Hummingbirds live a life of extremes. The flitting creatures famously have the fastest metabolisms among vertebrates, and to fuel their zippy lifestyle, they sometimes drink their own body weight in nectar each day. But the hummingbirds of the Andes in South America take that extreme lifestyle a step further.
Not only must they work even harder to hover at altitude, but during chilly nights, they save energy by going into exceptionally deep torpor, a physiological state similar to hibernation in which their body temperature falls by as much as 50 degrees Fahrenheit. Then, as dawn approaches, they start to shiver, sending their temperatures rocketing back up to 96 degrees.
It’s an intense process, says Andrew McKechnie, a professor of zoology at the University of Pretoria in South Africa.
“You’ve got a bird perching on a branch, whose body temp might be 20 degrees Celsius,” or 68 Fahrenheit, he said. “And it’s cranking out the same amount of heat as when it is hovering in front of a flower.”
Now, Dr. McKechnie and colleagues reported on Wednesday in Biology Letters that the body temperatures of Andean hummingbirds in torpor and the amount of time they spend in this suspended animation vary among species, with one particular set of species, particularly numerous in the Andes, tending to get colder and go longer than others. They also report one of the lowest body temperatures ever seen in hummingbirds: just under 38 degrees Fahrenheit.
On a trip to the Andes about five years ago, Blair Wolf, a professor of biology at the University of New Mexico and an author of the new paper, and his colleagues captured 26 of the little birds for overnight observation. They measured the hummingbirds’ body temperatures as they roosted for the night and found that almost all of them entered torpor, showing a steep decline in temperature partway through the night.
They also kept track of the birds’ weights, because hummingbirds, like many other birds, lose weight between dusk and dawn, as they burn through the calories they have consumed during the day.
The researchers were curious whether the birds were adjusting their temperatures to be close to the ambient air temperature. They also wondered whether the torpor of different species — six were represented — would look different, and whether longer, deeper torpor was connected with losing less weight.
“The extent to which birds can save energy by going into torpor might well affect how well they do at these high altitudes,” Dr. McKechnie said.
Indeed, they found that birds that used torpor only briefly could lose as much as 15 percent of their body weight. Birds who took a longer break, on the order of 12 hours, lost only 2 percent. Birds that reached lower temperatures lost a smaller percentage, too.
Some species, like the sparkling violetear, descended to a set temperature (in this case, roughly 46 degrees Fahrenheit) regardless of the ambient temperature. Others, like the black metaltail, seemed to be tracking the air and got very cold. One metaltail hovered around 38 degrees Fahrenheit, scoring the lowest recorded temperature of any hummingbird, to the researchers’ knowledge.
In fact, the metaltail, the black-breasted hillstar and the bronze-tailed comet, which are related species, all entered colder, longer bouts of torpor than the others. This could help explain why this group is more common at high altitudes — they have worked out ways to minimize the stress of living in an extreme environment.
These birds were held in captivity overnight, but Dr. McKechnie says he thinks that in a natural setting, there is more to learn about how hummingbirds save energy.
There are stories of hummingbirds in the Andes that will enter a cave during cold spells and not emerge for several days, a pattern that, if confirmed, would suggest that the birds are capable of hibernation, he notes. Similar to torpor, hibernation saves an organism energy, but it goes on longer than a single night.
“For me, the next step beyond this study would be to get a clear idea of where they roost,” Dr. McKechnie said.
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