Like humans, chimpanzees have molecules in their urine that reflect their physical condition. When they feel stress, for example, they make the hormone cortisol. The pancreas produces a substance called c-peptide in response to food. Its levels can reflect whether chimpanzees are getting enough energy. If a chimpanzee gets dehydrated, the protein creatinine builds up in its urine.

Scientists regularly gather urine from forest chimpanzees, but there, they need only go under a tall tree and hold out a leaf. On the savanna, Ms. Wessling would have to wait until a chimpanzee ambled away from where it had urinated. By the time she reached the spot, the urine might have already seeped into the ground or evaporated. “You basically watch your sample disappear,” Ms. Wessling said.

From 20 chimps, Ms. Wessling gathered 368 urine samples that were taken back to Germany for analysis.

The chimps’ c-peptide levels showed they ate a decent amount of food, and possibly termites to get additional calories.

While that was an indicator of a healthy diet, analyses of the two other compounds told another story. Many of the chimps had produced high levels of cortisol, indicating that life on the savanna could be very stressful. And their creatinine levels were also high, evidence that the heat of the savanna caused them to become dehydrated.

For all the ways that the Fongoli chimps tried to protect themselves from the heat, it still punished them.

“These chimps are sort of right at the edge of what they can do,” Dr. Pruetz said. “This really gives you the biological basis of it.”

The research was published earlier this month in the Journal of Human Evolution.

To scientists who study human evolution, the Fongoli chimpanzees offer some intriguing parallels to our ancestors millions of years ago. Studies of DNA indicate that our two evolutionary branches split roughly seven million years ago.

The earliest members of our branch (known as hominins) may have been chimp-like in some respects, growing fur and walking through forests on their knuckles.

Over millions of years, Africa’s rain forests retreated into patchworks, as savannas expanded. In eastern and southern Arica, hominins moved into open habitats, eventually reaching arid grasslands — places as daunting for survival as Fongoli.

“How and when hominins got better at coping with heat is a fascinating, unsolved problem,” said Daniel E. Lieberman, a paleoanthropologist at Harvard.

The results from Fongoli suggest that a chimpanzee-like ancestor might have eked out an existence on an east African savanna. Food might not pose the biggest challenge. Instead, they would be hard-pressed by the heat.

Early hominins might have used some of the strategies documented in Fongoli, like staying near water and shifting a lot of activity from day to night. But even so, early hominins would have still suffered stress.

That stress might have only been overcome when hominins evolved new physical adaptations. Humans have skin glands that let us sweat much more than chimpanzees, for example. The origin of our upright posture might also be intertwined with our struggle with heat.

Some researchers have proposed that early hominins began standing to aid in reaching fruit hanging from trees. Peter Wheeler, of Liverpool John Moores University, has suggested that an upright posture would have helped hominins stay cool in an arid environment. On the savanna, walking tall might mean walking cool.

Dr. Pruetz suspects Dr. Wheeler may be right, and she hopes to study the Fongoli chimpanzees more to test his idea. The chimpanzees may shift their posture — as far as they can with an ape anatomy — in order to cope with the high temperatures. It’s now possible to get close enough to measure the heat flowing from the chimpanzees with a thermal imaging camera.

“We really haven’t had that opportunity before,” she said. “There’s a lot of fun stuff we can do.”

Source – full article and photos – https://www.nytimes.com/2018/04/27/science/chimpanzees-savanna-evolution.html