Cheetahs: A Numbers Game
February 4, 2015
The life of a cheetah is a numbers game. It is a game of withdrawals and deposits. For every calorie a cheetah burns, it has to eat one. For every drop of water a cheetah uses, it needs to drink one.
It seems simple enough. Eat and drink what you can when you can and conserve your energy. But this numbers game of survival has gotten more difficult for cheetahs over the past few decades. Cheetah populations have dropped by more than 90% since 1900 and there are now fewer than 10,000 cheetahs left in the wild.
Biologists have been studying cheetahs for years in order to find out why they are dying out. They want to know what is unbalancing a cheetah’s deposits and withdrawals. For decades, scientists thought the speedy cats overheated when they run at top speed. Others contend larger stronger predators like lions and hyenas steal food that the cheetahs catch before the cheetahs can eat it. Now an international group of scientists says the changes humans have made to the cheetahs’ habitat are causing the decline.
The biologists, including North Carolina State University’s Johnny Wilson, studied several cheetahs in two South African wildlife reserves, observing their behavior and how much energy they consumed.
Wilson and his colleagues first had to capture the cheetahs. They fitted each cheetah with a radio collar so that they could track where the cheetahs travelled and what they were doing in a general sense — chasing prey, resting or walking around. They also gave each cheetah an injection of water with a radioactive isotope. Digestion and burning energy both require water so by testing the cheetah’s droppings for the radioactive isotope, Wilson and the other researchers could determine how much energy the cheetahs were using.
The results did not come out exactly as the researchers expected. Despite being the fastest land animal on Earth, reaching land speeds of about 60 miles per hour, the cheetahs did not expend an enormous amount of energy in the actual process of hunting. Specifically they measured how far the cheetah chased prey against how much energy the cheetah expended that day. They found that there was not a strong relationship between a longer pursuit and calories burned over the whole day.
The researchers found that cheetahs were, in fact, having their prey stolen by larger predators, but based on the energy they expended in the effort to catch new prey, that cheetahs were able to deal with this sort of competition. The researchers found that the cheetahs’ prey is stolen between 9 and 15% of the time. Theft forces cheetahs to hunt for a little more than an hour extra, expending 12% more energy than they normally would. Cheetahs usually make up for this with a larger meal, and the researchers project that cheetahs would need to lose about 50% of their kills in order for theft to become a real threat.
While food theft and longer pursuits did not significantly alter how many calories the cheetahs burned, the researchers found that the amount of walking cheetahs had to do to search for food strongly correlated with how much energy they burned in a day. Thus whatever causes the cheetahs to spend more time searching for food is responsible for the population decline.
The researchers hypothesize that human activities could fit this role. Obstacles like fences keep the cheetahs from traveling how they normally would. A cheetah could possibly jump it, but often it might have to spend time searching for a way around. Also, habitat fragmentation can decrease the amount of prey in a given area, forcing the cheetahs to search for longer before finding anything.
It’s important to note that the researchers did not directly study the effects of humans on cheetahs by monitoring the energy used by cheetahs closer to and further from civilization. Further study would be needed to determine human impacts for certain but humans having a role in the cheetah’s population decline is a logical extension of this research.
Many conservation groups are working to help the cheetahs bounce back, but this study, published in the journal Science, shows that we may have to rethink our own habits to make the numbers work for the cheetahs.
— Daniel Lane
Daniel Lane covers science, engineering, medicine and the environment in North Carolina.