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 abundance 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 nonreproductive 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 reproductive 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.

Mechanisms of Habitat Selection by the Hispid Cotton Rat (Sigmodon hispidus)

“…An explanation can be derived from results of foraging experiments in laboratory enclosures provisioned with stands of natural dietary items. Video tracking revealed that foraging movements by hispid cotton rats were nearly linear in these experiments (Randolph and Cameron 2001). Results from these feeding trials and the T/S analysis indicate that reproductive females forage in mixed patches by directed, linear movements.…”

“…In the current study, females preferred mixed habitats and avoided dicot habitats, whereas males showed no habitat preference. The importance of mixed habitats to reproductive females is underscored by the fact that their home ranges contained a higher proportion of mixed habitat than was available (Cameron and Spencer 1985) and they rely on a mixed diet of monocots and dicots (Randolph and Cameron 2001). …”

“…Reproductive female hispid cotton rats select both monocot and dicot plants to attain a diet nutritionally superior to either a monocot-only or a dicot-only diet (Kincaid and Cameron 1982;Randolph and Cameron 2001;Randolph et al 1991;Vivas and Calero 1988). In this coastal prairie habitat, dicots often occur in single-species clumps (G. N. Cameron and S. R. Spencer, in litt.).…”

“…Similarly, the strategy of mixing diet items, common to generalized herbivores (Pennings et al 1993; Pierotti and Annett 1987; Willig and Lacher 1991), and also characteristic of hispid cotton rats, is facilitated by occupation of mixed habitats. Hispid cotton rats obtain sufficient amounts of complementary nutrients only by ingesting both monocot and dicot plants (Randolph and Cameron 2001); monocots provide carbohydrates and dicots provide proximate and mineral nutrients, particularly protein, calcium, and phosporus, for reproduction and lactation (Randolph and Cameron 2001). Hence, mixed habitats would be of higher quality for hispid cotton rats because they contain the requisite food plants.…”

“…For example, protein concentration in food plants limits populations of hispid cotton rats (Doonan and Slade 1995;Parsons et al 2005;Randolph and Cameron 2001;Webb et al 2005), and affects selection of habitats (Eshelman and Cameron 1996). Hispid cotton rats also are able to select a nutritionally complete diet containing protein (Harriman 1977), and may be able to discern protein by odor and taste (Brot et al 1987;Heinrichs et al 1990).…”