Peru’s high Andes mountains are a hummingbird’s paradise, rich in wildflower nectar and poor in predators. But there is one problem: the cold.
Nighttime temperatures often drop below freezing in these rainy tropical highlands. How does a six gram bird that needs the nectar of 500 flowers a day just to survive get enough extra energy to keep warm all night long?
This is not the case.
Instead, as temperatures drop with the sun, these hummingbirds enter a state of suspended animation called torpor. One species, the black metaltail (Metallura Phoebe), chills at 3.26 ° Celsius, the coldest body temperature on record in a non-hibernating bird or mammal, researchers report Sept. 9 in Biology letters.
“They are cold as a rock,” says Blair Wolf, a physiological ecologist at the University of New Mexico at Albuquerque. “If you didn’t know better, you’d think they were dead.” Cooling to near-death temperatures allows hummingbirds to save valuable energy, allowing them to survive the cold night and prepare for food the next day, Wolf says.
Torpor had been seen in hummingbirds before, 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 cesspools.
Perched and erect, the birds pointed their beaks upwards, ruffled their feathers, and stopped moving. All species went into a sort of torpor, but the black metal tail cooled the most, from a daytime temperature of around 40 ° C to just above freezing.
During the day, the tiny but mighty hearts of these hummingbirds can beat 1,200 times per minute to fuel their hectic lifestyles. But during torpor, their heart rate drops to 40 beats per minute. “It’s an incredible drop,” says Wolf, and it could allow these high-altitude birds to reduce their energy use by about 95%. By not wasting energy trying to stay warm, these birds can thrive up to 5,000 meters above sea level. “It’s a remarkable adaptation.
As the sun rises, hummingbirds begin to run wild, heating up about a degree per minute by vibrating their muscles. “You see the bird shaking over there, then all of a sudden his eyes open and he’s ready to go,” Wolf says.