Apr. 4, 2002 Scientists have long been fascinated with how hummingbirds -- one of Earth's smallest, warm-blooded vertebrates -- can nutritionally maintain their high-metabolic daily lifestyles, let alone fly thousands of miles during migration.
University of Arizona ecologist Todd J. McWhorter has examined a less-studied facet of these amazing creatures -- their ability to consume several times their body mass in water every day. Humans, rats, domestic pigeons and gray parrots would succumb to 'polydipsia,' or water intoxication, before they could drink amounts of water anywhere near their own body mass.
In field and lab experiments with hummingbirds in North America and Palestine sunbirds in Israel, McWhorter strives to discover unique physiological mechanisms that permit such tiny nectar-feeding animals to cope with all the water they consume.
Floral nectar is composed almost entirely of water and simple carbohydrates, laced with trace amounts of protein and electrolytes. Hummingbirds supplement their diet with insects for protein. But flower nectar is the main course.
"Hummingbirds can take in between four and five times their body mass in nectar every twelve hours," McWhorter said. " They have a water turnover rate higher than aquatic amphibia, and nearly as high as that of freshwater fish. How do they deal with all that water?"
One idea -- first proposed in 1990 by Carol Beuchat, then of San Diego State University, UA ecologist William Calder and UA physiologist Eldon Braun - was that the birds absorb perhaps only a small fraction of their dietary water in their guts, leaving the rest to pass quickly and directly to the cloaca.
McWhorter and Carlos Martinez del Rio of the University of Wyoming discovered that isn't the case in hummingbirds. But it does happen in Palestine sunbirds, McWhorter since found.
McWhorter and Martinez del Rio mathematically modeled water absorption in broad-tailed hummingbirds by injecting specimens captured in the field with tritium, a harmless radioactive marker. In the laboratory, where daylight hours were carefully controlled to coincide with the hummingbirds' natural light cycle, they then measured nectar intake, marker decline and body mass in the birds.
McWhorter discovered that broad-tailed hummingbirds absorb about 80 percent of their dietary water in their gastrointestinal tracts, water that hence must be processed by the kidneys. They found that hummingbird kidneys are extraordinarily well suited to eliminating water.
What's more, where other researchers have emphasized that lack of water constrains hummingbird behavior and habitat selection, McWhorter and his colleagues emphasize a novel aspect of water-energy interaction: water overbundance in food. Well suited as hummingbird kidneys are to excrete large volumes of dilute urine, they have their limits.
"We speculate that the rate at which hummingbird kidneys can process water may impose limits to energy intake," he reported in 1999 in the Journal of Experimental Biology.
In other words, not lack of water, but too much water limits how many calories the birds can consume. For example, when hummingbirds are challenged by cold temperatures, they can't consume enough energy to stay warm and maintain weight. They lose weight, McWhorter found.
He has spent eight months over the past two years studying the same issues of water absorption and kidney physiology in Palestine sunbirds in Israel's Negev Desert. The small (6 gram) iridescent blue/green sunbird is also a nectar eater but of a different scientific order than the hummingbird. "We want to see if these different sorts of birds are dealing with their watery diet in the same way," McWhorter said.
"It appears that some of these birds -- Palestine sunbirds, especially -- may pass some of the water through their bodies without absorbing it. We're not sure how they do it at this point, but they appear to do so."
Researchers believe that tree-sap feeding aphids rid their systems of excess water without absorbing it in their systems when they excrete sugary "honeydew." But it has never before been documented in a vertebrate, McWhorter noted.
He discovered that, unlike hummingbirds, as sunbirds increase their water intake, the fraction of the amount of water they absorb drops. "Sunbirds can avoid absorbing up to 60 percent of the water in their diet."
Sunbirds live in the Negev the year round, and so experience very cold conditions, McWhorter noted. If their energy intake depended on how much water their kidneys could process -- which is the case in hummingbirds -- sunbirds wouldn't survive.
The sunbirds could be modulating how much water they absorb in a couple of ways, he suggests. One is by "aqua porin" water channels, or protein channels in the cell membrane. Aqua porins, which are regulated by anti-diuretic hormones, control how much water enters the cell. He and a colleague at George Fox University in Oregon recently discovered aqua porins in hummingbird gut tissue, but have yet to find the details of how these function.
Another water-regulating mechanism might be a "paracellular pathway," or "between cell" pathway. A University of Wisconsin scientist has shown that some birds control nutrient intake by modulating the permeability of junctions between cells in the gut. (As an analogy, think of the junctions between cells lining the gut as the six-pack ring that holds soda cans together, McWhorter said. The birds modulate the tightness of the junctions, or the permeability of the six-pack ring.)
After he completes his UA doctoral degree, McWhorter says he may join the University of Wisconsin scientist in post-doctoral research on paracellular transport.
Or he may work with UA physiologist Eldon Braun on another question that intrigues him: How do hummingbirds deal with all the sugar they consume?
"Floral nectars average between 20 percent and 30 percent by weight sucrose. Hummingbirds have very high blood sugar. If you had the blood sugar of a hummingbird that has fasted, you'd be in a diabetic coma. But these birds have none of the eye, liver, kidney or other problems associated with diabetes. If we can learn how birds deal with high levels of blood glucose, we might understand more about how to deal with diabetes."
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