Waterproof Fur

One of my programs for Third Grade classrooms is called "Fur, Feathers, Skins, and Scales".  It is a comparison of different types of body coverings found on Michigan's native animals.  Different types of body coverings have evolved to perform different jobs.  One of the things that we talk about is the idea that aquatic mammals often have "waterproof" fur that keeps the animals warm and keeps their skin dry even when swimming in ice cold water.  For example, this Muskrat (Ondatra zibethicus) climbed onto a shelf of ice after swimming  in a partially frozen river, shook like a dog and looked almost completely dry within seconds.

Muskrat looking completely dry seconds after emerging from a partially frozen river

On this Muskrat (below) you can see the water beading up on its fur.

A Muskrat (Ondatra zibethicus) with water beading up on its fur

Besides the Muskrat, two of the animals that I use to demonstrate this are the North American Beaver (Castor canadensis) and the North American River Otter (Lontra canadensis).

Beaver pelt

Otter Pelt

Both the Beaver and River Otter (and the Muskrat) have two layers of fur: an outer layer of long straight guard hairs and a dense interwoven inner layer.  The hair of both animals is oily and this combination of dense oily fur prevents most water from reaching through to their skin.  Both the Beaver and Otter groom often to keep their fur clean of debris and to spread oil (from glands located near their tail) evenly over their fur.

Looking closely at the fur, the beaver's fur is nearly twice as long as that of the otter.  Many of the outer guard hairs on the beaver pelt are more than an inch long.

None of the otter hairs measure more than a half an inch long, but the otter has many more hairs per square inch of skin than the beaver.  Both furs are equally good at repelling water.

Beaver fur (above) & otter fur (below)

In addition to the oil that the animals rub on their fur, the secret to waterproofing lies in the structure of the fur.  Looking at the hairs under a microscope, the hair of the beaver and otter look remarkably similar.  The guard hairs are straight and relatively thick.  The underfur is thinner than the guard furs and is wavy.  These wavy hairs interlock easily with each other and form a dense interwoven mat.  It is this dense matted layer, combined with the oil rubbed into the fur, that forms the waterproof barrier that allows these animals to swim in near-freezing water without becoming cold.

Beaver fur (magnified 2Xs) - the straight hairs are the outer guard hairs, the wavy hairs form the underfur

Beaver fur - another closeup at 2X magnification

Otter fur (2X magnification) - thick straight guard hairs and wavy underfur

Otter fur (2X magnification) - notice how the wavy underfur interlocks forming a matted layer