By analyzing carbon isotope ratios along the length of a single elephant hair, the researchers could see the twice-annual switch when rain falls and the elephants go from eating mostly trees and shrubs to eating mostly grasses, and then back again.
The study found the elephants' average grass intake during the study was 21 percent of their diet. During the dry season, grass was about 15 percent of their diet, but that skyrocketed to 50 percent grass during rainy seasons. Grass consumption peaked at 60 percent during a particularly good rainy season after a prolonged drought, Cerling says, noting the elephants must have thought, "Finally, they have opened the restaurant."
Cerling says the researchers don't know why "the Royals" a group of 20 elephants including the four studied wandered off the reserve during the April-May 2003 rainy season, but when they did, tail hair carbon isotope levels showed they were eating only trees and shrubs and no grass. That was highly unusual, because even during the dry season they normally eat 15 percent grass.
"The impact of overgrazing by cattle on the typical wet season diet of elephants is clear," the researchers wrote. "Competition with cattle results in poor access to high-quality grass forage because cattle keep the grass very short and out-compete elephants."
The findings provide a hint that as Kenya's population continues to burgeon, and as global warming produces more droughts, increasing competition for grass with domestic cattle might threaten the elephants' ability to bulk up for pregnancy.
"We'd need quite a bit of data before we could say that, but it is clear the grass is providing nutrients they really want and presumably need for successful reproduction," Cerling says. "With increased drought, they may run into a reproductive wall."
Hydrogen and Oxygen:
|Contact: Lee Siegel|
University of Utah