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How to measure chimpanzee party size? A methodological comparison


Chimpanzees (Pan troglodytes) live in fission–fusion societies where community members form temporary parties that frequently change in size and composition. Chimpanzee party size and composition have been widely studied to identify proximate causes of grouping patterns, and party size estimates are used to assess population sizes and densities. Numerous socio-ecological factors influence chimpanzee party size, but findings differ across studies. Various methods to measure party size exist, including direct observations, motion-triggered camera (MTC) observations, and nest counts. However, comparative analyses of these methods are lacking. Here, we assess relative differences in four commonly used party size methods and we examine socio-ecological factors influencing party size of unhabituated chimpanzees (P. t. verus) at Seringbara, Nimba, Guinea. We also assess which method(s) best reflect the influence of socio-ecological factors on party size. Using data collected over 69 months, we show that night nest counts resulted in relatively larger party size estimates than the other methods, and day nest counts resulted in relatively smaller party size estimates. Direct and MTC observations did not differ in relative estimates of party size and composition. Both fruit abundance and presence of estrous females positively influenced party size, but this effect was only evident when measuring party size with MTCs. Methods thus differ in relative party size estimates and their ability to assess the impact of socio-ecological factors. We conclude that MTC observations best represent party size and the effect of socio-ecological factors at Nimba. MTCs show promising potential for studying grouping patterns in unhabituated chimpanzees.

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We thank the Direction Generale de la Recherce Scientifque et l’Innovation Technologique (DGERSIT) and the Institut de Recherche Environnementale de Bossou (IREB) for research permission. For their help in the field, we thank Seringbara guides K. Doré, F. Doré, G. Zogbila, F. Zogbila, P. Cherif, H. Camara, N. Doré, D. Zogbila, Y. Zogbila, C. Samy and N. Gbouomy, and research assistants M. Reilly Plocek, M. Fitzgerald, P. Le Sommer, W. Edwards, J. Caraway, D. Montanari, N. James, I. Vélez del Burgo Guinea and D. Hassler. For statistical advice, we thank H. de Vries. We thank two anonymous reviewers for helpful and constructive comments on the manuscript. Research was supported by Grants from the Lucie Burgers Foundation for Comparative Behaviour Research (the Netherlands), Homerton College, Newnham College (Cambridge) and Claraz Stiftung to KK, and by MEXT-JSPS Grants #16H06283; LGP-U04, the Japan Society for the Promotion of Science (JSPS) Core-to-Core Program CCSN to TM.

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Correspondence to Kelly L. van Leeuwen.

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This research was non-invasive, complied with the laws of Guinea, and was approved by the Direction General de la Recherche Scientifique et l’Innovation Technologique (DGERSIT). Moreover, this research adhered to the guidelines as set down by the Division of Biological Anthropology, University of Cambridge, the host institute of the lead researcher (K. Koops) during the study.

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van Leeuwen, K.L., Matsuzawa, T., Sterck, E.H.M. et al. How to measure chimpanzee party size? A methodological comparison. Primates (2020).

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  • Chimpanzees
  • Party size
  • Fruit availability
  • Estrous females
  • Nest counts
  • Camera traps