Why does a Walrus Blush?
If you ever get a chance to be airborne above the Bering Sea on a sunny early summer day, you may be lucky and see small groups of large marine mammals hauled out on ice floes drifting north. If you head south around Nunivak Island and east into Bristol Bay, you will spot a larger congregation of these pink colored animals hauled out on an island off the coast near Togiak. As you begin to feel chilled in the plane, you might wonder how these apparently bare-skinned mammals can freely move from seawater to land and migrate north each year on snow-covered ice floes into the Chukchi Sea. These remarkable pinnipeds of the Arctic are walruses (Odobenus rosmarus), and they have evolved specialized anatomical and physiological adaptations to maintain a healthy body temperature in extreme environmental conditions.
How can nearly hairless walruses be exposed to the extreme temperature gradients between the seawater and the atmosphere and still maintain their thermal balance? Bald whales stay warm in cold seawater, but they do not crawl up onto ice floes into air at forty below nor bask on sunny rocks that may be 50 degrees F warmer than the sea. A polar bear can live comfortably in the same conditions that a walrus does, but polar bears have thick insulating pelts. The suspicion is that walruses keep a steady body temperature by - in effect - turning portions of themselves into blood powered radiators.
Various investigators have suggested that the circulation in the skin and hind flippers of pinnipeds plays an important role in maintaining the animal's thermal equilibrium. The skin of walruses becomes pallid when it is exposed to cold air or water and changes to pink when exposed to warmer ambient temperatures. This color change may indicate that the circulatory output to the skin is altered to control their total body heat loss.
Within their skin, arterio-venous anastomoses (AVAs), or connections between small arteries (arterioles) and small veins (venules) which can allow arterial blood to enter the venous circulation without passing through the capillary beds), are suspected to be important in thermoregulation. People feel the action of AVAs in their hands - sometimes painfully - when cold fingers heat up.) Specifically, AVAs may act as heat dissipation structures in the subcutaneous layers of the pinnipeds' general body surface and hind flippers by providing low-resistance pathways for high blood flow. The ability of walruses to maintain a certain temperature differential between their deep body temperature and their surface temperature depends on the insulative quality of their fat, the thickness of their skin and the skin blood flow (depending on the density and distribution of the dermal AVAs.) My hypothesis: given that a warm walrus can pretty well open up its surface vascular radiator, then the amount of heat emitted from the skin of walruses will be proportional to the density of AVAs in the dermal vasculature. "Hot spots" indicate many AVAs.
I am studying the structure, density and distribution of AVAs in skin samples I collected last spring from the walruses harvested by the Eskimo hunters of Little Diomede Island. This summer I head off to Round Island, one of the Walrus Islands in Bristol Bay, with an Everest model 12 portable infrared thermometer. This device will permit me to measure from a distance the surface temperature of the body and hind flippers of marked animals as they haul out on land. A skin anatomy/field work thermoregulation study . . . to be continued!
