From land to water: evolutionary changes in long bone microanatomy of otters (Mammalia: Mustelidae)

Adaptation to an aquatic lifestyle has occurred independently in numerous amniote lineages. Such ecological shifts commonly engender adaptive changes in the musculoskeletal system. Semi-aquatic taxa highlight the compromises needed to enable locomotion both on land and in water and during the initial stages of the transition from land to water. Interestingly, mustelids include some semi-aquatic taxa that are primarily terrestrial and others that are primarily aquatic. Using microtomography, the inner bone structure of the whole stylopod from eight different mustelid taxa with distinct swimming abilities and ecologies was submitted to qualitative and quantitative analysis. Three main patterns in the three-dimensional inner organization of bone were identified, which illustrate three steps in the process of secondary adaptation to an aquatic lifestyle. These include a tubular organization in (primarily) terrestrial forms, a thickening of the cortex and an extension of the trabecular network in most otters, and a strong thickening of the cortex and a much denser and more widespread trabecular network in the almost exclusively aquatic sea otter. Microanatomical data are in agreement with morphological data and highlight consistency between the forearm and hindlimb patterns. The osteosclerotic pattern of the sea otter shows similarities to those of some aquatic amniotes, but additional comparative three-dimensional microanatomical data are required to elucidate its functional significance.