Wild chimpanzees pick up ant-fishing behavior from a female immigrant
Photographer Bill Wallauer was following a group of chimpanzees in Tanzania’s Gombe Stream National Park one March day when a young female caught his eye. She had climbed a tree, inserted a thin, peeled branch into a hole and was fishing out carpenter ants. Wallauer, of the Jane Goodall Institute, took out his video camera and filmed the chimp as she slurped up insects for several minutes.
What Wallauer witnessed wasn’t supposed to happen. Though chimps in other areas use tools to collect carpenter ants, scientists studying the Kasekela chimp community at Gombe had rarely seen the behavior since Jane Goodall began her fieldwork there in 1960. Before Wallauer’s 1994 observation, researchers had seen only one other instance of the behavior, in 1978. This type of tool use was considered a fluke.
But when Robert O’Malley, a primatologist now at Kenyon College in Gambier, Ohio, went to Gombe in the late 2000s, he noticed many of the Kasekela chimps regularly fishing for ants. He wondered why, after decades with only a couple of sporadic sightings, ant probing had become a widespread habit. Because of meticulous record keeping at Gombe, O’Malley and his colleagues had a rare opportunity to reconstruct the origin of this behavior.
An adult female immigrant who joined the Kasekela group in the early 1990s, the team concluded, introduced ant fishing, a common practice in her previous community. The finding, reported late last year in Current Anthropology, marks the first time in the more than 50-year history of chimp field studies that anyone has documented the transfer of a cultural tradition from one wild chimp group to another.
“It’s something we’ve been waiting for forever,” says William McGrew, a cultural primatologist at the University of Cambridge in England.
Research in captive chimps had suggested that they are capable of cultural exchange, but no one knew whether apes do it naturally. Uncovering and analyzing cultural spread in chimps, humans’ closest living relatives, may give scientists a glimpse of what makes human culture special.
Between you and me
Most scientists who study chimps agree that the animals have a basic form of culture. A range of “customs” — everything from meal choices to tool use to courtship rituals — vary from group to group (SN: 11/17/07, p. 317). These traditions often arise within a community when one member invents something new and the behavior catches on. Primatologists have recorded many examples of this process. In the 1990s, for instance, a group of chimps in C´te d’Ivoire’s Taï National Park started eating the leaves of two plant species in a different way, nibbling a leaf into a crescent shape and then swallowing it whole. (The purpose of this “leaf cutting” is unknown.) And in 2011 Japanese researchers reported that a chimpanzee community in the West African nation of Guinea learned how to disable snares set by hunters.
A first clue that habits might actually pass between groups came from a field experiment reported in 2003. Over a decade, researchers periodically provided wild chimps in Guinea with piles of hard-shelled coula nuts in a forest clearing. The chimps knew how to use stones as hammers but were unfamiliar with these nuts. A 30-year-old female immigrant started smashing the new nuts right away. Eventually, other individuals followed. By the end of the study, about two-thirds of the chimps cracked coula nuts whenever the researchers offered some. The scientists suspect that the pioneering immigrant recognized coula nuts as edible because she came from a group that regularly dined on them.
The new study offers more persuasive proof of natural cultural exchange and is the only documented case among chimps involving no human intervention. Like a historian piecing together the past, O’Malley combed through Gombe’s records, including some of Jane Goodall’s original notes, to look for instances of ant fishing. He also watched videos taken by Wallauer in the 1990s and 2000s and collected his own observations from 2008 to 2010.
The Gombe files, along with interviews with long-term field assistants, confirmed that Wallauer’s 1994 encounter was the first time since 1978 that researchers had watched Kasekela chimps fish for ants. From 1994 through 2007, ant fishing was observed 11 additional times. The activity became even more frequent after 2008, when O’Malley witnessed 17 sessions of one or more chimps probing trees for carpenter ants. By 2010, all chimps born in Kasekela after 1981 were ant fishers, as were several females who moved into the community as adults.
Circumstantial evidence points to a female named Trezia as Kasekela’s original ant fisher: She was the first adult to proficiently probe for carpenter ants and the only adult before 2003 to eat the insects. Trezia moved to Kasekela in 1991, coming from the neighboring Mitumba group. Researchers had documented frequent ant fishing in Mitumba since the late 1980s, so Trezia probably knew how to probe for ants before she joined the Kasekela group. Once she became established in the community, young chimpanzees picked up her habit, passing it along to new generations.
“The whole pattern fits that it was introduced by another group,” says psychologist Andrew Whiten of the University of St. Andrews in Scotland. If so, ant fishing at Gombe offers the best evidence yet that chimps do swap traditions.
But Bennett Galef, an animal behaviorist at McMaster University in Hamilton, Canada, is skeptical that ant fishing at Gombe counts as an example of a cultural swap. Kasekela chimps already fished for termites, so it is not surprising that they started using the same technology on a new insect, he notes. “It really isn’t a transfer of a novel behavior.”
Rare and elusive
The fact that it took more than half a century to identify a possible instance of cultural transmission among any wild chimp groups raises the question of whether such an event is rare or simply difficult to document. The answer is probably both.
Studies in captivity and in the wild hint that cultural transmission across groups may be an unusual phenomenon. Though young chimps learn at least in part from watching their peers, adults don’t like to try new things. “They’re creatures of habit,” Whiten says. “Like humans, they get stuck in their ways.” Even innovations within a group rarely take hold. This may explain why adventurous youngsters took up ant fishing at Kasekela while older adults never bothered.
Migrations in general are uncommon — maybe two individuals will enter a group each year at Taï, for example — which also limits the chance for cultural exchange, says primatologist Christophe Boesch of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Because chimps tend to be conformists, newcomers usually adopt the traditions of their new group, he says. A recent experiment in captive chimps supports this claim. When presented with equally viable ways to solve a novel task, chimps at a sanctuary in the Republic of the Congo chose the method used by the most other chimps, researchers reported last year in Current Biology.
Prestige plays a role in learning, too. Just as teenage boys and girls copy Justin Bieber’s haircut or Taylor Swift’s sense of style, chimpanzees emulate high-ranking group mates. Because immigrants — always female since males don’t leave the group where they are born — have low status, chimps don’t normally pay attention to what they do. But Trezia rose through the ranks quickly, making her an attractive role model, O’Malley says.
Local environmental conditions at Gombe also helped ant fishing spread. Kasekela’s territory is home to a frequently visited spot that has several trees well stocked with carpenter ants, where multiple chimps can probe for the bugs simultaneously. Sparse ground vegetation makes it easy for the chimps to watch and learn from each other.
The introduction of ant fishing at Gombe underscores the social, psychological and ecological factors behind cultural transmission. But no matter how common or rare such swaps may be, they will always be hard to detect, O’Malley says. Researchers can’t observe all group members all the time, so key events may be missed. And to identify knowledge transfers between groups, scientists need to know the behavioral repertoires of multiple local communities. Gombe is one of the few field sites where more than one chimp group is monitored, he says.
Despite the difficulties, collecting more examples of cultural exchange may help reveal what behaviors are most likely to be traded, as well as when, why and how.
But time may be running out. Deforestation and hunting threaten the ape’s long-term survival. “Many communities have already disappeared and have taken their unique patterns of behavior with them,” O’Malley says. “As remaining populations become more isolated, there will be fewer opportunities for gene flow and cultural diffusion.” For scientists that will mean fewer opportunities to witness these events.
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