Eurasian Otter

Vision underwater is poor in silty water, so the whiskers do the work — stiff vibrissae that read the pressure wake a fish leaves behind. The guard coat traps a layer of air over dense underfur, and grooming is not vanity but maintenance of the only insulation the animal has; there is very little body fat.

British and western European populations collapsed from the late 1950s, and the cause was eventually traced to organochlorine pesticides accumulating up the food chain rather than to hunting or habitat alone. After those compounds were banned, otters recolonised catchment by catchment over four decades. Because they sit at the top of a freshwater food web, their presence is now used as a working readout of water quality.