The science of hot cheetahs

You may have heard that cheetahs abandon hunts because they overheat. But this theory is wrong, according to new research. In fact, cheetahs often overheat when they've hunted successfully. Here's why.

Despite being the fastest land sprinters on the planet, cheetahs are very inefficient hunters. In fact, less than 40 percent of all cheetah hunts end in kills, and the big cats even abandon hunts when their prey appear to be in range. For several decades, the prevailing theory to explain cheetahs' hunting failures states that the animals stop hunting because they overheat, and simply cannot exercise any further.

"This theory is very popular among tour guides, textbooks and a lot of lay media," says Robyn Hetem, a physiologist at the University of Witwatersrand in South Africa. "We were actually quite surprised to find that the theory traced back to single study."

In the early 1970s, a pair of researchers trained two cheetahs to run on treadmills. In their 1973 study, they had the cheetahs exercise on a treadmill while they took an array of measurements, including rectal temperature, metabolic rate and heat loss from evaporative cooling. They concluded that the animals stored metabolic heat — that is, they didn't lose any heat from evaporative cooling and instead kept it all in. When cheetahs run, the heat quickly builds up, capping off at 40.5 degrees C (104.9 degrees F), at which point further exercise becomes thermally impossible.

"As soon as their temperature got to 40.5 degrees, they refused to run," Hetem tells io9. "The authors tell stories of how the poor cheetahs would fall off the treadmill."

In the experiments, the cheetahs ran for about 2 km (1.2 miles) at an average speed of 10 km/h (6.2 mph), with maximum speeds of up to 30 km/h (18.6 mph). Based on their measurements and calculations, the authors surmised that a cheetah would stop sprinting after about 30 seconds during a real hunt — 30 seconds is the actual average hunting time of a cheetah in the wild. The conclusions made sense at first glance, but there's a major issue with the study: It didn't simulate real hunts, where cheetahs will sprint at up to 100 km/h (62 mph) for only several hundreds of meters.

The science of hot cheetahs

So Hetem and her colleagues decided to test if free-living cheetahs really do abandon hunts because they overheat. They implanted temperature-sensitive and movement-sensitive data loggers into six cheetahs (two of which were killed by leopards during the study) in the Tusk Trust Cheetah Rehabilitation Camp of the AfriCat Foundation in central Namibia. Over seven months, the researchers analyzed the temperature and locomotive activity of the four remaining cheetahs as they made 23 unsuccessful hunts and 40 successful hunts (the team directly observed five kills, and inferred the rest from finding the cheetahs on fresh kills and tracing the hunts back to the corresponding activity data).

During the study, the cheetahs' had an average daily body temperature of 38.3 degrees C (100.9 degrees F), with minimum and maximum temperatures of 37.3 degrees C (99.1 degrees F) and 39.5 degrees C (103.1 degrees F), respectively. Cheetahs, like most other mammals, humans included, have a 24-hour rhythm of body temperature, Hetem says. The researchers found that the cheetahs' temperatures didn't rise significantly during the hunts. Indeed, their temperatures at the end of both successful and unsuccessful hunts averaged 38.4 degrees C. The main factor affecting terminal temperatures was the time of day when the hunt occurred — that is, their body temperature when the hunt started, as regulated by the 24-hour temperature rhythm.

The team did find, however, that the cheetahs' body temperatures began to slowly rise after the hunt was over. Surprisingly, successful hunts resulted in a body temperature increase of 1.3 degrees C (2.3 degrees F), while unsuccessful hunts only increased body temperatures by 0.5 degrees C (0.9 degrees F). Furthermore, the cheetahs' temperatures seldom hit the supposed thermal exercise limit of 40.5 degrees C.

The researchers found that hunt activity, hunt duration, air temperature and other factors didn't affect the cheetahs' post-hunt temperatures. And to add another layer to the mystery: One of the cheetahs was hurt for a few days and couldn't hunt, so he instead fed off his siblings' kills. But his temperature profile still matched that of the hunting cheetahs.

The science of hot cheetahs

"It wasn't anything to do with the hunt that caused this increase, which made us think that one of two things could possibly be causing it," Hetem says. The researchers' first thought was that the cheetahs' temperatures increased as a result of eating. But this theory didn't add up because the cheetahs would actually rest after hunting, all the while their body temperatures increased.

The more likely cause of the increase is "stress hyperthermia." Among big predators, cheetahs are near the bottom of the hierarchy. After hunting, they are very tired and most vulnerable to other big cats, such as leopards and lions. And if they have a kill, it becomes even more likely that another predator will come along, so the animals must remain vigilant after hunting — cheetahs have previously been described as being nervous after a kill and alert while feeding. They experience hyperthermia because they are stressed out, similar to the way that antelope experience fear-induced hyperthermia.

On the other hand, the temperature increases seen in the 1973 study may have been due to the setup of the experiment. The animals were running for long periods of time — on a treadmill — and were getting their temperatures taken rectally, an experience which may well have been stressful.

But the question lingers: Why do cheetahs give up on hunts?

There are several theories floating around, Hetem explains. The most probable explanation: They ran out of energy. Sprinting is a very anaerobic exercise that results in the build-up of lactate acid, which disrupts the breakdown of glucose to energy. "The cheetahs may just run out of energy after 30 seconds of sprinting," Hetem says.

Hetem and her colleagues published their work in the journal Biology Letters.

Top image via Mark Dumont/Flickr. Inset images via Brenda de Witt.