1 - Adaptations to the semi-aquatic habit and habitat

Summary

Introduction

It has been asked by opponents of such views as I hold, how for instance, could a land carnivorous animal have been converted into one with aquatic habits; for how could the animal in its transitional state have subsisted?

Charles Darwin (1859), The Origin of Species

Mammals exhibit a fascinating array of adaptations that suit them to their chosen habit and habitats. Many species show an association with water but the aquatic environment is a challenging one and presents many problems for those mammals that have chosen to utilize it. Hence, many species show compromise in the extent of their adaptations to amphibious life. Eisenberg (1981) uses the term semi–aquatic to include those species that must spend part of each 24 hour period out of the water. I use the term ‘semi–aquatic’ mammals to exclude those species where the association with water is to a large extent obligatory; such a distinction is somewhat arbitrary, since even the most highly evolved semi–aquatic mammals (e.g. pinnipeds) retain some dependence on a terrestrial substrate for part of their life, and many species of otter (Lutrinae) seem to be inseparably tied to waterways despite being incompletely adapted. Semi–aquatic mammals are phylogenetically diverse; representatives are found in several mammalian orders, including Monotremata, Marsupialia, Insectivora, Artiodactyla, Carnivora and, most commonly, Rodentia. Species from some 24 families of mammals have an association with aquatic habitats, although they vary in the extent to which they treat water as a medium in which to forage or to escape from predators or simply to traverse.