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

Abstract

Objectives Grooming has important utilitarian and social functions in primates but little is known about grooming and its functional 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’, i.e. 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.

Ectoparasite defence in humans: relationships to pathogen avoidance and clinical implications

“…Studies of human grooming in societies suffering high ectoparasite loads are therefore sorely needed. Promisingly, one recent study documented grooming behaviours directed at ectoparasite removal, and hygiene behaviours such as washing and bathing, in six small-scale societies in which individuals have minimal or no access to modern medicine and public health [86]. In each of these societies, substantial time was devoted to grooming, equivalent to that expected for a primate of our body size and social group size [86].…”

Sleep in a comparative context: Investigating how human sleep differs from sleep in other primates

“…Comparative methods make it possible to rigorously investigate evolution along a single branch on phylogeny (McPeek, ; Nunn & Zhu, ; Revell, ), including the human lineage (Jaeggi et al, ; Nunn, ; Organ, Nunn, Machanda, & Wrangham, ; Vining & Nunn, ). Some of these methods, for example, integrate interspecific variation and phylogeny to predict human phenotypes, and then test whether observed human phenotypes depart from this expectation (Nunn & Zhu, ).…”

Musicality in human vocal communication: An evolutionary perspective

“…They found that terrestriality predicts grooming time better than group size. While these analyses were disputed by Dunbar and Lehmann [164], newer criticism of the vocal grooming hypothesis was presented by Jaeggi et al [163]; in their analyses, terrestriality was again found to be an important predictor of grooming time, and no evidence of vocal grooming as a less time-consuming, more efficient form of bonding (as stated in the original form of the vocal grooming hypothesis) was found.…”

“…Similarly, Dunbar's hypothesis of vocal grooming [137][138][139]161], which is based on a positive correlation between group size and grooming time, has also been heavily criticised in recent years [162,163]. Grueter et al [162] argue that the association between group size and grooming time is confounded with substrate: they contend that grooming time should be higher in terrestrial (vs arboreal) species, given higher exposure to ectoparasites.…”