Sex and the Single Forager: Gender‐Specific Energy Maximization Strategies in Fiddler Crabs

Abstract

The energy costs and benefits of foraging were quantified for male and female fiddler crabs. Benefits accrued during foraging were calculated using patch—specific intake rates, combine with an analysis of the energy value of food. Foraging costs per unit time were quantified by measuring O2 depletion of foraging animals. An energetics model constructed with these cost—benefit functions predicted the leaving rule, defined as the food concentration at which a crab should cease feeding in a patch, that maximized energy intake. A second model was designed for males feeding on fine (sieved) sediments, which reduced intake rate, and therefore changed foraging costs. Lastly, by building on a previous analysis of foraging mechanics, the model examined whether the structure of the foraging system constrains the ability of fiddler crabs to maximize energy intake. Analysis of sex specific intake rates and foraging costs indicates that females, although experiencing greater costs, also had higher intake rates, than males. The model predicted that females should acquire more energy from foraging in a patch, and should display a lower leaving threshold than males. The leaving rule prediction was tested and confirmed; observed leaving thresholds for all three treatments (females, males, and males on fine sediments) were statistically indistinguishable from model predictions. As predicted by the model, males feeding on fine sediments responded to increased costs by displaying a higher leaving threshold, relative to males on unsieved sediments. A sensitivity analysis, performed by using the 95% confidence interval of O2 consumption to bracket foraging costs, indicated the model to be robust; leaving threshold were extremely stable, with all the three predicted threshold remaining quite distinct, despite a 20% change in the cost function. Finally, the model showed that the architecture of the feeding system constrains the ability to implement energy maximizing strategies. Constraints are gender specific, and the associated energy penalty is frequently small.