Response of a Cotton Rat Population to Increased Density

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Abstract
Experiments were conducted in old-field enclosures to define some of the responses of a confined, nonreproducing cotton rat [Sigmodon hispidus] population to increases in density. Successive groups of alien animals were randomly added to a monitored resident population on 2 occasions. Aliens exhibited high rates of mortality and weight loss. The alien males were particularly affected, only 18% surviving the experiment. Residents could not be accounted for by trapping as easily as aliens. Residents concentrated in discrete areas of preferred habitat yet ranged over the entire enclosure, while aliens staved in heavy ground cover close to the walls. Analysis of the average distance between captures of each group on a daily basis revealed increases in resident movements and corresponding decreases in alien movements. Changes in spatial distribution, movement patterns, sex ratio and rates of mortality due to trap deaths and predation in the alien groups reflected behavior interactions that operate as a form of social exclusion.

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