Whenever I go to a zoo, I stand in front of the giraffes and watch with awe and respect at these wonderful creatures. I always consider them to be an extreme example of natural selection. I read this article and thought it would be worth mentioning in our blog.

The giraffe's elongated neck has long been used in textbooks as an illustration of evolution by natural selection, but this common example has received very little experimental attention. In the recent issue of the American Naturalist, scientists at the Mammal Research Institute in the Department of Zoology and Entomology at the University of Pretoria tested whether foraging competition with shorter herbivores could explain why giraffes feed mostly on leaves high in trees, despite being able to feed at lower levels as well.


"This [study] provides the first real experimental evidence that the long neck of the giraffe might have evolved as a consequence of competition, which provides support for a previously untested textbook example of natural selection," says Elissa Cameron (University of Pretoria), who coauthored the study with Johan du Toit (University of Pretoria and Utah State University).

Giraffes are well known for their unusual height, and they generally feed high in the tree canopy, above the height other herbivores can reach. Giraffes receive more leaves per bite by foraging high in the tree, but it's unclear whether this is caused by competition -- smaller browsers eating some of the leaves at lower heights -- or if more leaves grow at higher levels.

The scientists built low fences around trees in greater Kruger National Park to stop smaller browsers from eating leaves. After a complete growing season they observed that the number of leaves on the fenced trees was roughly the same, revealing that small browsers are responsible for most of the foraging. Therefore, the scientists argue, it is competition from other herbivores, such as kudu, that appears to drive giraffes to eat leaves high in the trees.


Posted by: Kelly    Source

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