In humans and other social mammals, more socially connected females often have higher fitness. Yet evidence linking female sociality to offspring survival remains inconsistent, and is limited to a handful of primate species in which females depend on close female kin for social status. Here, we examine the relationship between female social integration and offspring survival in eastern chimpanzees. We find that females that were more socially integrated with other females in the year before giving birth had higher offspring survival to age 1 (the period of highest mortality) and age 5 (the approximate age of weaning). Furthermore, social integration remained a strong predictor of offspring survival among females without close female kin. Our results thus add to a small set of studies linking sociality with offspring survival, here in the dispersing sex. As in humans, more socially connected female chimpanzees have higher offspring survival, despite primarily residing with non-kin.
In group-living mammals, individuals often form differentiated social bonds and differ in their overall sociality.
1 Recent decades have seen accumulating evidence that sociality is related to health, longevity, and reproductive success in humans and other animals.
2,
3,
4,
5,
6 However, among social mammals, evidence for a link between sociality and offspring survival (an important fitness component
7,
8) remains mixed for two reasons. First, evidence for a positive association is taxonomically limited: four studies in cercopithecine monkeys find that more socially integrated females, or those with stronger social bonds, have higher offspring survival (yellow/anubis hybrid baboons, Papio cynocephalus with Papio anubis admixture
9; Chacma baboons, Papio ursinus
10,
11; vervet monkeys, Chlorocebus pygerythrus
12). But studies in other taxa, as well as recent work in cercopithecines, report no such sociality-offspring survival link (yellow/anubis baboons
13; chacma baboons
14,
15; bighorn sheep, Ovis canadensis
16; Kinda baboons, Papio kindae
17). In some cases, more social females actually have lower offspring survival (eastern gray kangaroos, Macropus giganteus
18; yellow-bellied marmots, Marmota flaviventris
19), or have higher or lower offspring survival depending on other group dynamics (white-faced capuchins, Cebus capucinus
20).Second, a recent analysis of data from yellow/anubis hybrid baboons demonstrated evidence of reverse causality in the relationship between female sociality and offspring survival.
13 Females were most social with other females when they had young infants, and with males when cycling, including the cycle immediately following infant death. Controlling for this variation, the authors found no evidence for a positive association between maternal sociality and infant survival.
13 Instead, offspring production and mortality are likely to drive differential female sociality rather than the other way around in this
9,
13 and likely other populations of baboons.
10,
11,
15 Indeed, in primates and other mammalian species, females often show attraction to, and regularly interact with, other females’ offspring,
21,
22,
23 and thus this potential for reverse causation may extend beyond studies of baboons. Of the studies described above, only two
12,
13 use methods that clearly avoid or account for state-dependent social variation as a potential confound, suggesting a critical need for a reevaluation of the relationship between sociality and offspring survival in social mammals.One reason to expect that female mammals improve their offspring’s survival by being more socially connected is that female sociality could facilitate tolerance or agonistic support, leading to better feeding efficiency or a reduction in received aggression and infanticide in the postpartum period.
5,
6,
24 Social bonds, tolerance, and agonistic support might be particularly strong among kin due to inclusive fitness effects.
25 On the other hand, because more social females can also incur costs such as feeding competition and increased aggression from males,
26,
27 it could be that females in most species may simply not derive sufficient benefit from social bonds to depend on them during the particularly sensitive and energetically costly perinatal period.
28,
29 Social organization may also play a role; in species with high fission-fusion dynamics, females have the option to avoid larger foraging parties to reduce the costs of sociality,
30 and could, therefore, potentially benefit from becoming less social when pregnant or nursing.Here, we investigate the relationship between social integration (here defined as females’ overall affiliative tendency
5) and offspring survival using 37 years of long-term data on free-ranging eastern chimpanzees (Pan troglodytes schweinfurthii) in Gombe National Park, Tanzania. Male philopatry and female dispersal are observed in all known chimpanzee communities, although the degree of female dispersal varies; in Gombe around half of females in one community remain in their natal community to reproduce,
31,
32 although only a minority of natal females reside with female kin as adults.
33,
34 This dynamic allows us to investigate the role that kinship may play in mediating a link between sociality and offspring survival, and to determine whether there are direct benefits to sociality among females that lack close kin. Chimpanzees also have high fission-fusion dynamics,
33 and in eastern chimpanzees females are less gregarious than are males, more often traveling alone or with only their dependent offspring.
35,
36,
37 But female chimpanzees nevertheless form differentiated social relationships with other female
34,
38,
39 and sometimes male
40 community members. Female-female bonds are associated with cooperative behaviors like food sharing
41 and joint territorial defense,
42 which may impact fitness; the potential adaptive benefits females gain from bonds with males are less clear, but in other primate species these male social partners may provide offspring protection (e.g., Nguyen et al.
24). Females that co-reside with female kin enjoy some fitness-related benefits such as access to higher-quality core areas,
33,
34 higher dominance rank,
43 and earlier reproduction.
44 Yet many female bonds are formed with non-kin,
34,
38 and females without close kin in their community can still achieve high rank.
43 Therefore, this system allows us to examine the influence of dispersal pattern, kin availability, and fission-fusion dynamics on the relationship between sociality and offspring survival.
This content is taken from Duke UniversityList of Referenes
- Joseph T. Feldblum, Kara K. Walker, Margaret A. Stanton, Elizabeth V. Lonsdorf, Deus C. Mjungu, Carson M. Murray, Anne E. Pusey. Socially integrated female chimpanzees have lower offspring mortality. iScience, 2025; 112863 DOI: 10.1016/j.isci.2025.112863
