Abstract
Grasshopper mice, Onychomys leucogaster and O. torridus, in Arizona are taxonomically revised for the first time in 52 years. Age, secondary sexual and geographical variation are analyzed. Specimens were placed in one of six age-groups determined by wear of teeth, although wear-pattern is different in the two species. There is no significant size difference between the sexes. Characters useful in distinguishing the species are relative length of tail, length of toothrow, shape of teeth and features of baculum. Geographical variation in external measurements, cranial features and color is pronounced in O. leucogaster, but these characters vary independently of each other. Color of dorsal pelage parallels the color of substrate; other characters behave in a different, seemingly random pattern. Microgeographic variants of O. leucogaster are described, but only four are recognized by a trinomen. In O. torridus, geographical variation is more concordant; two subspecies are recognized.
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