Human grooming in comparative perspective: People in six small-scale societies groom less but socialize just as much as expected for a typical primate

• Authors: Adrian V. Jaeggi, Emory University; Karen L. Kramer, University of Utah; Raymond Hames, University of Nebraska Lincoln; Evan J. Kiely, Emory University; Cristina Gomes, University of Miami; Hillard Kaplan, University of New Mexico; Michael Gurven, University of California Santa Barbara
• Journal Title: American Journal of Physical Anthropology
• Volume: Volume 162, Number 4
• Publisher: Wiley: 12 months | 2017-04-01, Pages 810-816
• Type of Work: Article | Post-print: After Peer Review
• Publisher DOI: http://doi.org/10.1002/ajpa.23164
• Subjects: Anthropology, Cultural
• Keywords: comparative methods; evolution of language; grooming; hunter-gatherers; primates; Animals; Anthropology, Physical; Bayes Theorem; Biological Evolution; Grooming; Humans; Indians, South American; Language; Models, Statistical; Phylogeny; Primates; Social Behavior; South America

Abstract

Objectives: Grooming has important utilitarian and social functions in primates but little is known about grooming and its f unctional analogues in traditional human societies. We compare human grooming to typical primate patterns to test its hygienic and social functions. Materials and Methods: Bayesian phylogenetic analyses were used to derive expected human grooming time given the potential associations between grooming, group size, body size, terrestriality, and several climatic variables across 69 primate species. This was compared against observed times dedicated to grooming, other hygienic behavior, and conversation among the Maya, Pumé, Sanöma, Tsimane', Yanomamö, and Ye'kwana (mean number of behavioral scans = 23,514). Results: Expected grooming time for humans was 4% (95% Credible Interval = 0.07%–14%), similar to values observed in primates, based largely on terrestriality and phylogenetic signal (mean λ = 0.56). No other covariates strongly associated with grooming across primates. Observed grooming time across societies was 0.8%, lower than 89% of the expected values. However, the observed times dedicated to any hygienic behavior (3.0%) or “vocal grooming,” that is conversation (7.3%), fell within the expected range. Conclusions: We found (i) that human grooming may be a (recent) phylogenetic outlier when defined narrowly as parasite removal but not defined broadly as personal hygiene, (ii) there was no support for thermoregulatory functions of grooming, and (iii) no support for the “vocal grooming” hypothesis of language having evolved as a less time-consuming means of bonding. Thus, human grooming reflects decreased hygienic needs, but similar social needs compared to primate grooming.