Differential Allocation: Sex Ratio Adjustment in House Wrens
Keller, Christopher Lee
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Life history decisions are intimately affected by the interplay of parental investment and sexual selection~ The differential allocation hypothesis (Burley, 1986) states that a female should adjust her reproductive investment disproportionately based upon the attractiveness of the male in order to maximize fitness benefits for her and her offspring. Females can allocate differentially and maximize fitness benefits is by adjusting the offspring sex ratio, known as sex-allocation (Trivers and Willard, 1973). According to sex-allocation theory, if a female mates with an attractive male, the result is a selection pressure to have a greater ratio of males to females. This study examines the affect of perceived male quality on the offspring sex ratio in house wrens, although the mechanism for sex ratio adjustment is unknown. In this study, we used the number of available nest boxes in an assigned territory to determine male attractiveness. Since males compete for territory, female house wrens may use nest box availability as an indicator of male fitness. We hypothesized that females that mated with males with surplus nest box availability in their territory would have a male-biased offspring sex ratio. This experiment was motivated by the response to the technology-fueled influx of inconsistent sex allocation research in birds. However, we too obtained mixed results as only one out of two years showed an offspring sex ratio that was significantly different from parity. Our results, along with those of others, indicate that a deeper understanding of why and how sex allocation occurs is important for further research and appropriate application of the sex -allocation theory.