Body size and sex allocation in simultaneously hermaphroditic animals

Sex allocation theory predicts that hermaphrodites optimally divide resources between male and female function, and this strategy may vary with a wide range of individual and population traits. We present a model of reproductive strategies for simultaneously hermaphroditic animals that incorporates multiple sex allocation tactics depending on the traits of an individual, its current mate, and the population at large. We examined the effect of resource variation on sex allocation in a sperm-storing population with two body-size classes. We found that an individual's sex allocation depends on its own size relative to its mate; when stored sperm is displaced exponentially, small animals (with fewer resources) invest a greater proportion of resources in male function than do large animals, and animals of either size invest more in male function when mating with a large mate than with a small mate. Optimal sex allocation depends on the size distribution in the population, the disparity of resources between size classes, the cost of filling a sperm storage organ, and the shape of the sperm displacement function. A function with S-shaped returns to sperm transfer results in a contrary finding: large animals invest more resources in male function than small animals when the cost of filling a sperm storage organ is high. Under many conditions, pronounced sex allocation differences between individuals result in similar proportions of sperm displaced and paternity gained. Thus, variation in individual traits within a population leads to multiple sex-allocation strategies that can produce equal outcomes in the game of sperm competition. Copyright 2002.