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.
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.
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 and the giant hummingbird , which is nearly twice the size of the metaltail and is the largest member of the hummingbird family.
Not only did every species of hummingbird go into torpor, but several reached surprisingly chilly temperatures. One black metaltail hummingbird'sBiology LettersOn 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.