Iteroparous organisms face a fundamental life history tradeoff between investments in current and future reproduction. The optimal balance can vary across breeding attempts, depending on whether the present opportunity is above or below average. Differential allocation theory suggests that individuals should reduce their reproductive investment when they have a below average breeding opportunity if this reduction increases the probability of survival and of obtaining above average breeding opportunities in the future. However, studies testing for differential allocation in wild populations have yielded variable and conflicting results. In many cases, variation in population demographics could help to explain these inconsistencies. We illustrate this idea with data from a long-term field study of house wrens (Troglodytes aedon) where we monitored female investment while experimentally manipulating female perception of male mate quality by manipulating male territories.
Results/Conclusions
During the first several years of the study, females mated to a male randomly assigned one nest box in his territory laid significantly smaller clutches than females mated to a male randomly assigned three nest boxes in his territory, consistent with the differential allocation hypothesis. However, this difference in clutch size between the two treatments declined over time, as the mean clutch size of one-box females increased systematically to equal that of three-box females. There are several possible explanations for this inconsistency. We focus on one underappreciated possibility with broad, general implications: Differential allocation should be sensitive to demographics because the value of future offspring is discounted by population growth. When the population is declining, future offspring are more valuable; when it is increasing, they are less valuable. Simple demographic models show that the optimal clutch size should be less sensitive to variation in mate quality in increasing populations, reducing selection for differential allocation. Consistent with these models, differential allocation in response to our manipulations was high in years when regional wren populations were declining but decreased as populations began to increase. Future studies should consider the influences of variation in population demographics on differential allocation of parental investment.
