For a few brief weeks each spring, nourishing rains sprinkle the Samburu region of northern Kenya. It is the wet season, when the land turns verdant and elephants roam the savanna, feasting on tender, newborn grasses.
Known for their adaptability, elephants will subsist on whatever vegetation they can find, in wet season or dry. But a new study has tracked the giant herbivores' diets, analyzing carbon, nitrogen and hydrogen isotopes in their tail hairs.
The animals' bodies are tuned for the rush of nutrition, the study shows, relying on it to provide the energy needed to reproduce. Now changes in regional climate and cattle grazing practices could threaten that delicate balance.
Thure Cerling of the University of Utah and a team of researchers used Global Positioning System (GPS) collars to track movements of "the Royals," a family of elephants living in and around the Samburu and Buffalo Springs Natural Reserves in Kenya. The area is home to some 800 elephants, thanks to local rivers that provide a permanent source of water and vegetation.
Carbon locked away in trees and grasses has different isotopic signatures, and the team found that the hairs on the animals' tails preserved a detailed record of what they had eaten over the six-year period.
When water was scarce most of the year, the animals ate mostly leaves from trees and shrubs. About two to three weeks after the rains began, though, the elephants shifted their diet, consuming as much as half of the 200 pounds or so of daily vegetation intake in the form of grass, whose protein content is at its peak at this time.
"When it starts to get green, they don't switch," Cerling said. "Then after two to three weeks they do. The grass has to get long enough for them to twirl it around their trunks, like spaghetti on your fork."
On average, the elephants conceived three weeks after the height of their grassy feasts, setting in motion a 22-month-long gestation period that culminates in newborns arriving just as the wet season begins two years later.
When the Royals left the parks' protected confines, all that changed. During one rainy season, they wandered into an area heavily grazed by cattle. Grass was abundant, but the cattle had trimmed it too short for the elephants to consume, and the giant plant-eaters were forced to eat the leaves of woody plants instead.
Cerling stressed that it's too soon to say whether such shifts in diet will affect elephants' ability to reproduce. But humans are pushing their cattle further and further into elephant habitat every year in part of a worrying trend that could combine with altered rainfall patterns from climate change to place huge amounts of stress on the the already troubled animals.
"Elephants are very adaptable, but as we continue to degrade their habitat, they're going to run out of that ability to adapt," Darryl de Ruiter of Texas A&M University said. "They have to keep eating on a near-constant basis. If they're constrained within a smaller and smaller area, they're going to become more destructive to that environment."