Apes do the darndest things
Chimps display hunting, foraging skills that may have served early hominids well
FIRED UP APES Chimps hunt and forage with tools and can predict how wildfires will move. Whether these primates are an appropriate model for ancient hominid behavior is up for debate.
Alexandra Rosati
Wildfires dart through tall grass and tree stands at Fongoli, Senegal, during a roughly seven-month dry season. Chimpanzees living in this West African savanna coolly monitor the approaching fires from perches in trees or from ravines. As flames near, the apes retreat just enough to stay safe, sometimes climbing a tree or scurrying into nearby woods.
Because they predict how and where wildfires will move, Fongoli chimps don’t get burned, says anthropologist Jill Pruetz of Iowa State University in Ames. It’s not a simple task — flame height, fire intensity, wind direction and other factors demand consideration.
Researchers have long used chimp behavior as a window on how ancient hominids lived. Grasping how fire behaves under different conditions represents a mental stepping stone that human ancestors must have reached before learning to control and start fires, Pruetz and Thomas LaDuke of East Stroudsburg University in Pennsylvania proposed in the American Journal of Physical Anthropology in 2010. Members of the Homo genus, an evolutionary category that includes present-day humans, may have made campfires 1 million years ago or more (SN: 5/5/12, p. 18).
Since the Fongoli report appeared, research into chimp thinking and behavior has heated up. New investigations indicate that these apes possess sophisticated knowledge about food, fire and tools. Chimps don’t roast marshmallows around campfires, but they may appreciate that certain foods taste better when heated. Chimps also take a more varied, equal-opportunity approach to hunting than previously thought. And some chimps are surprisingly dexterous in their tool use, wielding twigs with a repertoire of hand grips to break open soil tunnels where tasty termites reside.
Chimps (Pan troglodytes) and their sister species, bonobos (Pan paniscus), are humans’ closest living relatives. Scientists suspect a common ancestor of people, chimps and bonobos lived in Africa 5 million to 7 million years ago. Genetic evidence suggests that chimps and bonobos split from a common ancestor about 2 million years ago.
Chimps have received the bulk of researchers’ attention, largely because chimps out-number bonobos in the wild. Also, bonobos inhabit a politically volatile part of Central Africa.
Some hominid researchers dismiss the assumption that today’s chimps or bonobos can illuminate ancient hominid behavior. But scientists who study chimps remain undeterred. If chimp communities are capable of generating distinctive cultural traditions (SN: 6/16/12, p. 18), mental life must run deep among these apes.
“Chimps are incredibly complex animals,” says Harvard University psychologist Alexandra Rosati. “It’s hard to know all that they are capable of doing and thinking, even after observing them for months or years in the wild.”
Chimps have exceeded researchers’ expectations for more than 50 years. Starting in the 1960s, Jane Goodall documented, to her surprise, how chimps in East Africa’s Gombe Stream National Park hunted monkeys for meat and used twigs to extract termites from mounds in the ground. She witnessed fierce male power struggles, killings of neighboring group members and intimate bonds that formed between females and their young. A half century later, researchers inspired by Goodall conduct field and laboratory work aimed at better understanding what makes chimps, and perhaps what made ancient hominids, tick.
Get cooking
Consider that wild chimps, including Fongoli’s flame-trackers, may know more about fire and heat than they let on.
Wild-born chimps living in a sanctuary in the Republic of the Congo understand that raw food can be cooked, and they appear to prefer cooked over raw, Rosati and Harvard psychologist Felix Warneken reported in the June 22 Proceedings of the Royal Society B.
“Chimps will even save food if they think it will be cooked later,” Rosati says.
True, chimps have little motivation to cook, because their menu in the wild largely consists of fruits and leaves with only occasional small prey, Rosati says. Plus cooking usually draws a crowd, creating opportunities for food theft.
Rosati and Warneken probed the hankering for cooked food in a group of 29 chimps. The researchers found that the chimps usually chose to eat sweet potato slices that were briefly roasted for 1.5 minutes in a dry pan rather than raw slices.
In one experiment, each of 16 chimps had 10 chances, or trials, to choose to wait one minute for three cooked potato slices or take one raw slice right away. The chimps waited for cooked slices in 84 percent of trials. Given the chance to wait for three raw slices, they did so in only 60 percent of trials.
A third experiment exposed chimps to a round-bottomed container that turned raw into cooked slices and a cylindrical container that left raw slices unchanged. The animals quickly learned to select the cooking device over its alternative to obtain potato slices. After watching a researcher place potatoes into both containers, a group of chimps chose potatoes from the cooking device and ate them in nearly 88 percent of trials. Chimps even put a new food, carrot slices, in the cooking device more often than in the other container.
Chimps rarely placed inedible wood chips in the round-bottomed container, Rosati says, suggesting that they understood that the vessel was for cooking food. In a series of trials, chimps carried raw potato slices across the testing room to place them in the cooking device more than half the time rather than eating raw slices right away (SN Online: 6/2/15). Finally, five of 13 chimps consistently saved raw potato slices for several minutes until an experimenter showed up with the cooking device.
An understanding that certain foods taste better after exposure to fire and heat may go back millions of years to a common ancestor of people and chimps, Rosati proposes. Knowing about naturally roasted fare, early fire-tamers got cooking right away, in her view. That was a big deal, because increased energy available from cooked foods would have fueled brain expansion in Homo species over the last 2 million years, as Harvard anthropologist Richard Wrangham has argued.
Wait for it
Researchers have mixed reactions to the possibility of cooking-savvy chimps.
If chimps understand what it means to cook food, including meat, “I wouldn’t be surprised,” says biological anthropologist Craig Stanford of the University of Southern California in Los Angeles. Captive chimps prefer cooked meat if they grow up eating it. And wild chimps have been observed collecting and eating charred seeds and nuts from areas recently engulfed by wildfires.
Anthropologist Julie Lesnik of Wayne State University in Detroit has a different interpretation of Rosati and Warneken’s experiments. Her takeaway from the studies: Chimps notice which distinctively shaped container yields better-tasting food. “I’m not convinced we wouldn’t see similar results with baboons, dogs or pigs tested in the same way,” Lesnik says.
Psychologist Michael Beran of Georgia State University in Atlanta agrees. Further trials should test if chimps understand that certain foods, such as banana slices, taste worse after a spell in the cooking device, he says.
Chimps’ ability to show patience in pursuit of more or better food, cooked or not, has also proved hard to pin down, Beran says. In the new cooking study, for instance, animals pointed to indicate whether they wanted to receive one raw potato slice right away or three raw or cooked pieces after waiting one minute. Those who chose to wait for more had no access to food until a minute was up, and therefore had no opportunity to change their minds. That type of pointing test has been used for several decades to study self-control in primates, birds and other creatures.
Story continues after video
CLEVER CHIMPS Over the years, chimps have revealed themselves to be innovative tool users with a penchant for cooked food. Several chimp behaviors have researchers wondering if apes are a good model for early hominid life.
Video and image credits: Lesnik et al/Am. J. Phys. Anthropol. 2015, C. Sanz & D. Morgan/Goualougo Triangle Ape Project, B. Wallauer/Jane Goodall Institute, Alessandra Kelley, Elvert Barnes/Flickr (CC BY-SA 2.0)
But many animals, including chimps, tend to point at the larger of two piles of food simply because it’s bigger, Beran says. To explore that possibility, Beran and his colleagues tweaked the standard pointing task. After pointing at a smaller amount of food — say, four pieces of cereal instead of 12 pieces — an animal immediately gets the smaller stash. If an animal points at the larger amount, an experimenter puts those cereal pieces within reach one at a time. Food accumulation stops when anything gets eaten.
Chimps show impressive self-control on this task, although some are more patient than others, Beran and colleagues reported last year in Animal Cognition. The researchers studied 19 chimps, ages 18 to 44, living in groups at two primate research centers.
Some animals almost always chose 12 delayed rather than four immediate cereal pieces or grape slices. Even when pieces of food were presented every 20 seconds — a long time to wait when staring at available grub — 14 of 19 chimps regularly held out for a larger snack than they would have gotten right away.
Chimps, and perhaps other apes, control their impulses and anticipate future rewards better than other nonhuman primates, Beran suspects. In 2013, he and his colleagues reported that 18 capuchin monkeys tested on the self-control task frequently pointed at the bigger food amount but typically failed to wait long enough to collect more food than was available right away.
Self-control, combined with powerful memories of past experiences and familiar landmarks, enables wild chimps to follow a plan. They’ve been known, for example, to retrieve nuts from nut-bearing trees and collect nut-cracking stones from rock-strewn areas, Beran says. The chimps will then take those items to places with flat boulders that can be used as natural anvils for breaking nuts open.
People call on self-control to achieve goals more distant than chimps can imagine, Beran says. Humans would store nuts and nut-cracking stones near boulder sites for future use, an option chimps don’t consider. Still, on a sliding scale of temptation control, chimps lie closer to humans than to capuchins, Beran suspects. Whether ancient hominids resembled modern chimps in this crucial ability is unknown, he says.
Female hunters
In the wild, as in Beran’s lab, grabbing a bite demands patience. Male chimps at Gombe and other forest sites can spend hours scaling and swinging through trees when hunting speedy colobus monkeys. Once the prey is caught and killed, patience flies out the window. Hunters eat much of the meat on the spot. They save small portions to dole out to females in exchange for sex.
Researchers have often thought of hunting as an infrequent, male-only affair among chimps. That assumption may apply to forest-dwelling chimps whose main prey are monkeys, but Fongoli’s savanna chimps practice a different, more inclusive form of hunting, Pruetz says.
At Fongoli, many community members, and especially females, jab pointed sticks into holes in trees where palm-sized primates called bush babies sleep during the day, she and her colleagues reported April 15 in Royal Society Open Science. The goal: immobilize a hidden bush baby, drag it out of its nest, kill it and eat it.
Fongoli chimps taper branch ends with their teeth before heading out to flush small bush babies out of their tree nests and grab them (SN: 3/3/07, p. 131). Pruetz’s team tracked this behavior from 2005 through 2014. Although females made up 43 percent of the Fongoli chimp community during that time, they hunted bush babies with sticks 175 times, compared with 130 such hunts by males. Female chimps averaged 10.6 hunts each, versus 6.8 hunts by each male chimp.
More than 4 million years ago, hominids may have participated in similar types of small-mammal hunts, Pruetz speculates. East African landscapes at that time resembled Fongoli’s savanna and could have supported hunters of both sexes, she says.
Whatever early hominids were up to, Pruetz rightly emphasizes chimps’ ingenuity in using tools to find food, Stanford says. While studying chimps in Gombe’s thick jungles, Stanford twice observed chimps poke sticks into holes on the sides of trees until woodpeckers flew out. The apes then used their sticks to break woodpecker eggs nestled inside. Yolks were scooped out and eaten.
Fongoli is not the only place where females use tools more frequently than males, Stanford adds. Researchers already knew that female chimps elsewhere take a special interest in termite fishing. In this practice, chimps jab twigs into large mounds where termites nest. Termites near the surface go into protective mode and bite the twigs. Chimps yank out their probes and lick off mouthfuls of protein-rich insects.
Grip masters
In a densely forested part of the Republic of the Congo called the Goualougo Triangle, chimps take termite fishing to another level. Insect-seekers deftly manipulate twigs with various hand grips. Goualougo chimps sometimes switch from one grip to another to break through hard sediment that seals openings to termite tunnels, Lesnik and her colleagues reported in the American Journal of Physical Anthropology in June.
Lesnik’s team used remote, motion-sensitive video cameras to monitor chimp activity at 10
Chimps often pushed through sealed openings of termite tunnels with a sturdy twig held in both hands or with four fingers of one hand tightly wrapped around the twig. Additional grips included pinching a twig between the thumb and the side of the index finger and bracing a twig between thumb and index finger so it could be held between any two other fingers. Those grips are somewhat similar to how people hold pencils.
After forcing open a termite tunnel, chimps typically inserted a smaller twig to clean out the passage and collect termites.
On 31 occasions, the scientists observed chimps switching grips while breaking into a termite nest. Adjustments usually involved going from a two-handed to a one-handed grip. Chimps switched away from pencil-like grips only three of 51 times they were observed. Those one-handed grips combined power and dexterity, enabling precise movements in tight spaces, Lesnik says.
If Goualougo chimps go to such great lengths to munch termites, it’s likely that ancient hominids did as well, Lesnik suggests. Evidence from hominid sites in South Africa that date to more than 1 million years ago supports that idea. Lesnik and several other researchers have reported that animal bones from those sites display microscopic damage consistent with being used as tools for digging into termite mounds and punching through sediment-covering termite tunnels.
“Chimps can set a benchmark of what early hominids were capable of doing, including making tools for termite foraging,” Lesnik says. Ancient hominids may have gripped those tools much like chimps do, she says.
Ardi and bonobos
Until recently, little was known about whether early hominids looked or acted like chimps today.
Then a team of paleoanthropologists discovered and studied the 4.4-million-year-old partial skeleton of an adult female hominid dubbed Ardi (SN: 1/16/10, p. 22). Remains of more than 100 individuals from Ardi’s species, Ardipithecus ramidus, confirm that these early hominids were not built like modern chimps or humans, Tim White of the University of California, Berkeley, and his colleagues concluded in the April 21 Proceedings of the National Academy of Sciences.
Ardi’s surprising physical appearance topples the long-standing assumption that the earliest hominids, and the last common ancestor of present-day people and chimps, were relatively chimplike, the researchers contend.
Ardi walked upright rather than on her knuckles, as chimps do, White’s group says. She moved
Looking at the same fossils, most chimp researchers disagree. A. ramidus was an apelike hominid, they say, that climbed trees well and probably ate a varied diet that would appeal to chimps, including fruit, nuts and seeds. That makes chimps a good, if imperfect, source for thinking about how she may have behaved, Stanford says.
Like Ardi’s extinct species, present-day bonobos are wild cards for ape researchers aiming to reconstruct ancient hominid behavior. “Humans are as closely related genetically to bonobos as to chimps, so we need much more research on bonobos,” Rosati says.
For now, investigators see some general differences between bonobos and chimps. Bonobos often resolve conflicts through sex, while chimps frequently fight. Bonobo females form strong bonds and wield more power over daily affairs than female chimps do. Unlike chimps, bonobos rarely use tools in the wild.
Chimps are predisposed to use sticks, stones and other objects as tools, says anthropologist Kathelijne Koops of the University of Zurich. Bonobos show little interest in turning inanimate objects into tools, Koops and her colleagues reported June 16 in Scientific Reports.
Not a chimp
Some researchers say that the shapes of certain skeletal parts — including the skull base, teeth and feet — show that 4.4-million-year-old Ardipithecus ramidus (center) was fundamentally different from both chimps (P. troglodytes) and humans (H. sapiens). That would mean, some say, that chimps provide no insights into ancient hominid behavior. Chimp investigators disagree.
Chimps in Uganda and bonobos in the Democratic Republic of the Congo had comparable access to stones, twigs and other potential foraging implements, according to Koops’ research. But only chimps used these resources as tools to procure food. And only young chimps showed a keen interest in playing with leaves and other forest knickknacks.
Curiously, though, captive bonobos become ardent tool users when prompted by experimenters, reports a team led by anthropologist Itai Roffman of the University of Haifa in Israel. At a German zoo and a U.S. sanctuary with grasslands and a large forest, 11 of 15 bonobos used researcher-provided branches, stones and deer antlers as tools to retrieve food that had either been buried underground, placed in hard containers or inserted in holes drilled in animal bones, Roffman’s group reported in the September American Journal of Physical Anthropology. The animals were shown where food had been placed and then left to their own devices.
Using wild chimps as guides, Koops speculates that early hominids inherited a chimplike tendency to find playthings early in life as a prelude to using tools later on. Bonobos are also good models for exploring how ancient hominids transformed natural objects into tools, Roffman and his colleagues conclude.
Whether or not Ardi’s species shared anything in common with chimps, Jane Goodall’s scientific heirs have much work left to do. As Fongoli’s fire monitors and Goualougo’s grip masters suggest, chimps and bonobos probably live more complex lives than anyone has imagined.
This article appeared in the September 19, 2015, issue of Science News under the headline, “Fired-up apes: Chimps show their smarts and fuel an evolutionary debate.”
