Sexual selection and extrapair fertilization in a socially monogamous passerine, the zebra finch (Taeniopygia gullata)

Abstract

We investigated paternal exclusion rate (the percentage of young reared by a male that were not his genetic offspring), and behavioral and reproductive variables influencing this rate, in a freely breeding laboratory population of zebra finches (Tae-niopygia guttala castanotis), a socially monogamous grassfinch. Prior to the experiment, each male founder was fitted with either two red bands (creating a phenotype previously demonstrated to be attractive to females) or two green bands (unattractive to females) as part of a unique combination of four leg bands. The overall paternal exclusion rate was 28%, as determined by multilocus, minisatellite DNA fingerprinting of 278 offspring reared by 26 males and their mates. Mean exclusion rates were 16% and 40% for red- and green-banded males, respectively. Exclusion rates were directly proportional to rates of female participation in unforced extrapair copulations (UEPCs) with red-banded males that occurred when females were fertile. Rates of fertile, forced extrapair copulations (FEPCs) and fertile UEPCs involving green-banded males either failed to influence exclusion rate or varied inversely with exclusion rate, indicating that extrapair fertilization (EPF) is under female control. Effort devoted by males to seeking EPFs increased exclusion rate. Results suggest that males place greater effort into seeking fertile versus infertile EPCs and that unattractive males accrue fitness gains through high parental investment (PI), whereas attractive males benefit through decreased PI and increased allocation to EPF.