How waterbirds limit heat loss through their legs
In many waterbirds, arteries carrying warm blood towards the feet lie close to veins returning cooler blood. Heat passes between these neighbouring flows before reaching the surroundings. This countercurrent arrangement helps conserve body heat while allowing the feet to remain cooler than the body's core.
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Recover heat before it reaches the environment
A bird standing in cold water has exposed limbs through which heat could escape. Closely arranged arteries and veins create an opportunity to transfer heat internally.
Warm blood travelling down the leg gives heat to cooler blood returning upward. The blood reaching the feet is therefore cooler than it would otherwise be, while returning blood is warmed before reaching the body.
The goal is not to keep every part equally warm. Maintaining a cooler foot reduces the temperature difference between the foot and its surroundings, limiting further heat loss.
- Outgoing blood transfers heat to returning blood.
- Less heat reaches the exposed extremity.
- The body retains more of its thermal energy.
Blood-flow regulation and behaviour also matter. The arrangement supports cold tolerance but does not make every waterbird immune to freezing conditions.