The apes of Senegal’s Fongoli savanna may offer
hints to how our own ancestors moved out of the
woodlands, shed their fur and started walking upright.
hints to how our own ancestors moved out of the
woodlands, shed their fur and started walking upright.
Nine years later, Erin Wessling can still remember the first time she visited Fongoli, a savanna in southeast Senegal.
“You feel like you walk into an oven,” she said.
Temperatures
at Fongoli can reach 110 degrees Fahrenheit or more. During every dry
season, brush fires sweep across the parched landscape, leaving behind
leafless trees and baked, orange soil.
“It’s really nuts,” said Ms. Wessling, now a graduate student at the Max Planck Institute for Evolutionary Anthropology.
Yet
Ms. Wessling and her colleagues keep coming back to Fongoli, despite
the harsh conditions. That’s because it’s home to some remarkable
residents: chimpanzees.
To
study them, scientists have mostly traveled to African rain forests and
woodlands, where the apes live in dense groups. The sparse populations
of chimpanzees that live on savannas in western and central Africa are
far less understood.
Ms. Wessling and her colleagues think there are important lessons to be learned from chimps like the ones at Fongoli.
Because
they are our closest living relatives, they may even tell us something
about our own deep history. Millions of years ago, our apelike ancestors
gradually moved from woodlands to savannas and began walking upright at
some point. The Fongoli chimpanzees demonstrate just how difficult that
transition would have been — and how that challenge may have driven
some major changes in our evolution, from evolving sweat glands to
losing fur and walking upright.
The savanna became the subject of long-term research in 2000, when Ms. Wessling’s undergraduate adviser at Iowa State University, Jill D. Pruetz, first paid a visit.
Surveying
Fongoli, Dr. Pruetz decided it would be a good place to observe the
differences between chimpanzee life on a savanna compared to forests. In
forests, for example, chimpanzees typically thrive on a diet of ripe
fruit. That’s a rare treat on a savanna.
But
Dr. Pruetz could not simply settle down right away and watch the
chimpanzees. At first, the sight of her frightened them off. So Dr.
Pruetz and her colleagues let the apes grow accustomed to their company.
That alone took four years.
At
last, in 2004, Dr. Pruetz and her colleagues could follow the
chimpanzees from dawn to dusk. “You just have to drink water all day,”
said Dr. Pruetz, now a professor at Texas State University.
The
team gradually built up a catalog of strange behaviors — ones rarely if
ever seen in others. Forest chimpanzees get enough water from the fruit
in their diet so they need less drinking water and can wander in search
of food. The Fongoli chimps, by contrast, required daily drinking water
and anchored themselves to reliable water sources in the arid
landscape.
And
while forest chimpanzees are active throughout the day, Dr. Pruetz
found that the savanna chimpanzees rest for five to seven hours. Dr.
Pruetz could often find them lurking in small caves in the dry season,
and when the rainy season arrived, the chimpanzees would slip into newly
formed ponds and bob there for hours.
Forest
chimpanzees typically spend all night in nests they build in trees. But
at Fongoli, the research team noticed that the chimpanzees often made a
late-night racket.
Staying
up all night to watch them, Dr. Pruetz discovered that they spent hours
after sundown searching for food. “It might as well have been a daytime
scene,” she said.
All
these odd behaviors suggested that the chimps were struggling to cope
with Fongoli’s harsh conditions. But all Dr. Pruetz’s observations
couldn’t reveal what was happening inside their bodies.
“I didn’t know
how stressed they were,” she said.
In 2014, Ms. Wessling set out to get an answer — by collecting chimpanzee urine.
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.”
