This time of year, the temperature routinely reaches 104 degrees in northern Botswana. The grasses recede, forcing herds of wildebeest to walk farther and farther from their only water source to graze. Humidity falls to about 10 to 15 percent.
âItâs not quite Death Valley, but itâs not quite far-off it,â said Alan Wilson, a biologist whose research examined how the wildebeest cope with such an inhospitable environment. âTheyâre on a physiological knife edge in terms of: How do they continue to survive?â
His research showed that these cow-like animals, also called gnus, have remarkable adaptations, enabling them to walk up to 50 miles over five days without drinking water. They can do this because their muscles work incredibly efficiently â far more than their body size would suggest.
âI donât think weâd get to 50 miles,â Dr. Wilson said, referring to humans.
His study, published Wednesday in the journal Nature, showed that this efficiency means wildebeest donât have to sweat or pant as much to release heat, even when theyâre running in heat higher than their body temperature.
âThey donât have the problem of overheating that they would have otherwise or having to use water to cool themselves,â said Andrew Biewener, an expert in the biomechanics at Harvard University who was not involved in the research.
Dr. Biewener said the study helps link muscle physiology and locomotion to the animalsâ behavior.
Wildebeest are like very skinny cows, said Dr. Wilson, professor of locomotor biomechanics at the Royal Veterinary College in London. âTheyâre a wee bit shorter than a cow and an awful lot lighter,â he added. A typical cow weighs about 700 kilograms (1540 pounds) and a wildebeest about 200 kilograms (roughly 440 pounds).
Dr. Wilson flew over wildebeest in a helicopter in the Makgadikgadi Pans National Park, shooting tranquilizer darts into 20 of them. When six of the animals were briefly down, he took a small muscle biopsy from each to be analyzed in a lab in London.
All 20 animals were fitted with collars that contained GPS, an accelerometer, gyroscope, magnetometer, a humidity sensor and a thermometer that measured the combined effect of solar radiation, air temperature and air velocity on the animal. The collars were left on for as long as 18 months.
Larger animals are expected to have more efficient muscles than smaller ones, meaning that their muscle fibers have the capacity to more efficiently produce energy. But the new study shows that efficiency is about more than just size. Previous research found that rabbit muscles were 27 percent efficient and mouse muscles 34 percent efficient.
The new research indicated that the muscles of large animals are generally more efficient, with cows at about 42 percent efficient. The wildebeest clocked in well above their size at 63 percent efficiency.
The muscle efficiency measurements are a significant contribution to a field that hasnât calculated new species since mice were first measured in 1994, said Robyn Hetem, a senior lecturer at the University of the Witwatersrand in South Africa.
âMost of what we know about muscle fibers comes from this classic paper,â she said, noting that to calculate the amount of heat released from muscles requires temperature measurements within .001 degrees Celsius.
Climate change is expected to make Botswanaâs environment even more extreme, with significant temperature increases expected across southern Africa.
Dr. Hetem said sheâs already worried about herds of wildebeest south of the animals Dr. Wilson studied. The government of Botswana in the 1960s built a veterinary fence in the north of the country to keep wild antelope from infecting cattle with foot and mouth disease.
But in protecting the cattle, the government blocked the wildebeest from traveling historic migration routes. That doesnât matter much during typical years, she said, but has led to mass die-offs during particularly dry ones, Dr. Hetem said.
The wildebeest can probably cope with some rise in temperatures, Dr. Wilson said, but noted that âitâs a challenging environment for all the animals.â