A penguin, a sled dog, and a manatee walk into a diner…

Well, maybe not. But even if they don’t frequent the same restaurants, they do share similar adaptations that help them overcome the challenges of life in cold environments.

As veterinarians, we’re often asked how dogs can tolerate walking barefoot on snow and ice. Accomplishing this is actually a two-part challenge: keeping the feet warm enough to prevent tissues from freezing but also minimizing heat loss so the body’s core temperature doesn’t drop. While there is an insulating layer of fat under a dog’s foot pads (think blubber on whales and seals), their key feet-feature is a specialized circulatory mechanism called a countercurrent heat exchanger.

Arteries, the type of blood vessels that carry blood away from the heart, transport heated blood to the extremities. The veins, which flow in the opposite direction, carry cooled blood back to the heart. Unlike other parts of the body where these vessels run separately, in cold resistant extremities (like a dog’s paw, a penguin’s foot, or a manatee’s tail) these vessels are tightly entwined, allowing heat to be exchanged by blood flowing in opposite directions (hence the term, countercurrent heat exchanger). As a result, heat from the arteries can be transferred directly to the veins to buffer the overall temperature of the foot and prevent freezing. Furthermore, while the normal physiologic response to cold is to decrease blood flow to the extremities to concentrate heat in the vital organs, because the venous blood returning to the body’s core gets a temperature boost, this arrangement allows maintenance of blood flow to the feet while decreasing impact on overall core temperature.  

So what then of the colorful booties the dogs wear? In reality, their primary purpose is for protection rather than retaining heat.  While the booties are useful to keep ice from building up between the dogs’ toes and to prevent foot abrasions from rough trail conditions, it is the extra warmth created by the countercurrent heat exchanger that allows them to travel comfortably despite the cold temperatures. So the next time you see a sled dog, a penguin, or a manatee, you can truly appreciate the incredible adaptations they have developed to survive and thrive in their unique environments.