Like other otter species, the North American river otter (Lontra canadensis) has a long, sinuous, streamlined body, highly modified for aquatic life. The limbs are short and powerful, the clawed paws are fully-webbed, and the flattish head terminates in a broad muzzle equipped with numerous sensitive whickers used for detecting prey underwater. To propel itself through the water, this species primarily combines hind-limb paddling with vertical undulations of its remarkably long, tapering tail. The body is covered in densely packed underfur, overlaid with longer guard hairs that trap a layer of insulating air when the otter is swimming underwater. Generally the sleek outer fur is brown to velvety black above, and paler greyish-brown to silver underneath, but there is considerable regional variation in this species’ appearance, with numerous recognised subspecies. Although the sexes are identical in overall appearance, the males are, on average, around five percent larger and darker in color than the females.