Abstract
The long-term effects of early life adversities on social capacities have been documented in humans and wild-caught former laboratory chimpanzees (Pan troglodytes). However, former pet and entertainment chimpanzees have received little attention to date. This study aimed to investigate the long-term effects of early life experience on 18 former pet and entertainment chimpanzees, based on social grooming data collected at a primate rescue centre over a 12-year period. Moreover, we also focused on the possible short-term effects that alterations to group composition might have on grooming patterns. For this purpose, we compared stable and unstable periods (i.e. where alterations to group composition occurred). We used two individual social network measures to analyse the grooming activity and the distribution of grooming among group mates for each individual. We could show that wild-caught chimpanzees were significantly more selective regarding their grooming partners and spent less time grooming when compared to their captive born companions. We also found that individuals who were predominantly housed without conspecifics during infancy spent less time grooming compared to those who were predominantly housed with conspecifics during infancy. Furthermore, we found that alterations to the group composition had short-term effects on the distribution of social grooming from a more equal distribution during periods with a stable group composition towards a more unequal and selective distribution during unstable periods. Thus, we conclude that the social grooming networks of former pet and entertainment chimpanzees are shaped not only by long-term effects such as early life experience, but also by short-term effects such as alterations to group composition. Remarkably, we found not only captive born chimpanzees but also wild-caught individuals to adjust their grooming to socially challenging situations by modifying their grooming distribution in a similar way.
Highlights
Adverse experiences in early infancy affect the behaviour as well as the physical and mental health of human [1,2,3] and non-human primates [4, 5] in the long term [6,7,8,9]
We calculated two different social network measures for each individual per time period and group composition, and ran linear mixed models (LMMs) to investigate the effects of anstable group composition, arrival age, sex, origin and predominant housing condition during infancy on social grooming networks
We found that chimpanzees who were predominantly housed with conspecifics during infancy (N = 62 data points) groomed their group mates significantly more than chimpanzees who were predominantly housed without conspecifics during their first five years of life (N = 57 data points; Fig 1; analysis of variance (ANOVA) post hoc test: F1,15 = 8.03, P
Summary
Adverse experiences in early infancy affect the behaviour as well as the physical and mental health of human [1,2,3] and non-human primates [4, 5] in the long term [6,7,8,9] This applies to individual as well as social behaviour [10]. Between the 1950s and the 1980s, thousands of infant chimpanzees were taken from the wild [21, 22] These orphans experienced the separation from their mother and most likely witnessed the killing of their mother and other group members [23, 24]. Several studies suggest that early life stress induces long-term morphologic changes in primate brains expressing a delayed effect once the vulnerable brain system reaches maturation [34, 35]
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