Via three studies, this thesis investigates how dimorphism in the crania and pelvis differ and how parturition, locomotion and mating system affect pelvic dimorphism. In the first study, cranial and pelvic size and shape dimorphism were compared and a series of size and shape dimorphism indices were formulated. The study was performed on two closely related fox species (genus Urocyon). Although no significant size and shape dimorphism in the crania of either species was found, significant dimorphism in the pelvis did exist: size was dimorphic in island foxes (but not in gray foxes) and shape was dimorphic in both species (though more pronounced in island foxes).

The second study employed a comparative phylogenetic approach to examine variation in the magnitude of pelvic size and shape dimorphism in association with mating system, relative offspring size and locomotor stance. Pelvic shape and size dimorphism varied with all variables in a complex mosaic of interactions. Mating system had a greater effect on size than shape dimorphism and locomotor stance had an effect on both size and shape dimorphism. Of the two offspring size ratios, the ratio of offspring mass to pelvic size had a different effect on shape and size dimorphism than the ratio of offspring mass to female mass.

The third study used a sample of laboratory mice to contrast dimorphism in body length and pelvic size and shape between males and parous and nulliparous females. Correlations between relative litter size (L/M) and relative offspring size (O/M) with body length and pelvic size and shape in parous females were also examined. Males were longer than nulliparous females but not parous females. Overall, females had the largest pelves, with parous females having the largest and males the smallest. Pelvic shape was also significantly different between groups and was most divergent between parous females and males than between nulliparous females and males. Pelvic shape differences between all females were associated with body length differences between females. O/M was correlated with shape in parous females such that females with the largest offspring had the most divergent pelvic shapes.