High in the Andes, thousands of meters above sea level, speedy hummingbirds defy near-freezing temperatures. These tiny flyers endure the cold with a counterintuitive trick: They lower their body temperature—sometimes as much as 33°C—for hours at a time, new research suggests.
“It’s a capacity that’s just incredible,” says Anusha Shankar, a physiological ecologist at the Cornell Lab of Ornithology, who was not involved with the study.
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.”
Wolf and his colleagues wanted to compare how different hummingbird species utilized torpor at higher elevations. So in March 2015, they traveled 3800 meters above sea level to the Peruvian Andes, where nightly temperatures dip near freezing. They captured 26 hummingbirds from six different species, including the 12-centimeter-long black metaltail (Metallura phoebe) and the giant hummingbird (Patagona gigas), which is nearly twice the size of the metaltail and is the largest member of the hummingbird family.
The team placed each bird in a small roosting enclosure near the campsite and inserted a tiny wire into their cloaca, an all-purpose hole that birds use to excrete waste, mate, and—in females—lay eggs. This wire tracked the birds’ body temperatures overnight, letting the scientists know when the animals cooled down and warmed back up.
Not only did every species of hummingbird go into torpor, but 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.
The closer a hummingbird’s internal temperature is to the outdoor air, the less energy they need to spend on any metabolic process, such as keeping warm and maintaining a normal heart rate, Shankar explains. Although hummingbird hearts can beat by 1000 to 1200 beats per minute in flight, this can slow to as low as 50 beats per minute in torpor, Wolf says.
Cooling down has a trade-off, though, as motionless birds make easy prey. Although this would be a big risk at lower altitudes, Wolf says, the high Andes are relatively predator free for these tiny birds. "They aren’t worth much as far as a meal goes.”
Wolf next wants to explore how the birds’ bodies adapt to such cool temperatures. In humans, doctors can artificially cool the body for a few hours, placing it in metabolic slow motion during cardiac surgery, but “hummingbirds are doing this naturally,” he says. “To do that every night is a pretty incredible feat.”