Introduction

Water shrews are the smallest of amphibious mammals. They are insectivores belonging to the family Soricidae and, although the aquatic species are larger than most terrestrial species, they have a body length of only 70–130 mm and a body mass of 8–56 g. Thirteen species of water shrews are currently recognized (Wilson & Reeder, 1993), belonging to four genera (Table 4.1), and they have a wide geographical distribution through the Nearctic, Palaearctic and parts of south–east Asia where they are associated with freshwater streams and marshlands. Shrews have excited considerable interest because of their large energy demands, high levels of activity and voracious appetites. Water shrews, with their habit of diving in cold waters for aquatic prey, and the consequent energy costs of this, provide an additional dimension to the study of the physiology and ecology of very small mammals. However, compared with their terrestrial counterparts, remarkably little is known about water shrews, and many species have rarely been sighted or captured. This review investigates the geographical and habitat occurrence of water shrews and the use they make of the aquatic mode of life.

Anatomical adaptations to a semi–aquatic mode of life

Convergent evolution has occurred several times in the Soricidae, and adaptations for a semi–aquatic mode of life have developed in four different genera (Sorex, NeomySy Chimarrogale and Nectogale) and in two different continents. It is noteworthy that the genus Crocidura (to which some 125 of the 233 shrew species are ascribed) possesses no aquatic examples.