Alternative reproductive strategies in the white-throated sparrow: behavioral and genetic evidence

Organisms exhibiting genetic polymorphism often also exhibit true alternative life-history strategies in which behavioral tactics are genetically fixed. Such systems are ideal for the study of the evolution of life histories because the consequences of selective episodes can be more easily identified. Here I report an interesting and classic example of a species exhibiting true alternative strategies. Due to a chromosomal inversion, male and female white-throated sparrows (Zonotrichia albicollis) occur as two distinct morphs, tan or white. Tan and white morphs mate disassortatively, and this mating pattern maintains the polymorphism in relatively equal proportions within the population. In comparison with tan males, white males are more aggressive, frequently intrude into neighboring territories, spend less time guarding their mates, occasionally attempt polygyny, and provide less parental care. White females are also more aggressive and solicit copulation from their mates twice as often as tan females. Multilocus DNA fingerprinting revealed that 31.8 of nestlings and 56.3 of broods from white male–tan female pairs were the result of extrapair fertilizations, whereas only 4.4 of nestlings and 6.3 of broods from tan male–white female pairs contained extrapair offspring. However, 4.4 of the nestlings (12.5 of broods) from tan male–white female pairs were attributable to conspecific brood parasitism, and brood parasitism was not a factor for white male–tan female pairs. For three excluded extrapair offspring, actual paternity was assigned to territorial white males, suggesting that, although white males are not preferred by females in previous mate choice studies, they do attempt to increase their reproductive success by securing additional matings via extrapair copulations. However, by doing so, white males also lose paternity at home. Behavioral and genetic evidence suggest that the two morphs use alternative strategies and that reproductive trade-offs may serve to maintain a competitive equilibrium between the morphs.