Eutherians give birth to relatively large infants. Females have larger pelves than males, though males typically have larger nonpelvic bones. Pelvic dimorphism is an adaptation for parturition. Metatherian females, who give birth to markedly small infants, should not show pelvic adaptations for parturition and should have smaller pelves than males. Adult females and males of the Virginia opossum, Didelphis virginiana, are compared. Males are significantly larger than females in absolute size for 14 of 16 pelvic and 8 nonpelvic measures. Sexes did not differ significantly for lower iliac length and posterior space of midplane. However, relative lengths for females are significantly larger than for males for these 2 measures and for the pubis. These pelvic dimorphisms in opossums cannot be adaptations for parturition. These dimorphisms result from asynchrony among skeletal elements in growth cessation and sexual differences in adulthood growth.
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