Explore the vast range of titles available.

Dominance hierarchies are widespread in animal social groups and often have measureable effects on individual health and reproductive success. Dominance ranks are not static individual attributes, however, but instead are influenced by two independent processes: 1) changes in hierarchy membership and 2) successful challenges of higher-ranking individuals. Understanding which of these processes dominates the dynamics of rank trajectories can provide insights into fitness benefits of within-sex competition. This question has yet to be examined systematically in a wide range of taxa due to the scarcity of long-term data and a lack of appropriate methodologies for distinguishing between alternative causes of rank changes over time. Here, we expand on recent work and develop a new likelihood-based Elo rating method that facilitates the systematic assessment of rank dynamics in animal social groups, even when interaction data are sparse. We apply this method to characterize long-term rank trajectories in wild eastern chimpanzees ( Pan troglodytes schweinfurthii ) and find remarkable sex differences in rank dynamics, indicating that females queue for social status while males actively challenge each other to rise in rank. Further, our results suggest that natal females obtain a head start in the rank queue if they avoid dispersal, with potential fitness benefits.

The role of biological and social influences on sex differences in human child development is a persistent topic of discussion and debate. Given their many similarities to humans, chimpanzees are an important study species for understanding the biological and evolutionary roots of sex differences in human development. In this study, we present the most detailed analyses of wild chimpanzee infant development to date, encompassing data from 40 infants from the long-term study of chimpanzees at Gombe National Park, Tanzania. Our goal was to characterize age-related changes, from birth to five years of age, in the percent of observation time spent performing behaviors that represent important benchmarks in nutritional, motor, and social development, and to determine whether and in which behaviors sex differences occur. Sex differences were found for indicators of social behavior, motor development and spatial independence with males being more physically precocious and peaking in play earlier than females. These results demonstrate early sex differentiation that may reflect adult reproductive strategies. Our findings also resemble those found in humans, which suggests that biologically-based sex differences may have been present in the common ancestor and operated independently from the influences of modern sex-biased parental behavior and gender socialization.

Significance Early social experience is critical to developing appropriate adult behavior. Mothers are particularly important social partners but may also facilitate or restrict access to others. Studies investigating human parental facilitation are limited and confounded by culture. Comparative studies therefore provide invaluable insight into how parents provide social opportunities for offspring. We investigated maternal subgrouping patterns by infant sex in one of our closest relatives, chimpanzees, and found that mothers with sons were more gregarious. Infants themselves may influence patterns later in infancy, but differences were apparent early in life, when mothers and infants are in almost constant contact. Furthermore, mothers with sons associated more with adult males, potential behavioral models, early in life. These differences foreshadow sex-typical adult social behavior. In many mammals, early social experience is critical to developing species-appropriate adult behaviors. Although mother–infant interactions play an undeniably significant role in social development, other individuals in the social milieu may also influence infant outcomes. Additionally, the social skills necessary for adult success may differ between the sexes. In chimpanzees ( Pan troglodytes ), adult males are more gregarious than females and rely on a suite of competitive and cooperative relationships to obtain access to females. In fission–fusion species, including humans and chimpanzees, subgroup composition is labile and individuals can vary the number of individuals with whom they associate. Thus, mothers in these species have a variety of social options. In this study, we investigated whether wild chimpanzee maternal subgrouping patterns differed based on infant sex. Our results show that mothers of sons were more gregarious than mothers of daughters; differences were especially pronounced during the first 6 mo of life, when infant behavior is unlikely to influence maternal subgrouping. Furthermore, mothers with sons spent significantly more time in parties containing males during the first 6 mo. These early differences foreshadow the well-documented sex differences in adult social behavior, and maternal gregariousness may provide sons with important observational learning experiences and social exposure early in life. The presence of these patterns in chimpanzees raises questions concerning the evolutionary history of differential social exposure and its role in shaping sex-typical behavior in humans.

The mother-offspring relationship is paramount in most mammals and infant survival often depends on maternal investment. In species with prolonged periods of development or co-residence, mothers may continue to influence their offspring’s outcomes beyond nutritional independence with benefits biased towards the philopatric sex. Chimpanzees (Pan troglodytes) are long-lived mammals with a protracted period of immaturity during which offspring continue to travel with their mothers. In contrast to most mammals, chimpanzees are also typically male philopatric. Here, we use over 50 years of demographic data from two communities in Gombe National Park, Tanzania, to examine the survival and longevity of both male and female chimpanzees that experienced maternal loss during three different age categories. Males who were orphaned between the ages of 0–4.99 years, 5–9.99 years, and 10–14.99 years all faced significantly lower survival than non-orphans and died earlier than expected. Females faced similarly reduced survival probabilities when orphaned between 0–4.99 and 5–9.99 years of age; however, females who experienced maternal loss between 10 and 14.99 years of age were no more likely to die than non-orphans. Females orphaned in this later age class also lived significantly longer beyond maternal loss than their male counterparts. As observed in other mammals, philopatric male chimpanzees may continue to benefit from their mother’s ecological knowledge, whereas maternal influence on female offspring likely fades as they prepare to emigrate. These results emphasize how maternal influence on offspring outcomes can extend well beyond weaning, particularly for the philopatric sex.

Highly differentiated and affiliative social relationships are observed in a variety of mammals, including primates, cetaceans, and social carnivores. Although there has been a transformation in our understanding of the form and function of social bonds in the past two decades, the role of early life social experiences in the development of bonds remains less clear. Few studies have examined whether social relationships during infancy and juvenility (aside from those between mothers and offspring) persist into adulthood. In this study, we used longitudinal data on wild chimpanzees (Pan troglodytes schweinfurthii) at Gombe National Park, Tanzania, to investigate the effects of party-level association during infancy and juvenility (hereafter immaturity) on affiliative relationships among adult males (the philopatric sex). In total, we examined behavioral data from focal follows between 1970 and 2015 (N = 23 adult males). We used maternal associations as proxies for the associations of immature males, and we measured adult male social relationships using party-level associations and grooming activity. We found that immature males that associated with individual adult males at higher rates had stronger relationships with those same adult males later in life. By contrast, rates of association between pairs of immature males did not predict the strength of their dyadic relationships as adults. Overall, these findings emphasize the importance of early socialization in male chimpanzee social development. These results also reinforce studies in mountain gorillas (Gorilla beringei) and bottlenose dolphins (Tursiops sp.), suggesting that the persistence of social relationships that do not involve the mother may be more likely to evolve in long-lived species where young adult males face challenges entering an adult hierarchy composed of stronger and/or more socially experienced competitors.

Establishing the order of a dominance hierarchy among female chimpanzees (Pan troglodytes) is complicated by the fact that they often forage solitarily, and aggressive interactions between them occur infrequently. Authors of previous studies have typically ranked females via the direction of submissive pant-grunts and the outcome of agonistic interactions. Given that higher rank correlates with higher reproductive success in female chimpanzees, assessing rank is important but may be limited by sparsely populated dominance matrices. I tested the hypothesis that rank predicts the direction of female approaches. There is a significant relationship among Gombe females between the frequency with which a female was approached and her dominance rank. Dominant females approached other females less often than they were approached. Though approached frequencies failed to meet the criteria for formal rank indicators, they may be useful as real indicators of subordination. Because approach interactions occur far more frequently than pant-grunts, they may be useful in assigning categorical rank when traditional dominance metrics are limited.[PUBLICATION ABSTRACT]

Chimpanzees exhibit cultural variation, yet examples of successful cultural transmission between wild communities are lacking. Here we provide the first account of tool-assisted predation (“ant fishing”) onCamponotusants by the Kasekela and Mitumba communities of Gombe National Park. We then consider three hypotheses for the appearance and spread of this behavior in Kasekela: (1) changes in prey availability or other environmental factors, (2) innovation, and (3) introduction. Ant fishing was recognized as habitual in the Mitumba community by 1992, soon after their habituation began. Apart from one session in 1978,Camponotuspredation (typically with tools) was documented in the Kasekela community beginning only in 1994, despite decades of prior observation. By February 2010, ant fishing was customary in Kasekela and with one exception was practicedexclusivelyby chimpanzees born after 1981 and immigrant females. We hypothesize that changes in insect prey availability over time and/or the characteristics of one popular ant-fishing site may have influenced the establishment of ant fishing. Though innovation cannot be completely ruled out, the circumstantial evidence suggests that a Mitumba immigrant introduced ant fishing to Kasekela. We submit that this report represents the first documented case of successful transmission of a novel cultural behavior between wild chimpanzee communities.

Infectious disease and other health hazards have been hypothesized to pose serious threats to the persistence of wild ape populations. Respiratory disease outbreaks have been shown to be of particular concern for several wild chimpanzee study sites, leading managers, and researchers to hypothesize that diseases originating from and/or spread by humans pose a substantial risk to the long-term survival of chimpanzee populations. The total chimpanzee population in Gombe National Park, Tanzania, has declined from 120–150 in the 1960s to about 100 by the end of 2007, with death associated with observable signs of disease as the leading cause of mortality. We used a historical data set collected from 1979 to 1987 to investigate the baseline rates of respiratory illness in chimpanzees at Gombe National Park, Tanzania, and to analyze the impact of human-related factors (e.g., banana feeding, visits to staff quarters) and non-human-related factors (e.g., sociality, season) on chimpanzee respiratory illness rates. We found that season and banana feeding were the most significant predictors of respiratory health clinical signs during this time period. We discuss these results in the context of management options for the reduction of disease risk and the importance of long-term observational data for conservation.

Researchers have reported a total of 31 infanticides in 4 different chimpanzee (Pan troglodytes) populations. Though infanticide is infrequent, low reproductive rates of females likely make it a strong selective pressure in the species. We report a new incident of intragroup infanticide in Gombe National Park, Tanzania, in which a community male attacked a 3.5-yr-old male. We then consider the infanticide in terms of adaptive and nonadaptive explanations for infanticide including the social pathology, by-product of male aggression, nutritive benefits, resource competition, and sexual selection hypotheses. The incident reported here is not well explained by any of them. While the infanticide is puzzling in terms of ultimate explanations for infanticide, it provides a good context in which to consider proximate mechanisms for offspring recognition. The incident provides some evidence that males may use their mating history with the mother to assess paternity likelihood.[PUBLICATION ABSTRACT]

Parental investment theory predicts that maternal resources are finite and allocated among offspring based on factors including maternal age and condition, and offspring sex and parity. Among humans, firstborn children are often considered to have an advantage and receive greater investment than their younger siblings. However, conflicting evidence for this “firstborn advantage” between modern and hunter-gatherer societies raises questions about the evolutionary history of differential parental investment and birth order. In contrast to humans, most nonhuman primate firstborns belong to young, inexperienced mothers and exhibit higher mortality than laterborns. In this study, we investigated differences in maternal investment and offspring outcomes based on birth order (firstborn vs. laterborn) among wild chimpanzees (Pan troglodyte schweinfurthii). During the critical first year of life, primiparous mothers nursed, groomed, and played with their infants more than did multiparous mothers. Furthermore, this pattern of increased investment in firstborns appeared to be compensatory, as probability of survival did not differ by birth order. Our study did not find evidence for a firstborn advantage as observed in modern humans but does suggest that unlike many other primates, differences in maternal behavior help afford chimpanzee firstborns an equal chance of survival.