Why do giraffes have
such long necks?
|By Kathy Wollard|
Why do giraffes have such long necks? Is it to reach higher leaves? asks Alejandro Carreras, a student in North Carolina.
While the origins of the giraffe's neck is still a mystery, many scientists think the giraffe backstory is less about feeding, and more about fighting.
Giraffes are the tallest mammals on Earth, with some males standing 19 feet high. Weighing in at 1,200 to 4,200 lbs., giraffes still manage to run at up to 35 mph. Besides their long necks, giraffes also have long, grasping tongues, which can extend 18 inches to reach tasty inner tree leaves.
The fossil record shows that giraffes evolved from a deer-like ancestor with a shorter neck. By about 1 million years ago, modern giraffes had appeared on the African savannah. Why did the long neck evolve? Until recently, the most popular theory involved finding food. Giraffe-like animals who were born with longer-than-normal necks were thought to have a big feeding advantage, since in times of scarcity, they could reach higher into trees to forage for leaves. Longer-necked individuals were more successful at surviving, and passed their long-neck genes onto their offspring. Over many generations, the modern giraffe evolved.
But many scientists thought this theory was more a "just-so" tale than good science. And in the 1990s, two scientists in Africa decided to compare the idea to the realities of giraffe life. They found that giraffes spent most of the food-scarce dry season feeding in low bushes rather than in tall trees. In the rainy season, when leaves were green and plentiful, giraffes were more likely to turn their attention treeward.
(A graphic illustration of giraffe feeding heights can be seen here.)
So while a long neck could be meal-friendly, it didn't seem to provide enough of a survival advantage in scarce times to account for its eventual dominance. According to Robert Simmons and Lue Scheepers, giraffes do "win by a neck." But the competition is less with other mammals for food, and more with other zebras -- for reproduction.
Like rams, male zebras butt heads over females, and the males that win, mate. Male zebras as young as 1 month old play-practice this sparring behavior. As zebras grow, the sparring – also called "necking" – turns serious.
An average male giraffe's neck weighs 200 lbs. and can stretch 6 feet long. Giraffes fight over females by swinging their necks and heads like a medieval ball and chain. The longer and heavier the neck, the more momentum behind the often bone-shattering head slams.
Simmons and Scheepers found that males with the longest, most massive necks tended to win the mating contests, allowing their genes to be passed down to future generations. Simmons believes that it was the competition for mates that pushed the evolution of the giraffe's neck, with longer-necked animals more successful at reproducing. Female giraffes have many of the same genes, so their necks are long, too. But the females' necks stop growing in adolescence, while young males go on to add nearly 100 pounds of neck weight as they reach adulthood.
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Copyright © 1999-2006 by Kathy Wollard & Debra Solomon