Smallest aquatic mammal

The world's smallest aquatic mammal is the American water shrew Sorex palustris, native to streams and lake fringes in the mountain ranges of contiguous USA, Alaska, and Canada. It measures 130–170 mm in total length (of which 57–89 mm is accounted for by its tail), weighs 8–18 g, and adult males are larger than females. It will dive for up to 15 seconds (although in experimental conditions it has been seen to survive dives over 40 seconds), but while diving it needs to swim continuously, otherwise its very light weight means that it would rise to the surface.

There are several species of water shrew, accommodated in four different genera. These are: Sorex, containing the three species of American water shrew (the other two being only marginally larger than S. palustris); Neomys, containing the three Old World species of true water shrew; Nectogale, containing only the Chinese elegant water shrew; and Chimarrogale, containing at least six species of Asian water shrew. All species in the latter three genera are notably larger than S. palustris.