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To survive frigid nights, hummingbirds cool themselves to record-low temperatures
Among vertebrates, hummingbirds have the highest metabolism for their size. With a metabolic rate roughly 77 times that of an average human, they need to feed nearly continuously. But when it gets too cold or dark to forage, maintaining a normal body temperature is energetically draining. Instead, the small animals can cool their internal temperature by 10°C to 30°C. This slows their metabolism by as much as 95% and protects them from starvation, says Blair Wolf, a physiological ecologist at the University of New Mexico, Albuquerque.
In this state, called torpor, a bird is motionless and unresponsive. “You wouldn’t even know it was alive if you picked it up,” Wolf says. But when the morning comes and it’s time to feed, he says, the birds quickly warm themselves back up again. “It’s like hibernation but regulated on an even tighter schedule.”
.....several reached surprisingly chilly temperatures. One black metaltail hummingbird’s body temperature dipped to 3.3°C, the lowest ever recorded in birds or nonhibernating mammals, the researchers report today in Biology Letters. (The Arctic ground squirrel, which hibernates for weeks at a time, can lower its body temperature below freezing.)
On average, hummingbirds in torpor reached body temperatures of 5°C to 10°C, 26°C or more lower than when they are active. In humans, when body temperature drops by just 2°C, we become hypothermic.
sciencemag.org |
