In the Andes mountains in South America, miles above sea level, hummingbirds have to survive in harsh conditions. At night the temperatures often drop below freezing, which is very sensitive for the small birds. To survive those conditions, the birds do something remarkable: they do not warm themselves up, but just cool down. Sometimes they even cool themselves with more than 30 degrees Celsius, for hours at a time. Scientists from the University of New Mexico write this in a paper in Biology Letters.
We already knew that hummingbirds have the highest metabolism of vertebrates, as much as 77 times that of an average human. That is why the birds have to eat almost continuously. When it cools down in the Andes at night, hummingbirds have a hard time maintaining their normal body temperature. That takes too much energy to sustain. In order to survive the freezing cold at an altitude of 3,800 meters, the birds developed a remarkable skill over the centuries. They cool themselves down to extremely low temperatures.
Hummingbirds do this to slow their metabolism by as much as 95 percent. This also prevents them from getting hungry. The closer their own body temperature is to that of the outside air, the less energy they have to put into things like staying warm and maintaining a normal heart rate.
(Alarming WWF Report: 68 percent of animals surveyed have disappeared since 1970.)
In numbers, the hummingbirds’ performance is nothing short of spectacular. Their heart rate slows down from 1,200 beats per minute to barely 50. Their body temperature drops to an average of 5 to 10 degrees Celsius. The black metal-tailed hummingbird even reached 3.3 degrees Celsius, according to the researchers. That is the lowest body temperature ever measured in birds. During the day, the body temperature of the hummingbirds is on average 26 degrees higher. By way of comparison: in humans there is already hypothermia with a drop of 2 degrees Celsius.
The researchers speak of a mini hibernation. The hummingbirds do not move a feather and do not respond to any impulses from their environment. This is not without risk, because the birds are easy prey for predators at night. However, relatively few predators swarm so high in the mountains.
In the morning, temperatures in the Andes soar. The birds then quickly warm up and start another day filled with uninterrupted meals.