The effect of dietary protein on habitat selection, growth, and reproduction of the cotton rat, Sigmodon hispidus

Previous studies have calculated that protein is one of the nutrients most likely to limit growth and reproduction of the hispid cotton (Sigmodon hispidus). Grasses are the primary food source of the hispid cotton rat yet these plant types are considered nutritionally inferior to forbs, particularly with respect to protein availability. Protein is among the nutrients considered most critical for growth and reproduction of many herbivorous animals. The hispid cotton rat is thus faced with the problem of acquiring an adequate supply of protein from habitats where this nutrient may be available in limiting quantities. No empirical studies have been conducted which experimentally manipulate naturally available levels of dietary protein and examine the effects on growth, reproduction, and habitat selection of grassland herbivores. The goal of this research was to determine whether natural levels of dietary protein yielded differential growth, reproduction, and influenced habitat selection of the hispid cotton rat. Animals were captured from the field and bred to produce litters in the laboratory. Litter mates of pups born in the laboratory were paired by sex and mass and raised on artificial diets which mimicked availability of protein in monocot (4% protein) and mixed (monocot and dicots, 11% protein) habitat patches. Individuals raised on the low protein diet had significantly lower growth rates and later onset of the first estrous cycle than individuals on the high protein diet. Preferred forage species of hispid cotton rats were fed as meals composed of either monocot or dicots to determine minimum passage time and protein extraction efficiency of the gut. There was no significant difference in minimum transit time between monocot and dicot meals. Protein was more efficiently removed from dicots than from monocots. A third series of experiments determined that hispid cotton rats shifted their pattern of habitat occupancy in response to experimentally increased protein availability under laboratory conditions. Similar results were not obtained during field trials perhaps as a result of a seasonal effect. Results from the above experiments are consistent with those for a proposed nutrient balancing foraging strategy of the hispid cotton rat.