Between-Group Competition Impacts Reproductive Success in Wild Chimpanzees

Abstract

Highlights d High neighbor pressure during pregnancy affects chimpanzees' offspring survival d Female chimpanzees reproduce slower when neighbor pressure is high d Reproductive rates are shorter when many males are present in a group d Within-group competition makes female reproduction slower

Long-term repeatability in social behaviours suggests stable social phenotypes in wild chimpanzees

“…Much research to date on social phenotypes has focused solely on association patterns (19,25), or on single forms of dyadic interaction (20, 34), with a minority of studies examining consistency in multiple forms of social behaviours (89,98,99). Our study shows that consistent individual differences in social behaviour extends to patterns of aggression and affiliation, both of which should influence fitness more than association alone (43,44,85–88,100). Indeed, both aggression and grooming involve direct, typically physical interactions with other group members, meaning that variation in these phenotypes will be important for factors such as rank acquisition (51,64,68,101,102), disease transmission (103–105) and group cohesion (45,106,107).…”

“…The three behaviours of aggression, grooming (affiliation) and association represent three important components of sociality for chimpanzees, facilitating social goals such as dominance rank attainment (82,83) and the formation of social bonds (42,84), both of which influence fitness (43,44,85–88). Our study expands on former research in this field in terms of its scale, both temporally and in the range of behaviours examined, in a wild population, demonstrating that social phenotypes are a phenomenon in long-lived species occupying complex social environments.…”

Interactions with conspecific outsiders as drivers of cognitive evolution

“…Those metrics could be used as independent factors in analyses, potentially comparing between different threat types, or could be combined to form an index of overall threat. Recent work on between-group interactions in chimpanzees, for instance, has developed a single 'neighbour pressure index', integrating estimates of between-group interaction frequency, territorial position (with intrusions to the territory core being viewed as more threatening than those on the periphery) and disputed resource value (as indicated by usage by the resident group) 18 . This index was then coupled with a group's competitive ability and within-group competition to consider influences on reproductive output 18 .…”

“…We argue that considerable cognitive variation may have remained unexplained because the SIH, and empirical tests of it, overlook a second major axis of social variation in animals: interactions with conspecific outsiders. From solitary species to those living in complex groups, in all animal taxa from invertebrates to primates, interactions with conspecific outsiders are commonplace [18][19][20] and their profound effects on sociality have become increasingly recognised [21][22][23][24] . Rival individuals, pairs or groups may compete for valuable resources such as mating opportunities, breeding positions, food or territories [25][26][27] .…”

Postweaning maternal care increases male chimpanzee reproductive success

“…This could be particularly pertinent in highly territorial, fission-fusion social systems, such as chimpanzees. Successful adult male chimpanzees need to not only navigate the social complexities of securing coalition partners to outcompete in-group rivals ( 19 , 44 ) but also maintain territorial defense by cooperating with the same individuals with whom they compete intensely over mating opportunities ( 23 , 45 , 46 ). Research in humans and rodents show that the extent of maternal nurturing received may affect offspring’s later adult social skills ( 47 , 48 ).…”