The evolution of mutual ornamentation

Abstract

Many conspicuous ornamental traits in animals are expressed in both males and females. Despite this, most research has focused on sexually dimorphic ornamentation. Mutual ornamentation has often been viewed as a result of either a nonadaptive genetic correlation between the sexes or similar selection pressures in both sexes. Here, we review the theoretical underpinning and empirical evidence for these ideas. Few studies have attempted to test empirically whether a genetic correlation between the sexes can constrain the evolution of sexual dimorphism, and the results have been mixed. By contrast, there is good evidence that mutual ornaments can have a signal function in both sexes, especially in terms of mate choice. Other possible signalling functions have received little attention. Social status signalling is especially likely to be important, because competition over nonsexual resources is more balanced between the sexes than sexual competition. There is a need for experimental studies that explicitly test these hypotheses simultaneously in both sexes.

Pairing dynamics and the origin of species

“…The stability of mating pairs is critical in these situations because reproductive success will be affected if one of the two mates decides to switch partners during this time. As for mutual mate choice, it is known to occur in a variety of taxa, including fish, birds, amphibians, insects and rotifers [38]. In addition, while mate choice is often thought of as the familiar situation where females choose males, the significance of male mate choice for the maintenance of female ornamentation is receiving increasing attention and support, even in polygynous species [38 -41].…”

Sexual Selection in Males and Females

“…The relative size of sexual swellings differs between females, so these differences may also signal individual variation in reproductive performance (55). Mutual mate choice by males and females also occurs in a number of monogamous birds and, like intrasexual competition, may lead to the evolution of similar ornaments in males and females (24,49,56).…”

Same trait, different receiver response: unlike females, male American goldfinches do not signal status with bill colour

“…Although the mutual selection and social selection hypotheses have gained much attention in recent years (Amundsen, 2000;CluttonBrock, 2007;, many studies have failed to find evidence that ornamentation functions as a social signal in both males and females (reviewed in Kraaijeveld, Kraaijeveld-Smit, & Komdeur, 2007). As such, there is growing acknowledgment that a complex interplay of selective processes may account for elaborate traits when expressed in both sexes (LeBas, 2006).…”