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For many animals, the juvenile stage of life can be particularly perilous. Once independent, immature animals must often complete the same basic survival functions as adults despite smaller body size and other growth-related limits on performance. Because, by definition, juveniles have yet to reproduce, we should expect strong selection for mechanisms to offset these ontogenetic limitations, allowing individuals to reach reproductive adulthood and maintain Darwinian fitness. We use an integrated ontogenetic dataset on morphology, locomotor performance, and longevity in wild cottontail rabbits (Sylvilagus floridanus, Allen 1848) to test the hypothesis that prey animals are under selective pressure to maximize juvenile performance. We predicted that (1) juveniles would accelerate more quickly than adults, allowing them to reach adult-like escape speeds, and (2) juveniles with greater levels of performance should survive for longer durations in the wild, thus increasing their reproductive potential. Using high-speed video and force platform measurements, we quantified burst acceleration, escape speed, and mechanical power production in 38 wild-caught S. floridanus (26 juveniles, 12 adults; all rabbits >1 kg in body mass were designated to be adults, based on published growth curves and evidence of epiphyseal fusion). A subsample of 22 rabbits (15 juveniles, 7 adults) was fitted with radio-telemetry collars for documenting survivorship in the wild. We found that acceleration and escape speed peaked in the late juvenile period in S. floridanus, at an age range that coincides with a period of pronounced demographic attrition in wild populations. Differences in mass-specific mechanical power production explained ∼75% of the variation in acceleration across the dataset, indicating that juvenile rabbits outpace adults by producing more power per unit body mass. We found a positive, though non-significant, association between peak escape speed and survivorship duration in the wild, suggesting a complex relationship between locomotor performance and fitness in growing S. floridanus.

The sacrum occupies a functionally important anatomical position as part of the pelvic girdle and vertebral column. Sacral orientation and external morphology in modern humans are distinct from those in other primates and compatible with the demands of habitual bipedal locomotion. Among nonhuman primates, however, how sacral anatomy relates to positional behaviors is less clear. As an alternative to evaluation of the sacrum's external morphology, this study assesses if the sacrum's internal morphology (i.e., trabecular bone) differs among extant primates. The primary hypothesis tested is that trabecular bone parameters with established functional relevance will differ in the first sacral vertebra (S1) among extant primates that vary in positional behaviors. Results for analyses of individual variables demonstrate that bone volume fraction, degree of anisotropy, trabecular number, and size‐corrected trabecular thickness differ among primates grouped by positional behaviors to some extent, but not always in ways consistent with functional expectations. When examined as a suite, these trabecular parameters distinguish obligate bipeds from other positional behavior groups; and, the latter three trabecular bone variables further distinguish knuckle‐walking terrestrial quadrupeds from manual suspensor‐brachiators, vertical clingers and leapers, and arboreal quadrupeds, as well as between arboreal and terrestrial quadrupeds. As in other regions of the skeleton in modern humans, trabecular bone in S1 exhibits distinctively low bone volume fraction. Results from this study of extant primate S1 trabecular bone structural variation provide a functional context for interpretations concerning the positional behaviors of extinct primates based on internal sacral morphology. Anat Rec, 302:1354–1371, 2019. © 2018 Wiley Periodicals, Inc.

Many derived aspects of modern human axial skeletal morphology reflect our reliance on obligate bipedal locomotion. Insight into the adaptive significance of features, particularly in the spine, has been gained through experimental studies that induce bipedal standing or walking in quadrupedal mammals. Using an experimental animal model (Rattus norvegicus), the present study builds on earlier work by incorporating additional metrics of the cranium, employing quantitative methods established in the paleoanthropological literature, and exploring how variation in mechanical loading regimes impacts axial anatomy. Rats were assigned to one of five experimental groups, including “fully loaded bipedal walking,” “partially loaded bipedal walking,” “standing bipedally,” “quadrupedal walking,” and “no exercise control,” and engaged in the behavior over 12‐weeks. From μCT data obtained at the beginning and end of the experiment, we measured foramen magnum position and orientation, lumbar vertebral body wedging, cranial surface area of the lumbar and first sacral vertebral bodies, and sacral mediolateral width. Results demonstrate that bipedal rodents generally have more anteriorly positioned foramina magna, more dorsally wedged lumbar vertebrae, greater articular surface areas of lumbar and first sacral vertebral bodies, and sacra that exhibit greater mediolateral widths, compared to quadrupedal rodents. We further document variation among bipedal loading behavior groups (e.g., bipedal standing vs. walking). Our experimental animal model reveals how loading behaviors and adaptations may be specifically linked, and implicates a potential role for developmental plasticity in the evolutionary acquisition of bipedal adaptations in the hominin lineage. Anat Rec, 2018. © 2018 American Association for Anatomy.