CONSEQUENCES OF DIET CHOICE BY A SMALL GENERALIST HERBIVORE

Abstract

We examined the energy, nutrient, time, and habitat consequences of diet choice by a generalist herbivore, the hispid cotton rat (Sigmodon hispidus) on the Texas coastal prairie. Benefit-cost and linear programming models were developed and used to test several specific hypotheses about diet choice, foraging strategy, and habitat use by cotton rats. Within cotton rat habitat, there were significant differences among both seasons and habitat patches in the abundance and biomass of foods available. Monocot patches typically were higher in standing crops of energy and nutrients than were either dicot or mixed patches. Relative to available biomass and nutrients in dicot foods, this cotton rat habitat was both spatially and temporally heterogeneous and dynamic. Although the abun- dance of dicot foods in a habitat patch did influence costs of foraging time, time costs of cotton rats foraging for suitable foods did not decrease when standing crops of energy and nutrients in these habitats increased. Higher requirements for energy and nutrients needed for lactation by reproductive cotton rats resulted in higher foraging costs than for non- reproductive female rats. A mixed diet incorporating both monocot and dicot foods was nutritionally superior to either a monocot-only diet or a dicot-only diet. A diet of only monocots would meet minimum daily energy requirements and would not exceed either maximum daily digestive capacity or maximum daily foraging time, but it would not meet minimum daily requirements for protein and phosphorus during reproduction. Conversely, a diet of only dicots would meet nutrient requirements for reproduction but would exceed maximum daily foraging time. Mixing of monocot and dicot plant parts in the diet met both net energy and nutritional requirements, and increased foraging time, as compared to a diet of only monocots. Diets of cotton rats foraging to maximize net energy intake each season differed from the observed diets. Likewise, diets of cotton rats foraging to minimize total daily foraging time differed from the observed diets. Thus, we conclude that repro- ductive females of this generalist herbivore neither maximized their energy nor minimized their time, but rather, consumed a mixed diet to balance intake of requisite nutrients. Accordingly, a diet choice that included a mixture of monocot and dicot foods was most efficient when foraging occurred in a habitat having a mixture of monocot and dicot plants. Mixed patches provided higher energy and nutrient benefits to foraging cotton rats than did either monocot or dicot patches. Finally, we observed that reproductive female cotton rats concentrated their activities in mixed-patch habitats.