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Are humans to blame for cheetah decline? Yes, say scientists.

An international team of researchers studied the energy expenditures of cheetahs in southern Africa and discovered that time spent looking for food is what really wears them down.

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    A male cheetah cub is weighed in a bucket at the Smithsonian Conservation Biology Institute in Front Royal, Va., Wednesday, Feb. 9, 2011. Cheetah populations have dwindled over the last century and a new study eliminates prey stealing as the primary source, making human activity a more likely cause for the demise of the cheetah.
    Jacquelyn Martin/AP
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We tend to view cheetahs as super-mammals: they have exceptional eyesight, are experts in stealth, can go days without rehydrating, and have the ability to accelerate from 0 to 60 miles in just 3 seconds. But these fast felines are more vulnerable than one might think.

The number of cheetahs has dropped to 10 percent of what it was a century ago. Until recently, scientists have theorized that theft of prey, or kleptoparasitism, by other predatory mammals, may have been putting undue strain on cheetahs’ energy stores. However, new research suggests that human activity may be causing the biggest drain on energy reserves.

"At some stage the cheetahs will lose more energy than they need during the day, and that pushes them over an energetic precipice that affects their viability," says John Wilson, a North Carolina State University biologist and an author of a new paper published online Thursday in the journal Science.

Kleptoparasitism seemed to be a likely cause given that such theft has been found to impact a similarly-sized predatory mammal, the African wild dog.

In this study, an international team of researchers set out to determine if prey stealing by larger predators like lions and hyenas could be leaving cheetahs exhausted and without a way to refuel. First, however, the researchers had to determine how much energy cheetahs actually put towards catching their prey.

An animal's level of breathing is directly related to how hard it is working, so the researchers sought to measure how much oxygen was in a cheetah's body in order to determine how much energy it was expending. Dr. Wilson and his colleagues injected “heavy water,” a variation of H2O but with the hydrogen isotope deuterium, into cheetahs from two wildlife parks in southern Africa. The researchers then followed the cheetahs and took samples from the animals' waste, allowing them to calculate the ratios of oxygen to the hydrogen isotope.

From these measurements, the team found that cheetahs didn't expend most of their energy going after a gazelle or hare. Instead, the distance cheetahs walked looking for prey was the biggest driver of how much energy they used.

This finding contrasts with what scientists have discovered about wild dogs. Cheetahs only spend a few seconds in quick pursuit of their prey and it seems that hunts make up a small portion of their daily energy budget. Wild dogs, however, go after their prey for longer periods of time, making their hunting strategy a more costly one when it comes to their energy stores.

The study also revealed that kleptoparasitism levels were low for cheetahs, indicating that they can handle the thievery of some of their prey by lions or hyenas and competition with these larger predators is likely not the problem when it comes to population depletion.

"For prey theft to be a factor, cheetahs would have to have their meal stolen over 50 percent of the time, which is not the case with the animals we studied," said lead author Mike Scantlebury, biologist at Queen's University in Belfast, in a release.

Knowing that cheetah's energy loss primarily comes from the time spent traveling and searching for prey, this raises the likelihood that the real issue for this species of cat is loss of habitat from man-made structures such as fences or lodges.

"We humans, for example, if we put fence in their territory, they might walk up to the fence and then walk right back to look for food," says Wilson. "And that's energy that they wasted."

And so it appears that human encroachment is causing the cheetahs to become weak to a point where they can no longer defend themselves in their own environment.

"The cheetahs aren't really weak," says Wilson. "It's us making them weak."

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