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Hummingbirds can enter a hibernation-like state at night, dropping their body temperature to under 4掳C in an effort to preserve energy for use during the day.

The birds are among the few animals, including night hawks and some small rodents, that are capable of torpor, in which body function reduces to a bare minimum for a few hours, says Blair Wolf at the University of New Mexico.

Previous studies have found that hummingbirds can drop their body temperature from 40掳C to about 17掳C at night, but now Wolf and his colleagues have found that some species can go much further, reaching 3.26掳C. 鈥淵ou鈥檇 think they鈥檙e frozen,鈥� he says. 鈥淭hey feel like a cold rock.鈥�

By day, the birds expend massive amounts of energy, hovering while consuming nectar from 400 to 600 flowers. At night, they can reduce energy expenditure by upwards of 95 percent. Even their heart rate drops, falling from 1000 beats per minute down to around 50, says Wolf.

He and his team studied six species of hummingbirds thriving at up to 4000 metres above sea level in the Peruvian Andes. They trapped individuals overnight and inserted tiny probes into the cloaca (hind orifice) to monitor body temperatures from evening to morning, including the 12 night-time hours when outdoor temperatures dropped to between 2 and 6掳C.

Most of the birds, representing all six species, entered torpor in bouts lasting from two to almost 13 hours, says Wolf. Body temperatures varied, but it was the black metaltail species that came closest to outdoor temperatures, hitting a low of 3.26掳C.

At around sunrise, the birds started quivering, increasing body temperature by more than 1掳C per minute. Although reheating requires a lot of energy, the amount saved by staying in torpor exceeded this cost. The team weighed the birds and found that those spending less time in torpor lost more weight, indicating they had used more energy in the night.

Even so, torpor doesn鈥檛 come without a price, says Wolf, as the birds鈥� inert state makes them easy targets for predators. High up in the tropical Andes, however, 鈥渋t鈥檚 a pretty predator-free environment鈥�, he says. 鈥淭his is an adaptive strategy that works for them.鈥�