An Analysis of the Community and Population Dynamics of the Helminths of Sigmodon hispidus (Rodentia: Cricetidae) from Three Central Texas Vegetational Regions1

During the period August 1974 to October 1975, an ecological study of the helminth parasites of Sigmodon hispidus was conducted in the area of San Marcos, Texas. A total of 355 S. hispidus were examined, 95% of which were infected. Twelve species of helminths were recovered, including four species of cestodes, three species of trematodes, and five species of nematodes. Parasitism in S. hispidus was found to vary significantly with the season of the year, and with such hostrelated factors as habitat, size, and sex. Cotton rats (Sigmodon hispidus) are distributed throughout Central America and the southern United States and are usually the most numerous mammals where they occur (Hall and Kelson, 1959). They may be found within their range wherever there is sufficient thick, screening vegetation to provide protection and concealment (Bunn, 1941), and they may competitively exclude other rodent species from their home range (Raun and Wilks, 1964; Terman, 1974). As a result of their wide range and relatively dense populations, cotton rats have often been the subject of helminthological surveys and their helminth fauna is relatively well known. The rats themselves have also been the subject of a considerable number of general ecological studies. However, the effects of ecological factors upon the helminth fauna of the cotton rat have received little attention. Harkema and Kartman (1948) included discussion of differences in mean density of parasites over the four seasons in their study of North Carolina cotton rat helminths. Coggins and McDaniel (1975) reported the effects of season and host sex on helminth population dynamics in North Carolina, basing their findings on 130 cotton rats collected over a 2-year period. Layne (1968) studied the effects of habitat upon infections of Capillaria hepatica in Florida mammals, including S. hispidus. Kinsella (1974) reported differences in the helminth fauna of cotton rats collected from several habitats in Florida, and included a checklist of the helminths of S. hispidus. Mollhagan (1978) investigated the effects of five western Texas habitats upon primarily the helminthofaunal similarity between the habitats but used relatively small sample sizes and did not investigate the effects of season or of the length or age of the hosts and, as a consequence, was not able to demonstrate the effects of host-related factors. The purpose of this study was to obtain a checklist of the helminths of Sigmodon hispidus in the area of San Marcos, Texas, as well as to investigate the effects of such factors as the season of collection, and the sex, size, and habitat of the host upon the incidence, mean density, and diversity of the helminth infections. 1 Portion of a thesis submitted by James L. Martin to the Graduate School of SWTSU in partial fulfillment of the requirements for the degree of Master of Science (copies available from University Microfilms). Present address: P.O. Box 1166, College Station, Texas 77840.