Intra-sexual competition modulates calling behavior and its association with secondary sexual traits

Abstract

The expression of elaborate sexual displays is associated with individual quality ensuring reliable information about the bearers. However, the associated cost of expressing enhanced sexual traits is expected to change according to environmental circumstances. Specifically, the cost of maintaining or producing a signal is predicted to increase when environmental conditions are unfavorable, which may lead to a reduction in signal expression as shown in several species. Here, we compared the calling behavior of male red grouse Lagopus lagopus scoticus living in an area of experimentally increased intra-sexual competition to that of males living in a control area. Levels of intra-sexual competition were experimentally manipulated by testosterone implants in a subset of captured males. In addition, we compared the association between two sexual traits, calling behavior and comb size, of males living in these two areas. Although call frequency was not affected by different levels of intra-sexual competition, males from the control area performed shorter calls than individuals from the treatment area. Additionally, a positive association between comb size and call duration was found only for males in the area of lower aggressiveness. We suggest that environmental conditions influence the expression of multiple plastic sexual traits, depending on the costs and the information conveyed about different individual qualities. Although sexual signals are considered reliable indicators of individual quality, environmental heterogeneity may modulate their expression and reliability. We experimentally manipulated levels of intra-sexual competition in a wild population of red grouse by increasing testosterone levels using implants in a subset of males. We observed that the social context shapes the expression of sexual traits. In less competitive conditions, males performed shorter calls and call length was positively related to the size of the supra-orbital comb, a relevant secondary sexual trait. Under higher intra-sexual competition, this relationship was decoupled and investment was directed at increasing call length.