Deal with the sugar<\/h2>\n
To fuel their sky-high metabolic rate, hummingbirds suck down about 80% of their body weight in nectar each day. That\u2019s the equivalent of a 150-pound person drinking nearly a hundred 20-ounce Cokes daily\u2014and nectar is often much sweeter than a soda.<\/p>\n
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.<\/p>\n
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\u2019s 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\u2019re less exposed to circulating sugars.<\/p>\n
Other, as yet unknown strategies to cope with high blood sugar may one day yield practical benefits for managing diabetes in people. \u201cThere could be a gold mine in the genome of the hummingbird,\u201d says Welch.<\/p>\n
Do a metabolic flip<\/h2>\n
By the end of its nightly fast, a hummingbird has nearly depleted its sugar stores\u2014which poses an opposite metabolic challenge. \u201cHow does it wake up and fly?\u201d Welch asks. \u201cThere\u2019s nothing but fat available to burn.\u201d<\/p>\n
Hummingbirds have evolved to be remarkably nimble at switching their metabolism from sugar-burning to fat-burning, he has found. \u201cThis requires an enormous shift in the biochemical pathways that are involved,\u201d Welch says\u2014and it happens in mere minutes, far more quickly than other organisms can manage. \u201cIf we could have that kind of control over our fuel use, we\u2019d love that.\u201d<\/p>\n
Sugar isn\u2019t the only challenge posed by a nectar-rich diet. After all, nectar is mostly water\u2014and 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. \u201cThey pee almost distilled water,\u201d says Carlos Martinez del Rio, an ecophysiologist now retired from the University of Wyoming.<\/p>\n
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. \u201cThey go into what, in a human, would be considered acute renal failure,\u201d says Martinez del Rio. \u201cHummingbirds have to do this, or they would [pee] themselves to death.\u201d<\/p>\n
Fly higher\u2014gradually<\/h2>\n
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\u2019s largest, which can live in the Andes Mountains at elevations over 14,000 feet\u2014higher 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.<\/p>\n
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.<\/p>\n
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\u2014which turned out to be a separate, hitherto unrecognized species\u2014migrate 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\u2014a round trip of roughly 5,000 miles\u2014but also the need to adapt to thinner air as they travel.<\/p>\n
Their secret? Do it gradually. \u201cIt looks a lot like how human mountaineers summit something like Mount Everest, with bursts of climbing and pauses to acclimatize,\u201d Williamson says. \u201cThe journey takes months.\u201d<\/p>\n
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.<\/p>\n