The energy cost of singing in wild rock hyrax males: evidence for an index signal

In many species individuals advertise their quality via vocal communication. Although sound production requires energy, it is still unclear to what extent vocalizing increases metabolic rate. A substantial energetic cost of vocalization may suggest that it serves as a handicap. In this study, we evaluated the energetic cost of vocalization in the context of other activities in a free-living mammal. In the rock hyrax, Procavia capensis, males produce loud complex songs that convey multiple types of information about the singer. We used the doubly labelled water method to estimate field metabolic rate (FMR), in males that were collar-fitted with miniaturized recorders. Higher-ranked males sang longer songs, which also featured higher sound energy. However, contrary to our predictions, the duration of singing was negatively correlated with FMR, whereas the duration of other social sounds, typically associated with social conflicts, had a positive effect on FMR. We suggest that higher-ranking males expend overall less energy due to lower intrinsic metabolism, and thus the singing duration effect on FMR is a reflection of social rank. Rock hyrax singing is therefore assessed as requiring a high effort over a short period but not as a lengthy activity, matching the category of index signals. Singing may also serve as a handicap to the signaller, not because of its energetic cost but because it exposes the singer to potential predation. Our study is the first to record the activities and vocalizations of a free-living mammal in such detail. We discuss the costs and benefits of hyrax songs, suggesting that singing males trade safety from predators for increasing their reputation.