Measuring the amount of biomass eaten showed that, in words of the report, ordinarily "giraffe feeding efficiency is reduced at low heights" because of competition with smaller animals.

Relatively short herbivores have an advantage in selecting more desirable shoots, which grows lower. They have narrow muzzles, allowing them to pluck out the tastier treats from among the acacia's thorns.

"The smaller-bodied ones take up the quality" items, reducing the amount of better food available to big animals.

In the Serengeti, large creatures move along before the smaller ones, eating the less desirable food. In some respects, du Toit said, "the bigger ones are victims of the smaller ones."

Giraffes are left with higher forage, which is not as nutritious per pound eaten. A big animal would benefit from low-level material, the study shows, but it isn't available.

"The bigger-bodied animals take bigger bites," giving them wood and fiber as well as leaves. They were forced to adapt, growing long necks to reach the less nutritious reaches where they had no competition.

That, du Toit said, is "what natural selection's all about."

---

E-mail: bau@desnews.com