Abstract

One of the most obvious adaptations of animals reintroduced to an aquatic environment is the difference in bone density. Numerous marine mammals and marine reptiles exhibit changes in bone density that correlate to their habitat (ecological niche) and dietary specializations, not phylogenetic relationships. Increased bone density (either pachyostosis, osteosclerosis, or pachyosteosclerosis) was observed early in the transition of terrestrial taxa to the aquatic environment. Animals such as early cetaceans and sirenians clearly exhibit these adaptive features and even retain many terrestrial characters such as hind limbs and behaviors such as paddle swimming). The increase in bone density is a more energetically efficient hydrostatic mechanism for buoyancy for marine mammals with large lung volumes. As the taxa became more specialized for the aquatic environment morphologically (evolving fins, flippers, and flukes) and behaviorally (evolving an oscillating swimming mechanism), variation in bone density correlates with their ecological niche. Modern sirenians retain increased bone density, allowing these large-sized mammals to remain submerged in shallow waters to feed on sessile littoral foods (sea grasses). However, the bone density in modern cetaceans became more osteoporotic, allowing them to swim faster and hunt faster moving prey. Pinnipeds live in a wide range of habitats (from cold to warm waters) and demonstrate varying feeding mechanisms, ranging from filter feeding on krill, bottom feeding on mollusks, and/ or catching fast moving prey. Bone density is one vital character that can be used to predict the specific ecological niche and feeding preference for pinnipeds. Some early hominids have been shown to have an increase in bone density. These heavier, thicker bones would make it easier for early Homo to hunt in waters for littoral food sources and would compensate for the lack of stability from bipedalism.

Keywords: Pachyosteosclerosis; Osteosclerosis; Pachyostosis; Bone density