The Differential-Allocation Hypothesis: An Experimental Test
Time-budget estimates of parental investment in captive populations of color-banded zebra finches demonstrate that relative attractiveness correlates with the apportionment of parental investment as predicted by the differential-allocation hypothesis: attractive birds incur smaller parental expenditures; unattractive birds have larger ones. This result occurred in two experiments: color banding was used to manipulate female attractiveness in one experiment and male attractiveness in another. Attractiveness was correlated with participation in parental behaviors but not with defensive and agonistic behaviors. The mean paternal expenditures (PE) were nearly identical in populations with color bands and in those without. Overall, males contributed 43%-45% of the total time expenditure during daytime hours. If parental investment increases nonlinearly with PE (the accelerating-cost hypothesis), estimates of expenditure are conservative estimates of differences in parental investment, at least at the extremes. Results reported here, combined with those reported previously, collectively show that unattractive birds have higher parental investment per offspring, shorter life spans, and lower long-term reproductive success than same-sex attractive birds. Results lend support to the idea that reproductive effort has a cost, a seldom-tested evolutionary principle; they show how individuals may profitably manipulate a finite supply of reproductive resources; and they demonstrate a mechanism by which sexual selection can operate in species with substantial biparental care.