The high Andes mountains of Peru are a hummingbird’s paradise, rich in wildflower nectar and low in predators. But there’s one problem: the cold.
Nighttime temperatures often dip below freezing in these rainy tropical highlands. How does a six-gram bird that needs nectar from 500 flowers a day just to survive get enough extra energy to keep itself warm all night?
Instead, as temperatures drop with the sun, these hummingbirds enter a state of suspended animation known as torpor. One species, the black metaltail (Metallura phoebe), chills to 3.26° Celsius, the coldest body temperature ever recorded in a bird or non-hibernating mammal, researchers report September 9 in Biology Letters.
“They’re cold as a rock,” says Blair Wolf, a physiological ecologist at the University of New Mexico in Albuquerque. “If you didn’t know better you’d think they were dead.” Cooling to near-death temperatures lets the hummingbirds save precious energy, allowing them to survive the cold night and gear up to feed the next day, Wolf says.
Torpor had been observed before in hummingbirds, but Wolf and his colleagues wanted a more detailed picture. They placed 26 individuals of six different species in cages overnight and inserted the equivalent of miniature rectal thermometers into their cloacas.
Perched and upright, the birds pointed their bills upwards, fluffed their feathers and stopped moving. All of the species entered some kind of torpor, but the black metaltail cooled the most, dropping from a daytime temperature of about 40° C to just above freezing.
During the day, these hummingbirds’ tiny-yet-mighty hearts can beat 1,200 times a minute to power their frenetic lifestyle. But during torpor, their heart rates plummet to as low as 40 beats a minute. “It’s an astounding drop,” Wolf says, and it could allow these high-altitude birds to cut their energy use by about 95 percent. By not wasting energy trying to stay warm, these birds can thrive as high as 5,000 meters above sea level. “It’s a remarkable adaptation.”
Around sunrise, the hummingbirds start revving up, warming about a degree a minute by vibrating their muscles. “You see the bird quivering there, then all the sudden its eyes pop open and it’s ready to go,” Wolf says.