Chimpanzee (Pan troglodytes schweinfurthii) Group Sleep and Pathogen-Vector Avoidance: Experimental Support for the Encounter-Dilution Effect


Sleep is essential for survival, yet it represents a time of extreme vulnerability, including through exposure to parasites and pathogens transmitted by biting insects. To reduce the risks of exposure to vector-borne disease, the encounter-dilution hypothesis proposes that the formation of groups at sleep sites is influenced by a “selfish herd” behavior, where individuals dilute risk by sleeping with other group members. To investigate this hypothesis in the context of chimpanzee (Pan troglodytes schweinfurthii) sleep site selection, we employed four light traps that we also baited with nontoxic chemical attractants to capture insects throughout the night. Across 74 nights with 294 traps set, we collected 66,545 individual insects. Consistent with the encounter-dilution hypothesis, we found that insect exposure, inferred by absolute numbers of insects caught in nighttime traps, was strongly influenced by the grouping of traps. Specifically, single traps caught more insects—including vector transmitting female mosquitoes—than grouped traps, and the number of insects caught increased with increasing distance between grouped traps. Moreover, ground sleep sites caught fewer insects than arboreal sleep sites. In addition, traps associated with Cynometra alexandri trees resulted in significantly lower catch rates than Pseudospondias microcarpa–associated traps. Our results suggest wild chimpanzees use group sleep as a strategy to avoid biting insects that serve as hosts for vector-borne diseases.

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  • 10 January 2020

    The original version of this article unfortunately contained a mistake in the authorgroup section.


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We are grateful to the Government of Uganda, the Uganda Wildlife Authority, and the National Research Council. We would like to thank the two anonymous reviewers and the editor for their commentary and suggestions that significantly improved the quality of the original manuscript. We are thankful to the staff at the Semliki Chimpanzee Project, particularly Moses Comeboy, and Duke University for funding.

Author information

DRS, KDH, and CLN conceived and designed the experiments. DRS, LAL, KG, SW, BL performed the experiments. DRS, BJW analyzed the data. DRS wrote the manuscript; other authors provided editorial advice.

Correspondence to David R. Samson.

Additional information

The original version of this article was revised: Author Samantha Wylie’s family name was incorrectly presented as “Wiley”.

Handling Editor: Joanna M. Setchell

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Samson, D.R., Louden, L.A., Gerstner, K. et al. Chimpanzee (Pan troglodytes schweinfurthii) Group Sleep and Pathogen-Vector Avoidance: Experimental Support for the Encounter-Dilution Effect. Int J Primatol 40, 647–659 (2019) doi:10.1007/s10764-019-00111-z

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  • Chimpanzee
  • Disease vector
  • Encounter-dilution
  • Sleep
  • Sociality