Broadening the semiaquatic scene: Quantification of long bone microanatomy across pinnipeds

Investigations of bone microanatomy are commonly used to explore lifestyle strategies in vertebrates. While distinct microanatomical limb bone features have been established for exclusively aquatic and terrestrial lifestyles, identifying clear patterns for the semiaquatic lifestyle remains more challenging. Pinnipeds and mustelids provide an ideal framework for studying the diversity of semiaquatic adaptations. Here, we tested whether their humerus and femur showed significant differences in microanatomical and biomechanical parameters along the diaphysis relative to locomotor strategies. We μCT-scanned extant pinnipeds and included semiaquatic and generalist mustelids to build a reference for the amphibious lifestyle. Statistical analyses showed that comparisons away from the growth center distinguished species with non-weight-bearing hind limbs from those with all-limb weight-bearing. Among the latter, otariids and semiaquatic mustelids shared highly compact bones and a reduced medullary cavity, though otariids exhibited a more gradual medullocortical transition with more trabeculae. Contrary to the prevailing assumption, our findings indicate bone mass increase can also be associated with fast-swimming predators diving beyond shallow depths. Phocids, which cannot bear weight on their hind limbs, include species that spend 70%-80% of their lives at sea. These species exhibited extremely spongious bones: low compactness, thin cortex, and a trabeculae-filled medullary region-reminiscent of the osteoporotic-like condition seen in some extant cetaceans. Additionally, the shared humerus-femur pattern, regardless of locomotor type, suggests that skeletal adaptations can be systemic in carnivorans. This new comparative dataset broadens the spectrum of microanatomical patterns associated with an amphibious lifestyle and provides a foundation for better deciphering extinct species' locomotion.