EXAMINATION OF THE NITROGEN LIMITATION HYPOTHESIS IN NONCYCLIC POPULATIONS OF COTTON RATS (SIGMODON HISPIDUS)

1. Nitrogen-containing nutrients have long been considered a frequentl y limiting resource to the growth of herbivore populations (nitrogen l imitation hypothesis). To explore this hypothesis, we examined the rel ationships between availability of essential amino acids and concentra tions of phenolics in the diets of hispid cotton rats (Sigmodon hispid us) in central Oklahoma and the intrinsic characteristics of their non -cyclic populations. We posited that lower quality proteins (i.e. esse ntial amino acid composition) and elevated phenolic levels (protein di gestion 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 int ervals. 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 seaso n, concentrations of essential amino acids were as much as 43% greater in diets of cotton rats from high-density populations. Dicots, typica lly higher in protein than monocots, were an important component of di ets and were preferred forage in all seasonal collections. Seeds and a rthropods were frequently utilized by cotton rats as additional high-q uality sources of essential amino acids. Concentrations of total pheno lics in the diet (greater in diets from low-density populations) were consistent with the nitrogen limitation hypothesis. 3. Density was con sistently 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 repr oductive activity was greater in high-density populations during the b reeding 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 n itrogen limitation hypothesis where levels of essential amino acids an d phenolic compounds in the diet during the breeding season may determ ine annual peak densities of cotton rats that can be supported in thei r habitat. However, we could not rule out the involvement of other env ironmental variables such as overhead cover (as well as other unmeasur ed variables) as contributing factors to determining annual peak densi ties.