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"How come penguins (and some other birds) can't fly?" asks Garrett Poliey,

One of the most common human dreams is of flying. Unlike in everyday life, where our feet are pinned firmly to the earth, in a dream we can lift off, soar over rooftops, ride imaginary air currents. We envy birds, because they can take off into the air whenever they please.

So it's a real paradox that some birds are as earthbound as we. Out of the nearly 9,000 species of birds on Earth, about 40 kinds are flightless. And here's another twist: Flightless birds evolved from birds that could fly. How come? As much as humans love to fly in dreams, flying in the real world is an energy-hogging way for animals to get around. Yes, it allows birds to migrate thousands of miles in the winter, helps them find food near and far and facilitates a quick escape from hungry cats and other predators. But the ability to fly comes at a cost. The cost is not in dollars and cents, but in physiology. Birds evolved from reptiles that walked on the land. Flying required myriad adaptations -- a highly specialized body structure with unique flight muscles and feathers, wings, beaks instead of teeth, and a limit on body weight. To top it all off, birds need to eat enormous amounts of food to fuel their energy-wasting flying. Pricey changes, from the point of view of evolution -- sort of like turning an ordinary sedan into a sleek, gas-guzzling sports car. Scientists say flightless birds evolved in isolated areas of the planet, such as ocean islands, from regular birds that flew in and stayed. On their deserted island home, the usual predators didn't exist. When baby birds were, by chance, born with small flight muscles or stunted wings, it wasn't a given that they would die young. Over millions of years, wings atrophied in some species, until the island had its own species of flightless birds. Examples include the emus of Australia and the kiwis and kakapo parrots of New Zealand. Birds that can't fly have their own survival strategies. Ostriches are huge, up to 8 feet tall and 350 pounds. They can kick hard, then race away at 40 mph. Kiwis hide in burrows and feed at night, using their remarkable sense of smell. Still, when boats brought new predators -- such as people and their pet cats -- to isolated islands, many flightless birds, like the dodo, the great auk and the 1,000-pound elephant bird of Madagascar, were hunted to extinction. Penguins are the largest group of flightless birds living today. Scientists say they evolved from flying birds that came to roost in isolated areas of the Southern Hemisphere. Wings of penguin ancestors evolved into thin, hard paddles, perfectly suited for "flying" through cold ocean waters. Flightless penguins were also free to develop stout bodies padded with fat to keep them warm even at the frigid bottom of the world, Antarctica.

# posted by Medical Information : 2:58 AM