Correlation of Bone Density in Semi‐Aquatic and Aquatic Animals with Ecological and Dietary Specializations

Abstract

A successful transition from a terrestrial to an aquatic environment requires the acquisition of unique adaptations that fit the distinct habitat. Pachyosteosclerosis is a combination of pachyostosis (thickening of cortical bone) and osteosclerosis (thickening of medullary bone). This accumulation of bone increases the density and weight of the animals that exhibit these characters. Pachyosteosclerosis is commonly found in aquatic and semiaquatic animals such as sirenians (sea cows), odobenids (walruses), otariids (sea lions), phocids (true seals) and cetaceans (whales). Increased bone density occurred early in the transition from a terrestrial to a marine environment, present before many of the other morphological characters such as flukes, flippers and fins and behavioral characters such as oscillatory swimming associated with aquatic life appear. Pachyosteosclerosis helps to counteract buoyancy, allowing taxa to remain submerged longer in order to feed while expending little energy. This is clearly demonstrated in sirenians, which are known to feed on sea grasses in shallow waters near the coastline. In most other aquatic mammals, such as modern cetaceans, pachyosteosclerosis is followed by an osteoporosis‐like bone lightening for increased swimming speeds that allows for the active hunting of fast‐moving prey. Pinnipeds live in a wide range of habitats (from cold to warm waters, from marine to fresh water) and demonstrate varying feeding mechanisms, ranging from filter feeding to bottom feeding to catching fast prey. Increased bone density is seen in multiple species of seals in different geological ages from the Miocene (~ 23mya) to present. The morphological and ecological significance for seals with thicker bones is that they swim slower and hunt in shallower water, while seals with lighter bones swim faster and hunt fast moving prey. Bone microstructural disparity allows different species of seals to successfully live in sympatric habitats due to their ecological and dietary specializations. Bone density is one vital character that can be used to predict the specific ecological niche and feeding preference for pinnipeds. Some early hominids, such as Homo erectus, have been shown to have increased density in the bones also. These heavier, thicker bones would make it easier for early Homo to hunt in coastal waters for littoral food sources and would compensate for the lack of stability from bipedalism.Support or Funding InformationFunding Provided by Torres Advanced Enterprise Solutions LLC and Just‐Julian Fellowship