RC: A new view of the origin of bird flight emerges

elmo 2003-03-27 22:56:06

你好!
我是韩国小姐
我3/26 参加考试...在rc题里发现了我一前读过文章...
清你们参考!

Taking Wing
A new view of the origin of bird flight emerges
By Kate Wong
WINGING IT: Ground birds often seek out trees and other elevated spots for safety. Juveniles not yet capable of flight accomplish this by running up the inclines, flapping their wings to enhance traction. The way these birds employ their developing wings may demonstrate the process by which avian flight evolved.BOZEMAN, MONT.--It's not often that a presentation given to the Society of Vertebrate Paleontology elicits coos and clucks of sympathy. These are, after all, the scientists who study Tyrannosaurus rex and other fearsome beasts of the past. But that's exactly the reaction Kenneth Dial got when, at the group's annual meeting last October, he showed video footage of a fuzzy little partridge chick with its wings taped to its sides trying to climb a tree--only to tumble down into Dial's waiting hands. Unfettered, however, the chick flapped its tiny wings while climbing and steadily made its way up. After teasing the audience for its sentimental display, the University of Montana biologist returned to the matter at hand: explaining how this and other experiments involving ground-dwelling birds led him to hatch a new hypothesis regarding the origin of avian flight. Traditionally, scholars have advanced two theories for how bird flight evolved. One of these, dubbed the arboreal model, holds that it developed in a tree-dwelling ancestor that was built for gliding but started flapping to extend its air time. The other, known as the cursorial theory, posits that flight arose in small, bipedal terrestrial theropod dinosaurs that sped along the ground with arms outstretched and leaped into the air while pursuing prey or evading predators. Feathers on their forelimbs enhanced lift, thereby allowing the creatures to take wing. As the idea that birds descended from dinosaurs gained acceptance by all but a few paleontologists, so too did the cursorial hypothesis. But both the arboreal and the cursorial scenarios have explanatory gaps. As far as tree dwellers go, of the hundreds of nonavian gliding vertebrates around today, not one flaps its appendages. And why would natural selection have favored the development of little protowings in a theropod equipped with heavily muscled legs for running across the ground? Neither theory, Dial asserts, adequately addresses the step-by-step adaptations that led to fully developed flight mechanics. Dial's eureka moment came after learning that partridges and their fellow ground birds routinely abandon terra firma in favor of trees and other elevated spots for safety. Although these animals appear to fly up into trees, he found on closer inspection that in many cases they were actually running up--legs bent and body pitched toward the tree--while flapping their wings. Subsequent research revealed that wing flapping assists in this vertical running by sticking the bird to the side of the tree, much as a spoiler helps to press a race car to a track. Although the adult ground birds are generally perfectly capable of flying up trees, their preference for running may stem from a time early in life when they couldn't yet fly: before a baby ground bird has the ability to launch itself into the air, the only means it has for getting off the ground is vertical running. And as Dial's experiments show, when a juvenile is trying to evade a predator this way, the aid of even a partially formed wing can mean the difference between life and death. Perhaps a bird ancestor's protowing conferred the same benefit, he suggests, and therefore natural selection favored its development. Over time, wings evolved to the point of enabling not only vertical running but, when employed by an animal running across the ground, flight. So far Dial's model has ruffled few feathers. Living animals do not necessarily make good models of extinct ones, however. \\"Is that the way bird ancestors did it? Well, maybe, maybe not,\\" comments Kevin Padian of the University of California at Berkeley. \\"But [Dial] is showing that it's possible.\\" For his part, Dial is leaving it to the paleontologists to figure out whether his theory of the genesis of avian flight jibes with future fossil finds--or whether it's for the birds.

Feathered dinosaurs

Four wings good
Jan 23rd 2003
From The Economist print edition

Chinese scientists have found a four-winged, feathered dinosaur

IT IS not always the biggest dinosaurs, such as Tyrannosaurus rex, that are the most exciting. This week sees the announcement of a 77cm fossil dinosaur find that has, remarkably, four feathered wings. It was discovered by a group headed by Xu Xing of the Chinese Academy of Sciences, and is described this week in the journal Nature.
Dr Xu's team found two fossils of a new species that they named Microraptor gui, in the Liaoning province of China, about 300 kilometres north-east of Beijing. Microraptor is a theropod dinosaur, and a close relative to the velociraptor that was made famous by the film “Jurassic Park”. Its feathers have an asymmetrical shape very similar to modern bird feathers. Dr Xu says that Microraptor's feathers would have allowed it to glide through the air.
Because the dinosaur's feathers go all the way down to the bottom of Microraptor's legs, the researchers believe that it did not live on land. Running would have been difficult for an animal that was dragging its leg feathers on the ground. And dirt-covered feathers would also have made flying impossible.
As no one has yet worked out a way in which hind wings could be used for flapping in flight (and it is certainly hard to imagine how a bird with four wings would actually fly) researchers think that Microraptor must have been a tree-dweller that could glide.
Dr Xu supposes that it, or a similar four-winged dinosaur, might be an intermediate stage between earlier theropods that could not fly at all and Archeopteryx, a theropod that is thought to have been able to fly and is also the earliest identified bird species. Archeopteryx, like modern birds, had only two wings; Dr Xu guesses that the rear wings atrophied as bird ancestors moved from gliding to fully-fledged flight.
This gives ammunition to adherents of the arboreal school of thought, who hold that powered flight developed from gliding. The competing school holds that flight evolved on the ground, as some sort of powered running. Last week, researchers reported that partridges could run up steep hills more easily if they flapped their wings. This provides an evolutionary advantage as an intermediate step to flight, supporting the running school of thought.
While Dr Xu's results seem to be a stroke in favour of the arboreal school, a note of caution is in order. The last time such a fossil find was announced, in the October 1999 issue of National Geographic, it turned out to be a falsified specimen—the head and body of a primitive bird had been glued to the legs and tail of a theropod. But since Dr Xu and his collaborators were those who uncovered the hoax, there is good reason to believe that some dinosaurs had feathers.

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Ornithology

Chicken run
Jan 16th 2003
From The Economist print edition

Birds that don't fly may be using their wings to run uphill

HOW did the chicken cross the road? According to scientists, at least, it flapped to the other side. Kenneth Dial has been studying bird flight for almost 20 years, and has been intrigued by birds such as grouse and pheasants, which spend much of their time on the ground—despite having wings. On the face of things, that seems odd. Flying birds are hatched naked, coddled in the nest, but do not start proper aviation lessons until they have been plumped up to the size of their parents. Ground birds, though, are hatched with feathers, and must survive from the day they crack out of the shell.
Dr Dial wanted to find out if ground birds might be using their wings. In his flight lab at the University of Montana in Missoula, he and two assistants recorded the activity of chukar partridges using high-speed video. The results of his work are reported in this week's issue of Science.
The film, which captures movement at up to 1,000 frames per second, revealed something unobservable to the naked eye: the birds seemed to use their wings to help them run up steep slopes. He dubbed this “wing-assisted incline running”, or WAIR, and set out to quantify its usefulness. It seems that new hatchlings could run up slopes steeper than 45° if they beat their wings vigorously, and four-day-olds could scale 60° inclines. By the time the birds had reached 20 days, some could make sheer vertical ascents of more than five metres, and could even get round overhanging slopes of up to 105°.
It seemed that wings were a great help. But Dr Dial wanted to be sure, so he compared the accomplishments of normal partridges with birds that had their wing feathers clipped to half their normal surface area, and to even less fortunate fowl whose main flight feathers were entirely plucked. In the youngest, clipping and plucking feathers did not make much difference. But when the birds were four or five days old, the less feathered started to show their deficiencies. Plucked birds could never ascend more than a 60° incline. Clipped birds, meanwhile, could not get up 90° slopes.
Dr Dial reckons that the wings act rather like spoilers on a racing car, improving traction. Wing strokes press the animal towards the ramp and help the bird stick to the surface rather than slipping over it. To confirm this, Dr Dial measured the direction in which the birds were forced by their wings, by mounting accelerometers on each animal's torso. He found that the birds are, for a substantial part of the wingbeat cycle, forced into the incline. Rather than flapping from back to belly, their wings are moving more like limbs—reaching forward and scooping backward.
Up to this point, Dr Dial has not weighed in on the debate about the evolution of flight. Now he feels he must. There are two divided camps. On one side are the people who believe that the first proto-birds climbed into trees and that, as they glided down, they figured out how to flap. But Dr Dial argues that none of the gliders that currently share the earth with us—such as frogs, lizards and squirrels—actually bother to flap. “Gliding is perfectly functional in its own right,” counters Dr Dial. The second camp argues that bipedal dinosaurs with partial wings learnt to use them to fly.
Both camps are claiming victory, believing that support for their theory is found in Dr Dial's results. Dr Dial, though, thinks his data point to an avian third way: that wings are for more than flying (or gliding, for that matter). Chickens everywhere can rest happy in the knowledge that their wings are more than just beer snacks.

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