Parental investment strategies in black brant on the Yukon-Kuskokwim Delta, Alaska.

摘要

Parental investment theory predicts that parental effort in current reproduction comes at a cost to future reproduction (Trivers 1972). Costs incurred by parents include reduced future reproductive output and survival. Parents that can optimize this trade-off to maximize lifetime reproductive output will have a fitness advantage over those parents that cannot. To more fully understand how individuals maximize this trade-off we addressed two main aspects of parental investment: first, how parents manipulate their investment in relation to offspring sex; and second, how parents manipulate their investment in offspring through secondary reproductive strategies.;Growth conditions during early life can affect an individual's fitness. We determined how differential growth rates during early life resulting from a sexual size dimorphism affected survival in a precocial bird, the Black Brant (Branta bernicla nigricans). Adult male brant are approximately 10% larger than females resulting in increased growth rates during development. Therefore we hypothesized that male brant goslings would suffer greater mortality during the brood rearing period compared to females. Supporting our hypothesis, we found that male brant goslings relative to female goslings suffered a 17% reduction in survival likely resulting from increased growth rates. We conducted a simulation to determine how gender biased survival would affect population level sex ratio. We found that a female biased population level sex ratio was never overcome even though previous work demonstrated a male biased post-fledging survival rate. This result explains a proportion of the 17% of females that permanently emigrate from their natal area, as a substantial proportion of females never acquire mates.;Additionally, we examined three hypotheses concerning sex biased parental investment in a precocial bird, the Black Brant. Based on Fisherian sex ratio adjustment, we hypothesized that the population level primary sex ratio in brant would be biased towards males. Contrary to our hypothesis we found that the population level primary sex ratio did not differ from parity (50.39% male, n = 645). Next we examined how female fitness, or quality, is related to primary sex ratio. Given that high quality females produce high quality offspring in Black Brant, we hypothesized that high quality females would produce more of the non-dispersing gender, or females. Our model selection analysis revealed that, overall, female quality had modest support for predicting offspring sex ratio (Total Akaike Model Weight = 29.94%) suggesting further study is warranted. Finally we examined how females change their behavior in response to primary sex ratio of their brood. Male brant have a lower prefledging survival rate when compared to females, likely resulting from greater size and associated relative difficulty in meeting nutritional requirements. Therefore, we hypothesized that parents would take male biased broods to areas with increased juvenile growth rates. We found that parents with male biased primary sex ratios tended to take their broods to brood rearing areas with increased growth rates. Furthermore, we found that female parents with male biased primary sex ratios had experienced higher growth rates themselves, suggesting they experienced better foraging conditions during their own growth period. These two factors together suggest that brant make decisions about parental investment in response to the primary sex ratio of their broods.;The second aspect of parental investment theory we addressed was how parents invest in offspring through secondary reproductive strategies. Conspecific brood parasitism is a secondary reproductive strategy in which females lay eggs in the nests of another female of the same species (Weller 1959). Numerous methods have been proposed to indirectly determine conspecific brood parasitism in birds. In this study we used a discriminant function analysis based on egg morphology in an attempt to detect nest parasitism in Black Brant ( Branta bernicla nigricans). True parasitic and host eggs were classified using microsatellite genotypes. We detected 32 parasitic eggs in a sample of 486 eggs from 126 families. As expected parasitic eggs differed from host eggs in volume: mean absolute residual volume of parasitic eggs = 1.56 +/- 1.24 while that for host eggs = 1.65 +/- 1.42. We used a more appropriate jackknife technique (posterior error probability = 28.86%) as well as an independent validation data set (posterior error probability = 29.70%). In both cases, host eggs tended to be classified as parasitic. We suggest when developing a predictive function based on egg morphology for predicting conspecific brood parasitism that an appropriate validation technique be used to validate the function.;Finally we determined characteristics associated with conspecific brood parasitism to address which individuals were adopting this secondary reproductive strategy. (Abstract shortened by UMI.)