Biology ETDs

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This study evaluates the potential for competitive interactions between two species of cotton rat along the Rio Grande valley in New Mexico. Field work was carried out on Sigmodon hispidus and S. fulviventer from April, 1976 to August, 1976, at six study sites. Field data gathered included number of juveniles, number of pregnant females and vegetation diversity. Further studies included morphological analysis of museum specimens and a behavioral study of animals brought back to the laboratory.

Population parameters were compared across the study sites for the cotton rats. Neither species has a sex ratio significantly different from 50:50, but the per cent of pregnant females is significantly larger for S. hispidus than for S. fulviventer. No significant differences were seen in per cent of juveniles present, or in average litter size of the females.

A discriminant function analysis of the vegetation characterizes the habitats by the abundance of Scirpus paludosus. Habitats abundant with S. paludosus should contain S. fulviventer while habitats lower in abundance should contain S. hispidus. Dietary preferences of the cotton rats were determined by identification of clippings left by the cotton rats. No differences in preferred food items were observed.

Two separate morphological analyses were performed, one from skull and skin data, and the other from skeletal data. Sigmodon hispidus has longer hind legs, a higher radius:ulna ratio, and a short mandibular tooth row. These skeletal differences are discussed in relation to their possible behavioral/ecological significance.

Inter- and intraspecific interactions between the species showed no species differences and no well-defined avoidance/aggressive behavioral patterns.



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Degree Name


Level of Degree


Department Name

UNM Biology Department

First Committee Member (Chair)

James Smith Findley

Second Committee Member

Frederick W. Taylor

Third Committee Member

Manuel Carl Molles

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