Are simakobu (Simias concolor) loud calls energetically costly signals?

Male vocal displays play an important role in sexual selection in many species. If there are costs or constraints, calls may convey honest information about the caller. We studied the loud calls of male simakobu (Simias concolor), a sexually dimorphic primate that resides in one-male groups, on Siberut Island, Indonesia.

We combined behavioral and ecological data with acoustic analysis to determine the influence of energy costs and status on calling effort. Based on all-day follows (n = 107) and loud-call recordings (n = 186), we determined how calling rates and durations varied in relation to short-term (time of day, temperature, travel distance) and long-term (fruit availability, physical condition) measures of energy availability and expenditure.

We found that males avoided calling at the hottest times of the day, and produced shorter calls at higher temperatures, supporting the hypothesis that calls are influenced by thermoregulation costs. A peak of longer duration calls just before sunrise, however, could not be explained by energy constraints, but we hypothesize that this pattern is related to the fact that dawn calls occur in choruses, motivating males to call longer than their neighbors. Contrary to predictions, calling rates increased with decreasing fruit availability and did not appear to be energetically costly.

We hypothesize that calling rates instead reflect strategies involving feeding and/or mate competition, signaling the motivation, rather than fighting ability, of the caller. Overall, results only weakly support the hypothesis that the timing and duration-but not rate-of simakobu loud calls are influenced by energy costs. Am J Phys Anthropol 161:44-52, 2016. © 2016 Wiley Periodicals, Inc.