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Evolution

Wings are not enough

Birds adapt to lack of food and oxygen during flight

Fabio ColombiniAmerican woodstorks, common from the United States to ArgentinaFabio Colombini

Many fossils found in China in the last few years are helping us to understand better how and when birds appeared and started flying. One of the most recent ones, presented in September 2009 in Nature, is the Anchiornis, an animal with feathers and four wings that lived some 150 million years ago, 10 million years before the Archaeopteryx, which had been considered the earliest bird up to then. The Anchiornis (at least until an older fossil appears) started the formation of a group of animals characterized mainly by their ability to fly, sometimes over thousands of kilometers, like migratory birds. “Today, 90% of bird species fly,” says the biologist José Eduardo Bicudo, a professor at the Biosciences Institute at the University of São Paulo and the principal author of the book Ecological and environmental physiology of birds, published in February by Oxford University in England. His studies, along with those of other experts, indicate that birds became able to fly not only because of their wings and their feathers, suitable for flying, but also because they acquired physiological adaptations that enabled them to fly for several weeks at high altitudes, where there is little oxygen. This is far above the altitude humans can reach, other than by airplane.

“The physiological principle is simple: the less cargo carried during the trip, the better,” says Bicudo. Before departure, the muscles that help the animal to fly gain volume, but they atrophy later, during the course of the journey. Another unique feature is digestive efficiency: “Migratory birds can increase or diminish the production of digestive enzymes, depending on whether they have a lot of food or only a little. If no food is available, digestive system cells die and the digestive tract shrinks to half its initial volume. When the fast comes to an end, the intestines and the liver make new cells and reacquire their normal volume.”

The sight of birds of prey gliding over the Himalayas at 9 thousand feet may be a fine spectacle for us, but for the birds, it probably spells discomfort: at very high altitudes, it is extremely cold and there is little oxygen. “They overcome these difficulties through their respiratory efficiency,” Bicudo tells us. In an article published in 2006 in Integrative and Comparative Biology, Douglas Altshuler, from the California Institute of Technology, and Robert Dudley, from the Smithsonian Institute, described the physiological mechanisms that enable flight at high altitudes –  that extend beyond the air sacs (bags connected to the lungs and the bones that make the skeleton lighter). Birds’ lungs extract almost all the oxygen from air and bird hemoglobin has a greater capacity to connect and disconnect itself from oxygen than the human version.

Known for their sharp eyesight, birds can also have a relatively refined sense of smell. “Many species of marine birds detect dimethylsulfate, a substance generated by decomposing fish. They use this for navigation and to look for food. Albatrosses fly in a relatively random fashion until they find a school of fish exhaling dimethylsulfate,” Bicudo informs. Besides their well developed sense of smell, even though for decades it was disregarded, another surprising idea is that the brains of mammals and of birds have equivalent functional structures, despite being morphologically quite different – which means that saying someone is birdbrained, meaning unintelligent, is actually inaccurate. “Doves can memorize 400 color patterns,” Bicudo argues. It is also by means of their nervous system that birds detect the axis of the Earth’s magnetic field and identify north or south.

From the desert to the pole
These flight peculiarities, which even experts were unaware of until recently, explain why birds have spread over the entire planet, adapting to environments as diverse as the deserts and the freezing Poles. This group now has highly distinct species, ranging from the harpy eagle, whose wingspan may cover as much as 2.5 meters, to the saffron finch, smaller than an adult’s hand.

The older fossils confirm that dinosaurs gave rise to birds, which first appeared in low latitude equatorial regions, such as China and Brazil, even though the moist forest soil in the latter did not preserve the fossils. Bicudo believes that many species that now pass through our continent, such as the fish eagle, one of the 33 migrant species already seen in the São Paulo state savannas, actually originated here. Humming birds, which he has been studying for years, exemplify this irradiation: South America is home to some 90 species, Central America to no more than 15, and North America to 5 or 6. “Brazil is a depositary of hummingbirds,” he says.

In 2001, the biologist Claudia Vianna and he discovered that the swallow-tailed hummingbird’s pectoral muscle, which accounts for one third of the bird’s body volume, produces a protein, HmUCP, that allows the bird to re-warm itself rapidly and reach its most comfortable temperature within 30 to 40 minutes, before taking flight. At night, after a day of non-stop flying, the hummingbird’s temperature drops brutally: from 40o C to close to the ambient temperature – sometimes down to 15o C. The drama occurs the following day: when it wakes up, it needs to reach a temperature that will enable it to fly and resume its quest for food. After this, its wings start beating on average 700 times per minute and its heart, 1,400 times. Until this time, the equivalent proteins had only been identified in mammals (see Pesquisa Fapesp no. 69, October 2001). Bicudo and his team did not progress much along this line of work with the hummingbirds, as obtaining authorizations to take blood samples from them proved to be harder than catching them, but in 2005, Denise Loli and he found similar proteins, that helped to warm up the body, in bees of the Bombus genus and of the indigenous stingless bees of the Melipona genus, indicating, perhaps, that flying animals – birds, insects and bats – may have many physiological mechanisms in common.

The project
Comparative study of the interrelations of ontogenetic and environmental factors on the endothermy of the two-color Melipona of the Lepetier type (nº 2002/13973-2); Type Doctoral Grant (Denise Loli); Coordinator José Eduardo Pereira Wilken Bicudo – IB-USP; Investment R$95,369.33 (FAPESP)

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