfurthii) in Kibale National Park, Uganda. In interindividual contrasts, older females had higher cortisol levels, as did females that were low-ranking for their age. Over time, cortisol was significantly predicted by rates of aggression, particularly female-targeted aggression. The effect of male-female aggression was most pronounced for cycling, swollen females, who are the most frequent targets of sexual coercion. On the other hand, lactating females were strongly affected by group size, such that cortisol levels were elevated when parties contained more females and fewer males. Fruit consumption was an important covariate for lactating females, who experienced higher cortisol when the diet was poor. Our data indicate that both social and energetic factors contribute to stress variation in female chimpanzees, but that energetic stress increases in significance for females facing high reproductive costs. Our study also contributes to growing evidence that direct and indirect competition, while subtle in their expression, can have substantial impacts on female chimpanzees.