Genetic Variation and Parentage in a California Population of Acorn Woodpeckers

Abstract

-We examined genetic variation in a California population of the highly social Acorn Woodpecker (Melanerpes formicivorus) using starch-gel electrophoresis of 580 blood samples collected between 1975 and 1982. Eighteen genetic loci were resolvable from blood, 2 of which (Mpi and Np) were polymorphic. One additional locus (Est-2) showed variation that was not easily interpreted as an outcome of simple Mendelian inheritance. Average heterozygosity (0.032) was comparable to that of other bird species. Both variable loci showed a slight but not significant deficiency of heterozygotes relative to Hardy-Weinberg expectation. Of 186 nestlings from 62 nests, the genotypes of 4 nestlings (2.2%) from 2 nests (3.2%) were inconsistent with the genotypes of the group breeding males, thus indicating kleptogamy (cuckoldry) in our study population. The low level of genetic variation and the high degree of relatedness between males within groups prevented us from making definitive conclusions concerning the patterns of within-group paternity. Nonetheless, at 1 one-female nest the brood was probably, but not certainly, multiply sired. Received 23 January 1984, accepted 13 September 1984. THE genetic structure of natural populations of social vertebrates is a topic of much current research. Of particular interest is the question of whether social structuring results in decreased effective population size and increased inbreeding, which could in turn promote more rapid rates of evolutionary divergence (Wilson et al. 1975, Bush et al. 1977, Schwartz and Armitage 1980) or the evolution of cooperative or altruistic traits (Brown 1974, Johnson and Brown 1980). Electrophoretic and other genetic markers have proved useful in investigating alternative reproductive strategies. For example, Hanken and Sherman (1981) found that multiple matings by female Belding's ground squirrels (Spermophilus beldingi) result in multiple paternity of most litters. Gowaty and Karlin (1984) suggest that extrapair copulations and intraspecific nest parasitism result in frequent kleptogamy (Gowaty 1982) in the Eastern Bluebird (Sialia sialis). Joste (1983) used electrophoretic 1 Present address: Department of Biology, Memphis State University, Memphis, Tennessee 38152