Friday, December 22, 2006

 

Evolution: Winning by a neck - Giraffes avoid competition

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 January 2007 issue of the American Naturalist, researchers 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 researchers built low fences around trees in greater Kruger National Park to stop smaller browsers from eating leaves. After a complete growing season they found 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 researchers argue, it is competition from other herbivores, such as kudu, that appears to drive giraffes to eat leaves high in the trees.

Source: University of Chicago / PhysOrg

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Based on the American Naturalist paper:

"Winning by a Neck: Tall Giraffes Avoid Competing with Shorter Browsers"

Elissa Z. Cameron and Johan T. du Toit

Cameron, E. Z. & du Toit, J. T. 2007. Winning by a neck: tall giraffes avoid competing with shorter browsers. The American Naturalist 169, 130-135.

Abstract

With their vertically elongated body form, giraffes generally feed above the level of other browsers within the savanna browsing guild, despite having access to foliage at lower levels. They ingest more leaf mass per bite when foraging high in the tree, perhaps because smaller, more selective browsers deplete shoots at lower levels or because trees differentially allocate resources to promote shoot growth in the upper canopy. We erected exclosures around individual Acacia nigrescens trees in the greater Kruger ecosystem, South Africa. After a complete growing season, we found no differences in leaf biomass per shoot across height zones in excluded trees but significant differences in control trees. We conclude that giraffes preferentially browse at high levels in the canopy to avoid competition with smaller browsers. Our findings are analogous with those from studies of grazing guilds and demonstrate that resource partitioning can be driven by competition when smaller foragers displace larger foragers from shared resources. This provides the first experimental support for the classic evolutionary hypothesis that vertical elongation of the giraffe body is an outcome of competition within the browsing ungulate guild.

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See "The Giraffe's Short Neck" from New York's The Nature Institute. Excerpt:

Once scientists began thinking about animals in terms of evolution, the giraffe became a welcome - and seemingly straightforward - example. It is as if the giraffe's long neck was begging to be explained by evolutionary theorists.

One of the first evolutionary thinkers, Jean-Baptist Lamarck, offered a short description of how the giraffe evolved in his major work, Philosophie Zoologique ('Zoological Philosophy'), which was published in 1809:

It is interesting to observe the result of habit in the peculiar shape and size of the giraffe: this animal, the tallest of the mammals, is known to live in the interior of Africa in places where the soil is nearly always arid and barren, so that it is obliged to browse on the leaves of trees and to make constant efforts to reach them. From this habit long maintained in all its race, it has resulted that the animal's forelegs have become longer than its hind-legs, and that its neck is lengthened to such a degree that the giraffe, without standing up on its hind-legs, attains a height of six meters. (Quoted in Stephen Jay Gould 2002, p. 188 - The Structure of Evolutionary Theory)

In Lamarck's view, we must imagine a situation in the past where the best food for browsing mammals was higher up in trees, the lower vegetation having been eaten by other animals. The ancestors of the giraffe - which we should imagine like antelopes or deer - needed to adapt their behavior to this changing environment. As Lamarck wrote, "variations in the environment induce changes in the needs, habits and modes of life of living beings ... these changes give rise to modifications or developments in their organs and the shape of their parts" (p. 179). So Lamarck imagined that over generations the habit of continually reaching for the higher browse produced in the giraffe's ancestors a lengthening of the legs and neck.

A little over sixty years later, Charles Darwin commented on giraffe evolution in the sixth edition (1872) of his seminal book, Origin of Species:

The giraffe, by its lofty stature, much elongated neck, fore-legs, head and tongue, has its whole frame beautifully adapted for browsing on the higher branches of trees. It can thus obtain food beyond the reach of the other Ungulata or hoofed animals inhabiting the same country; and this must be a great advantage to it during dearths.... So under nature with the nascent giraffe the individuals which were the highest browsers, and were able during dearth to reach even an inch or two above the others, will often have been preserved; for they will have roamed over the whole country in search of food.... Those individuals which had some one part or several parts of their bodies rather more elongated than usual, would generally have survived. These will have intercrossed and left offspring, either inheriting the same bodily peculiarities, or with a tendency to vary again in the same manner; whilst the individuals, less favoured in the same respects will have been the most liable to perish.... By this process long-continued, which exactly corresponds with what I have called unconscious selection by man, combined no doubt in a most important manner with the inherited effects of the increased use of parts, it seems to me almost certain that an ordinary hoofed quadruped might be converted into a giraffe. (Darwin 1872, pp. 177ff.) (More)

Citation: In Context #10 (Fall, 2003, pp. 14-19); copyright 2003 by The Nature Institute

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*The Mammal Research Institute strives to "be internationally recognised for research and teaching on the biology and ecology of African mammals; and to maintain a focus that is relevant to Africa, and to southern Africa in particular, with regard to conserving the diversity of our indigenous mammal fauna in the context of sustainable human development."

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