One set of genes that the birds seem to leave untouched are those responsible for their internal clock. “It’s important for them to do things at the right time when they’re in torpor,” Shankar says. To be ready to meet the day, for example, the birds begin to rouse from their torpor about an hour before sunrise, well before visible light cues.
Deal with the sugar
To fuel their sky-high metabolic rate, hummingbirds suck down about 80 percent of their body weight in nectar each day. That’s the equivalent of a 150-pound person drinking nearly a hundred 20-ounce Cokes daily—and nectar is often much sweeter than a soda.
The human gut is incapable of absorbing sugar that fast, which is one reason why consuming too much soda or Halloween candy upsets the stomach, says Ken Welch, a comparative physiologist at the University of Toronto at Scarborough. Hummingbirds cope with the onslaught by having leaky guts so that sugars can enter the bloodstream between gut cells instead of only through them. This gets sugar out of the gut quickly, before it can cause upset. That rapid transport, and probably other adaptations as well, allows hummingbirds to reach blood sugar levels as much as six times higher than those seen in people, Welch says.
That much sugar in the blood leads to serious physiological problems in people. It causes more sugar molecules to glom onto body proteins, a process known as glycation; in the long run, excess glycation causes many of the complications of diabetes, such as nerve damage. It’s still unclear how hummingbirds avoid the problems of glycation, Welch says, but clues are beginning to emerge. One study, for example, found that bird proteins contain fewer of the amino acids most prone to glycation than mammal proteins, and those that remain are often tucked deep within the protein where they’re less exposed to circulating sugars.
Other, as yet unknown strategies to cope with high blood sugar may one day yield practical benefits for managing diabetes in people. “There could be a gold mine in the genome of the hummingbird,” says Welch.
Do a metabolic flip
By the end of its nightly fast, a hummingbird has nearly depleted its sugar stores—which poses an opposite metabolic challenge. “How does it wake up and fly?” Welch asks. “There’s nothing but fat available to burn.”
Hummingbirds have evolved to be remarkably nimble at switching their metabolism from sugar-burning to fat-burning, he has found. “This requires an enormous shift in the biochemical pathways that are involved,” Welch says—and it happens in mere minutes, far more quickly than other organisms can manage. “If we could have that kind of control over our fuel use, we’d love that.”
Save water—or not
Sugar isn’t the only challenge posed by a nectar-rich diet. After all, nectar is mostly water—and birds that drink in so much liquid must get rid of most of it, without losing electrolytes. As a result, hummingbird kidneys are highly adapted to recapture electrolytes before they are excreted. “They pee almost distilled water,” says Carlos Martinez del Rio, an ecophysiologist now retired from the University of Wyoming.
But that brings a further problem: If a hummingbird kept producing dilute urine overnight, it would die of dehydration before morning. To avoid that, hummingbirds shut down their kidneys every night. “They go into what, in a human, would be considered acute renal failure,” says Martinez del Rio. “Hummingbirds have to do this, or they would piss themselves to death.”
Fly high—gradually
The metabolic demands on a hummingbird are tough enough at sea level. But many species live at high elevations, where thin air contains less oxygen and offers less resistance to push against when hovering. Consider the giant hummingbird, the world’s largest, which can live in the Andes Mountains at elevations over 14,000 feet—higher than many helicopters can fly. To cope with these conditions, the birds have evolved more hemoglobin-rich blood, says Jessie Williamson, an ornithologist at Cornell University.
But some of the birds face an even steeper challenge, as Williamson found. Giant hummingbirds are large enough that researchers can attach satellite tracking tags, as well as smaller geolocators. So Williamson and her colleagues decided to fit the birds with trackers. After thousands of hours spent trying to capture birds with netting, the researchers managed to attach trackers to 57 birds using custom-made harnesses of elastic jewelry cord.
Though they recovered tracking data from only eight birds, even that tiny sample had a big surprise: Some of the birds lived in the high Andes year-round, while others—which turned out to be a separate, hitherto unrecognized species—migrate to the Andes annually from breeding grounds along the coast of Chile. That means they face not only the obvious challenges of a long migration—a round trip of roughly 5,000 miles—but also the need to adapt to thinner air as they travel.
Their secret? Do it gradually. “It looks a lot like how human mountaineers summit something like Mount Everest, with bursts of climbing and pauses to acclimatize,” Williamson says. “The journey takes months.”
As tracking technology becomes lighter and cheaper, researchers like Williamson hope to follow smaller hummingbird species as well. That, together with other progress in research technology, may offer plenty of new surprises about the biology of these tiny, amazing birds.
Bob Holmes is a science writer based in Edmonton, Canada, and a special contributor for Knowable Magazine.
This article originally appeared in Knowable Magazine, a nonprofit publication dedicated to making scientific knowledge accessible to all. Sign up for Knowable Magazine’s newsletter.
Knowable Magazine, 2024. DOI: 10.1146/knowable-092524-1 (About DOIs)
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