Territorial males do not discriminate between local and novel plumage phenotypes in a tropical songbird species complex: implications for the role of social selection in trait evolution

Whether novel traits involved in animal communication are favored or not by social selection depends on how receivers respond to them. If alternative traits arise at different locations or the same traits are perceived differently, then populations may diverge even when occupying similar environments. Aggressiveness towards bearers of novel versus familiar traits is informative about how male-male competition may drive phenotypic evolution, yet few studies have tested the function of signaling traits in social selection across populations. We assessed the role of a black pectoral band during territorial contests in two allopatric species of Neotropical passerine birds in the genus Arremon either having or lacking this plumage trait. Field experiments using taxidermic mounts and playback of conspecific songs revealed that males of both species were equally aggressive towards phenotypes bearing or lacking a pectoral band. Although evaluating physiological and social costs of bearing alternative traits and assessing female preferences is required to further examine the roles of social and sexual mechanisms in the evolution of plumage and geographic variation, our results indicate that social selection via male-male interactions is an unlikely driver of phenotypic divergence among populations of Arremon. Social selection via intrasexual competition for resources may drive phenotypic divergence and population differentation in animals, but few studies have tested predictions of this hypothesis by examining how alternative traits varying geographically are perceived in contexts of male-male competition. We assessed the behavioral response of territorial males in two species of Neotropical songbirds to taxidermic mounts having or lacking a black pectoral band, a geographically variable plumage trait. Irrespective of their own phenotype, males of both species were similarly aggressive to potential intruders bearing or lacking the pectoral band. In contrast with work suggesting male-male interactions may mediate trait evolution, our results indicate that pectoral bands in this system are unlikely targets of social selection via contests in which males compete for territories, and call for studies assessing long-term costs of bearing alternative traits or female choice as selective forces driving phenotypic evolution.