Cold Toes

Day 134: Have you ever wondered why birds' toes don't suffer from frostbite except during extreme freezing events? The major arteries (carrying warm blood from the heart to the extremities) and veins (carrying cooled blood back to the heart) split into multiple smaller vessels at the leg and wing joints nearest the body and are closely intertwined to form a "re-heating" network. Outgoing blood transfers as much as 85% of its heat to the chilled blood returning to the body. This process is known as "countercurrent circulation." It has also been noted that birds in colder regions tend to have smaller beaks and feet, reducing the amount of exposure they receive. Some small birds (hummingbirds, for example) may enter a state of torpor to survive cold nights, with their normal body temperature dropping as much as forty degrees! In torpor, a hummingbird's heart rate may drop from 500 beats per minute to 50 or less. The bird may look like it's dying when in torpor, but in fact, it's executing a physiological strategy which helps it survive bitter weather.