Abstract
Cotton rats, genus Sigmodon, occur commonly in grassland habitats <br /><br />throughout the southern United States and Mexico. The most widespread species <br /><br />is S. hispidus, which is known to occur in California, although previous studies <br /><br />give reason to believe 5". arizonae also occur in the state. Cotton rats were studied <br /><br />in California using a combination of karyology and skull morphology. Results <br /><br />indicate two species of Sigmodon occur in California: S. hispidus in extreme <br /><br />southeastern California and the Imperial Valley and S. arizonae along the Col- <br /><br />orado River north of the Palo Verde Mountains.
