Pelvic sexual dimorphism and its association with cranial morphology in Neotropical spiny rats (Rodentia, Echimyidae)

Pelvic morphology in mammals is shaped by several factors, including the functional commitment with parturition. Due to this, some mammalian clades, like primates, show a detectable evolutionary relationship between the magnitude of pelvic sexual dimorphism and cranial morphology of neonates. However, this correlation is less understood within the most diversified mammalian order, Rodentia. The present study aims to describe the general patterns of sexual dimorphism in pelvic shape among Neotropical spiny rats (Echimyidae), assessing its hypothesized coevolution with cranial morphology. We investigated the main sources of sexual variation in pelvic shape among 12 echimyid taxa using linear morphometry and subsequently tested the phylogenetically controlled correlation between male and female pelvic morphologies and craniometric variation. Results reveal that pelvic sexual dimorphism is concentrated in the pubis, with females showing a shorter and thicker pubic symphysis and a thinner cranial pubic ramus than males, a pattern already reported in other mammals and evidently related to obstetric demands. An evolutionary association was found between the intensity of pelvic femininization, the increase in pelvic sexual dimorphism, and the acquisition of relatively large crania and relatively inflated braincases; all of these traits are notably developed in the semi-fossorial broad-headed spiny rat, Clyomys laticeps . We argue that selection on the pelvis of female spiny rats, associated with their cranial morphologies and constraints linked to locomotory specializations, probably resulted in a correlated selective response in the pelvis of males. Our findings suggest that the magnitude of pelvic sexual dimorphism in echimyids is likely associated with obstetric demands imposed by cranial morphology.