Abstract
In species in which males signal competitive ability through secondary sexual traits, males with different levels of trait expression may adopt different reproductive tactics to maximize their reproductive success. In fission–fusion social systems, the most dominant males often roam widely in search of females in oestrus, and thus exhibit different patterns of sociability from subordinate males that utilize alternative reproductive tactics. Giraffes, Giraffa camelopardalis, are rare among mammals in that they are sexually dimorphic in colour, and colour is hypothesized to function as a signal of males’ social status by displaying their competitive ability. Here we analysed the coat colour and sociability of 66 wild male giraffes over 12 years at Etosha National Park in Namibia to test two premises underlying this hypothesis. First, we found that males did not all darken at the same rate or to the same degree, and colour variation increased with age. This suggests that colour is not solely an age-based trait but could be a secondary sexual trait. We then showed a distinct difference in the sociability of both young and pale males compared to darker males. Both younger and paler old males tended to be more gregarious while darker males were more solitary. This is consistent with a system where darker, more dominant males roam looking for females in oestrus. Younger or subordinate males may delay roaming or use an alternative tactic, such as remaining in groups with females to gain copulations when a more dominant male is not present. Our results are consistent with the hypothesis that male giraffe coat colour functions as a signal of social status through competitive ability, but deeper study into movement patterns and the costs and benefits associated with darker colours is required.