1. Nitrogen-containing nutrients have long been considered a frequently limiting resource to the growth of herbivore populations (nitrogen limitation hypothesis). To explore this hypothesis, we examined the relationships between availability of essential amino acids and concentrations of phenolics in the diets of hispid cotton rats (Sigmodon hispidus) in central Oklahoma and the intrinsic characteristics of their non-cyclic populations. We posited that lower quality proteins (i.e. essential amino acid composition) and elevated phenolic levels (protein digestion inhibitors) in diets of cotton rats from low- compared to high-density populations, especially during the breeding season, would be supportive of the nitrogen limitation hypothesis. Replicated low- and high-density populations were censused by live-trapping at 3-month intervals. Samples of stomach digesta were collected from cotton rats in similar habitats adjacent to trapping grids to determine the botanical and nutrient composition of their diets.
2. During the breeding season, concentrations of essential amino acids were as much as 43% greater in diets of cotton rats from high-density populations. Dicots, typically higher in protein than monocots, were an important component of diets and were preferred forage in all seasonal collections. Seeds and arthropods were frequently utilized by cotton rats as additional high-quality sources of essential amino acids. Concentrations of total phenolics in the diet (greater in diets from low-density populations) were consistent with the nitrogen limitation hypothesis.
3. Density was consistently higher in the high-density populations throughout the study. Other demographic and body condition parameters were similar between low- and high-density populations in the non-breeding season, but reproductive activity was greater in high-density populations during the breeding season. Total number of juveniles recruited into the trappable population over the entire study was about five times greater in high- compared to low-density populations.
4. Our data did not refute the nitrogen limitation hypothesis where levels of essential amino acids and phenolic compounds in the diet during the breeding season may determine annual peak densities of cotton rats that can be supported in their habitat. However, we could not rule out the involvement of other environmental variables such as overhead cover (as well as other unmeasured variables) as contributing factors to determining annual peak densities.