Annual variation in the offspring sex ratio of Saltmarsh Sparrows supports Fisher's hypothesis
The Auk, 135(2), pp 342-358, 2018-4
Evolutionary theory suggests that natural selection should favor the ability of animals to modify their offspring sex ratios when the fitness benefits of producing one sex over the other vary in relation to environmental conditions. The Saltmarsh Sparrow (Ammodramus caudacutus) is an extreme habitat specialist, with breeding behavior influenced by the tidal cycle. The challenges of nesting in the harsh environment of salt marshes and the unique promiscuous mating system of the Saltmarsh Sparrow provide a context for offspring sex ratio manipulation. We investigated adaptive sex ratio manipulation in this system across multiple sites and years using a mixed modeling approach. We collected data on nest initiation and nestling survival from 370 nests of 210 females during 2011-2015 in 4 marshes in the northeastern United States. Using molecular techniques, we determined the sex of 990 offspring and characterized variation in site-and population-level sex ratios. Using binomial linear mixed-effects models, we tested the influence of environmental, temporal, and maternal factors on offspring sex ratios. Across years and sites, we found an even offspring sex ratio of 1.03:1, with an alternating pattern of interannual variation between male and female bias at both the population and site level. The fluctuating sex ratio mirrored that of the adult sex ratio in the preceding year. Sex ratios did not vary as a function of timing of breeding within the breeding season or in relation to tidal flooding. Offspring sex was also independent of female condition at the time of nest initiation. Taken together, our findings suggest that female Saltmarsh Sparrows do not manipulate the sex ratio of their clutches in relation to environmental, temporal, or maternal factors. Our finding of a 1:1 offspring sex ratio and interannual variation in offspring and adult sex ratios in a wild bird population is more consistent with the predictions of Fisher (1930) than with those of Trivers and Willard (1973).
offspring sex ratio; maternal condition; temporal effects; environmental effects; Saltmarsh Sparrow; reproductive investment