Introduction

Foraging in both terrestrial and aquatic habitats exposes secondarily aquatic mammals to a range of environmental conditions not usually encountered by mammalian specialists. This is due in part to the very different physical properties of air and water. Depending on temperature and salinity, the density of water approximates 800 times that of air and its viscosity is nearly 60 times that of air. The heat capacity of water is approximately 3500 times higher than air and its heat conductivity 24 times greater. Consequences of these physical properties are different locomotor and thermoregulatory demands for runners and swimmers (Dejours, 1987). Remarkably, semi–aquatic mammals have maintained lifestyles that require locomotor and thermoregulatory proficiency in these two disparate environments.

Morphological specialization is one obvious way in which mammals have responded to the challenges presented by terrestrial and aquatic environments. Several notable trends include body streamlining and enlargement of the propulsive surfaces in animals specialized for aquatic locomotion (Fish, 1993). Comparison of terrestrial, semi–aquatic and marine mammals (Fig. 2.1) shows that both features change in a predictable manner. Fineness ratio, an index of body streamlining (Hertel, 1966; Webb, 1975), approaches the optimum value of 4.5 as we move from secondarily aquatic mammals to obligate marine mammals (Fish, 1993; see Fig. 2.1). Many obligate aquatic mammals also show a reduction in limb length and expansion of the propulsive surface area. In contrast, modification of the appendages of semi–aquatic mammals may be limited by the dual role they must serve.