The Yellow-nosed Cotton Rat, Sigmodon ochrognathus, in Arizona

In southeastern Arizona the yellow-nosed cotton rat, Sigmodon ochrognathus, inhabits grassy, rocky slopes, in or near the oak belt in the same habitat as Thomomys umbrinus and Reithrodontomys fulvescens. The cotton rats form extensive surface runway systems con- necting surface nests and burrow openings. Surface nests, constructed of grass and placed in dense thickets, are 7 to 16 in. in diameter. Abandoned burrows of Thomomys umbrinus are used by cotton rats and appear to. be a limiting factor in their distribution. Wild animals are much more active during daylight hours than at night. However, one captive male ran an exercise wheel an average of 31,836 ft per night for 100 consecutive nights. In the wild, S. ochrognathus feeds extensively on blue grama; in captivity a diet of salad greens and dry lab chow is adequate. Caged animals do not require free water if fresh plant material is provided every 2 or 3 days. Females breed as early as 45 days of age and 2 individuals had gestation periods of 33 and 36 days. Litter size varies from 2 to 6 and young can be weaned by the 15th day, probably sooner. The furred newborn are precocial with the eyes opening during the first 24 hr. At 2 days of age they can climb the hardware-cloth sides of the cage. Growth is rapid. After 100 days, there is little' increase in total and tail-length; after 40 days, in hind foot-length. By 2 V/2 month of age, the skull has attained over 90% of adult size in greatest length, zygomatic breadth, mastoid breadth, and length of upper toothrow. Estimations of the ages of wild-taken specimens indicate that animals rarely live for more than 4 months in the wild since 96% are less than 4 months old and 83% are less than 2V2 months old. 'Characters for distinguishing S. ochrognathus from S. minimus and S. hispidus in Arizona are listed. Certain distinguishing features of the baculum, interparietal, foramen ovale, and foramen magnum have not been previously reported. The yellow-nosed cotton rat, Sigmodon ochrognathus, has a limited range in the Southwest and, judging from the few specimens in collec- tions, is an uncommon animal. Our own recent field work in Arizona has enabled us to recognize the signs and habitat of this species and once aware of these, we were able to obtain numerous specimens from 15 localities in southeastern Arizona. Discussions of the habitat, be- havior, growth, and general characteristics of this species are based upon field observations in Arizona and upon 7 wild-caught animals and