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Sex differences in maternal sibling-infant interactions in wild chimpanzees

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Abstract

Apart from the crucial bonds between mothers and offspring, siblings are an important first social partner in most primate species. Socioecological theory predicts that sibling-infant interactions may differ depending on whether the older immature sibling is male or female. Here, we used 24 years of long-term data from wild chimpanzees in Gombe National Park, Tanzania, to characterize maternal sibling-infant relationships and investigate sex differences therein. Since young female primates typically exhibit higher rates of interest in infants, we hypothesized that older sisters would be more likely than older brothers to groom, play with, and carry infant siblings. Alternatively, due to male-biased philopatry in this species, older brothers may be equally or more likely to interact with these potential future allies. We also examined whether sibling-infant interactions differed according to sibling age and sex of the infant. For both play and grooming, we found a significant interaction between sibling age and sibling sex. Older brothers had increased likelihood of playing and grooming with infant siblings as they themselves aged. Additionally, male-male siblings played significantly more than any other dyad type. Older sisters had decreased likelihood of playing with age and maintained relatively consistent likelihood of grooming; they also spent more time with their infant siblings at later ages. Instances of carrying young infants were exceedingly rare and did not differ by sibling sex. Thus, the sex combination of sibling-infant dyads may have substantial consequences for the social development of both individuals, which we argue is an important focus of future research.

Significance statement

The role of siblings in nonhuman primate social development is relatively understudied, and socioecological theory predicts that male and female siblings may interact differently with infants. We conducted the most detailed investigation to date of sibling relationships in wild chimpanzees and found distinct differences in how immature brothers and sisters interact with infant siblings. Older brothers have an increased likelihood of playing and grooming with infant siblings as they themselves age. We also found that male-male siblings played more than all other dyad types, which may have consequences for fostering alliances in adulthood. Older sisters show decreased likelihood of playing with age and maintain relatively consistent likelihood of grooming; they also spend more time with their infant siblings at later ages. We argue that a renewed focus on understanding the importance of siblings on primate social development is warranted.

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Acknowledgments

We are grateful to Federica Amici and Anja Widdig for their invitation to the topical collection “An evolutionary perspective on the development of primate sociality.” The authors thank the Jane Goodall Institute and Tanzania National Parks (TANAPA) for initiating and continuing the over 57-year research tradition at Gombe. Special thanks are due to Jane Goodall and Anne Pusey for their initiation and preservation of the family follow dataset, respectively. We are extremely grateful to the Gombe Stream Research Centre staff, the Government of Tanzania, Tanzania National Parks, Tanzania Commission for Science and Technology, and Tanzania Wildlife Research Institute for continued support of this research. Thanks are also due to Karen Anderson, Victoria Fiorentino, and Sophia Reji for data management, and to the numerous assistants who participated in long-term data entry. We also thank Anne Pusey, Sean Lee, Kaitlin Wellens, and three anonymous reviewers for helpful comments on earlier versions of this manuscript. This work was supported by grants from the Leo S. Guthman Foundation, the National Geographic Society, and the National Institutes of Health (5R00HD057992).

Funding

This study was supported by grants from the Leo S. Guthman Foundation, the National Geographic Society, and the National Institutes of Health (5R00HD057992); the long-term data collection is supported by the Jane Goodall Institute.

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Authors and Affiliations

  1. Department of Psychology, Franklin and Marshall College, 415 Harrisburg Avenue, Lancaster, PA, 17604, USA

    E. V. Lonsdorf & M. A. Stanton

  2. Center for the Advanced Study of Human Paleobiology, The George Washington University, 800 22nd St NW, Washington, DC, 20052, USA

    M. A. Stanton & C. M. Murray

Authors
  1. E. V. Lonsdorf
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  2. M. A. Stanton
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  3. C. M. Murray
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Correspondence to E. V. Lonsdorf.

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The authors declare that they have no conflicts of interest.

Ethical approval

This study was completely observational in nature and the chimpanzees were well-habituated to human observation. Permission to conduct behavioral data collection at Gombe National Park was granted and approved by the relevant governing bodies in Tanzania: Tanzania National Parks, the Tanzanian Wildlife Research Institute, and the Tanzanian Commission for Science and Technology.

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Communicated by A. Widdig

This article is a contribution to the Topical Collection An evolutionary perspective on the development of primate sociality - Guest Editors: Federica Amici and Anja Widdig

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Lonsdorf, E.V., Stanton, M.A. & Murray, C.M. Sex differences in maternal sibling-infant interactions in wild chimpanzees. Behav Ecol Sociobiol 72, 117 (2018). https://doi.org/10.1007/s00265-018-2531-5

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  • DOI: https://doi.org/10.1007/s00265-018-2531-5

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