Primate colour predicts social status and aggressive outcome

Abstract Colourful signals in insects, fish, amphibians, lizards and birds have attracted considerable attention among evolutionary biologists. While primates display the most conspicuous secondary sexual coloration in mammals, little is known about its function. In several types of organisms, variation in colour predicts differences in social status. For this study, I experimentally investigated the relationship between scrotal colour and social status in adult vervet monkeys Cercopithecus aethiops sabaeus , by analysing the social interactions between pairs of unfamiliar males matched for size, but differing in scrotal colour. Differences in scrotal colour predicted eventual social status, as males with a dark scrotum dominated those with a pale scrotum. Experimental modification of scrotal colour failed to confirm that scrotal colour alone signalled social status, but supported the finding that males with scrota of similar colour were more antagonistic towards each other than males differing in colour. These results represent the first experimental evidence for a relationship between colour and social status in primates.