Urine Washing and Sniffing in Wild White-Faced Capuchins (Cebus capucinus): Testing Functional Hypotheses

Abstract

Urine washing (UW) is taxonomically widespread among strepsirhines and platyrrhines, yet its functional significance is still unclear. We used 2274 h of focal follows of 35 adult and subadult wild white-faced capuchins (Cebus capucinus) to test 1) the intergroup signaling, intragroup social signaling, and thermoregulatory hypotheses for UW and 2) the hypothesis that individuals sniff each other’s urine and other traces to gather socially significant information. Males engaged in significantly more UW than females. All 5 $α$-males engaged in more UW than subordinate males did, including 4 $α$-males that increased their UW rate above that of their male groupmates after their rise to $α$ rank. Males engaged in significantly less UW while in view of other males than at other times. Male-male sniffing rates do not correlate with either aggression rate or dominance rank distance. Urine washing rates did not increase while subjects were in parts of their home range where more intergroup encounters occurred. Urine washing rates were highest early in the morning and late in the afternoon, presumably when temperatures were coolest. The data do not support either the thermoregulatory or social signaling hypothesis. We suggest that experiments with captive capuchins are necessary to resolve the issue of the function of urine washing in the taxon.