Sex ratios and sexual selection in socially monogamous zebra finches

An experiment was performed in which adult sex ratios of zebra finches, Taeniopygyia guttata castanotis, were varied to test possible effects of adult population sex ratios on sexual selection intensity and mating system dynamics in species with biparental care. The possibility that sex ratio influences the success of social mating patterns (leading to polygyny when males are rare and polyandry when females are rare) was not supported. Results did support the prediction of the differential allocation hypothesis that individuals of the abundant sex would increase their relative parental expenditure (PE). Although total (male + female) PE did not vary between treatments, relative male PE was significantly higher in the male-biased treatment (MBT; sex ratio 64% male) than in the female-biased treatment (FBT; sex ratio 36% male). In both treatments, male PE contributions contributed to female reproductive rate. Results also supported the prediction of the differential access hypothesis that individuals of the abundant sex would experience greater intensity of selection on sexually selected attributes. Male beak color, a sexually selected trait, influenced male social parentage in the MBT but not in the FBT. Finally, broods in the FBT displayed higher hatching asynchrony and lower hatching success; we believe this was caused by early onset of incubation, a tactic used as a defense against intraspecific brood parasitism, which was much higher in the FBT. Population sex ratios may be an important factor affecting female ability to influence male parental investment patterns.