Hummingbirds have scooped another record: they are not only tiny but can reach body temperatures below that of any non-hibernating mammal and any other bird.
The hummingbird is among a number of small creatures, including certain bats, that can enter a state known as daily torpor, a phenomenon where they turn down their metabolism and body temperature to save energy.
Unlike hibernation, this is not a prolonged state: after a night in torpor, the hummingbird’s metabolic rate rises again, with its body temperature reaching around 40C.
Now researchers studying six species of the bright birds in the Andes, around 3,800 metres above sea level, have found the body temperature of the black metaltail hummingbird can fall to as low as 3.3C.
Prof Andrew McKechnie, of the University of Pretoria, South Africa, a co-author of the study, said torpor was vital to hummingbirds’ survival, as they would have had to burn large amounts of energy to maintain a body temperature of around 40C during the cold Andean nights. “They wouldn’t be able to store up enough fat at the end of the day to provide sufficient fuel to last them for the entire night,” he said.
McKechnie said with the new study revealing that the body temperatures of hummingbirds could fall to those more characteristic of hibernating animals, it could be that the diminutive creatures were not only able to enter torpor, but to hibernate. “It would be big news if they did,” said McKechnie. “Only one avian hibernator has ever been reported.”
McKechnie said the mechanisms behind torpor were of interest for biomedical applications. “At one stage Nasa was quite seriously asking whether it would be possible to induce a torpor-like state, or hibernation-like state, in humans, in order to get beyond the vicinity of Earth,” he said.
Writing in the journal Biology Letters, McKechnie and colleagues reported how in Peru in March 2015 they captured 26 hummingbirds of six different species and placed them in tents without food for at least one night, tracking changes in body temperature and mass. Air temperatures fell to as low as 2.4C.
The team found that 24 of the 26 birds, covering all six species, entered torpor; however their lowest body temperature varied between individuals and between species.
The body temperature of one black metaltail hummingbird fell to just 3.3C which, they said, was not only a record for hummingbirds, and indeed all birds, but was lower than the record for non-hibernating mammals. The previous record for birds was 4.3C, reported for the common poorwill, the only species of bird known to hibernate.
The time in torpor varied: the giant hummingbird spent on average 5.7 hours in torpor at night, compared with 10.6 hours for the black metaltail. The longer hummingbirds spent in torpor, the lower their loss of body mass.
McKechnie said further analysis showed that the differences in torpor between the various species was, at least in part, rooted in evolution. “You have different species using different [torpor] patterns, despite them all being in common conditions at the same site, experiencing the same weather,” he said.