Comparing pelvic dimensions of precocial and altricial Leporidae species to explore the idea of human “secondary altriciality”

Human infants exhibit a unique combination of precocial and altricial traits, often described as "secondarily altricial," reflecting their high dependency at birth despite belonging to a typically precocial primate lineage. One hypothesis posits that this altriciality evolved to alleviate obstetric constraints caused by bipedal locomotion and large brain size. To test whether pelvic morphology corresponds to life history patterns, this study examines pelvic dimensions in two species within the Family Leporidae: one hare (precocial) and one rabbit (altricial), which share similar relative brain sizes and locomotor patterns, minimizing confounding variables. Nine pelvic measurements were collected from 24 adult specimens. Accounting for body size, analyses revealed that pelvic dimensions significantly differed between the altricial and precocial species. The precocial hare exhibited significantly larger pelvic dimensions in all but two measures compared to the altricial rabbit. These findings support the hypothesis that pelvic morphology can adapt to differences in life history patterns, with expanded dimensions in precocial species helping to ease parturition of more developed offspring. This suggests that secondary altriciality in humans may reflect an evolutionary compromise between developmental maturity and constraints of the bipedal pelvis. Further research on additional taxa is needed to evaluate these trends and their implications for understanding human birth evolution.