Copying the Seagull’s Leg

A seagull does not freeze, even while standing on ice. How does this creature conserve its body heat? Part of the secret is in a fascinating design feature found in a number of animals that dwell in cold regions. It is called the countercurrent heat exchanger.

What is a countercurrent heat exchanger? To understand it, picture two water pipes strapped closely together. Hot water flows in one pipe, and cold, in the other. If both the hot water and the cold water flow down the pipes in the same direction, about half of the heat from the hot water will transfer to the cold. However, if the hot water and the cold water flow in opposite directions, nearly all the heat will transfer from the hot water to the cold.

When a seagull stands on ice, the heat exchangers in its legs warm the blood as it returns from the bird’s cold feet. The heat exchangers conserve heat in the bird’s body and prevent heat loss from its feet. Arthur P. Fraas, a mechanical and aeronautical engineer, described this design as “one of the world’s most effective regenerative heat exchangers.”¹³ This design is so ingenious that human engineers have copied it.