Sexual selection on plumage and behavior in an avian hybrid zone: 
experimental tests of male-male interactions.

McDonald, D.B., R.P. Clay, R.T. Brumfield, and M.J. Braun. 2001. Evolution 55: 1443-1451.

Abstract: In western Panamá, an unusual hybrid zone exists between white-collared manakins, Manacus candei, Aves, and golden-collared manakins, M. vitellinus. Unidirectional introgression of plumage traits from vitellinus into candei has created a region in which all definitively plumaged males are lemon-collared. These males are nearly indistinguishable from white-collared candei genetically and morphometrically, but strongly resemble golden-collared vitellinus due to the introgression of secondary sexual plumage traits, particularly the lemon collar. The introgression could be explained by sexual selection for golden-collared traits, or by a series of mechanisms that do not invoke sexual selection (e.g., neutral diffusion, dominant allele). Sexual selection on male-male interactions implies behavioral differences among the plumage forms -- specifically that golden- and lemon-collared males should be more aggressive than white-collared males. In contrast, the non-sexual hypotheses predict behavioral similarity between lemon- and white-collared males, based on their nearly identical genetics. We tested the sexual selection hypothesis experimentally, by presenting males with taxidermic mounts of the three forms. As response variables, we monitored vocalizations and attacks on the mounts by replicate subject males. Both golden-collared and lemon-collared males were more likely to attack than were white-collared males -- as predicted under sexual selection, but not by the non-sexual hypotheses. Lemon-collared males were more vocally reactive than either parental form, contrary to the prediction of the non-sexual hypotheses. Our study demonstrates that sexual selection on male-male interactions may play an important role in the dynamics of character evolution and hybrid zones.