Sex and age affect vulnerability of desert rodents to owl predation

Pelvic morphology was examined to distinguish sexes of three prey species eaten by great horned owls, and age classes of one of these species. Comparisons of sex ratios in prey populations with frequencies of males and females in owl diets indicated that male Dipodomys ordii and female Peromyscus maniculatus were more vulnerable to owl predation than the opposite sexes of these species. Furthermore, juvenile Microtus montanus were more vulnerable than adults and, within the juveniles, females were more vulnerable than males. We examined morphological (body size) and behavioral (degree of activity) attributes of these prey species, and concluded that either morphology or behavior may facilitate differential vulnerability in Dipodomys , whereas behavioral differences between sex and age classes are more likely to account for differential vulnerability in Microtus . For Peromyscus there are no body size or activity differences between sexes that are consistent with differential vulnerability, but body weights of female Peromyscus are more variable than weights of males, which could promote female-biased vulnerability if predators select large prey.