The Significance of Female Mimicry in Male Contests

In many animal species, the two sexes are very similar in appearance. One hypothesis for such sexual indistinguishability is that the members of one sex benefit from resembling those of the other sex. Here we focus on the adaptive significance for males of resembling females. We suggest that female mimicry may directly enhance the competitive ability of a male. This is because males that conceal their sexual status may hide important information to their opponent (i.e., that they are males and represent a threat). We use game theory to present this hypothesis. The hypothesis is supported in an experiment with pied flycatchers (Ficedula hypoleuca), a species that previously has been shown to have difficulties in discriminating between female-like males and females. In aviary trials involving contests between black males and female-like males over a nest site, we show that female-like males were the first of the competitors to initiate attacks and were also the winner of the contest in nearly all cases. The same result was found when we used two female-like males of which one was painted black. Apparently, female-like males gain an advantage because they are in control of when to initiate a fight because of the opponent's poor sex recognition ability. We discuss our hypothesis in relation to delayed plumage maturation theory and the evolution of sexual indistinguishability. We suggest that concealment of sex in competitive interactions may be especially relevant to animals in which males provide resources that are attractive to females, such as a territory or a nuptial gift. In such cases, resource-holding males may benefit from signaling their true sex in order to attract females, whereas male intruders may benefit from concealing their sex. Concealment of sex would be easy to achieve in species in which sexual signaling is canalized through nonpermanent signals. However, female mimicry may also be evolutionarily stable in animals in which a high proportion of resource-holding males possess permanent sexual signals but intruders mimic the sexual signals of females-that is, if social status is relatively highly predictable. Examples of the latter situation may be exemplified by some bird species that possess delayed plumage maturation.