Thousands of miles from Wisconsin, a Senegalese female climbed up a savannah tree and prepared herself for a hunt. Swiftly, she chose her weapon—transforming a nearby tree branch into a sturdy spear. With great force, she jabbed the wooden spear into the hollow spaces of the tree, hoping to immobilize potential prey. While the huntress failed to land many successful kills, her actions have captured the attention of scientists around the world—the Senegalese huntress is not a woman, but rather one of our close cousins, the female chimpanzee.
For the first time, scientists are reporting that chimpanzees from the southeastern region of Senegal are using weapons to hunt small mammalian prey. And, based on their observations, it's the adult females and young chimpanzees, an unusual hunting group, that are most likely to engage in this behavior. Together, these observations offer new details about early human learning.
""We know from genetic data that chimpanzees are our closest living relatives,"" said Karen Strier, UW-Madison professor of anthropology. ""So, they can provide comparative insights into our evolutionary past.""
The similarities between chimpanzees and humans extend beyond our genes. According to John Hawks, UW-Madison assistant professor of anthropology, chimpanzees maintain complex social lives and transmit information through social learning.
Based upon previous studies, most scientists believed chimpanzees reserved tool use for capturing insects and cracking open troublesome nuts, not hunting vertebrates. As Jill Pruetz, professor of anthropology at Iowa State University and other scientists discovered, chimpanzee tool use is far more sophisticated than scientists previously acknowledged.
Initially unaware of the unique behaviors of the Senegalese chimpanzees, Pruetz and others were drawn to study the chimpanzees living in a region in southeastern Senegal due to its unique habitat. Known as the Fongoli region, it is a relatively dry area containing both savannah and woodland ecosystems.
After four years of living among the Fongoli chimpanzees, Pruetz and other scientists were surprised to witness the emergence of a unique behavior among the female members of the Fongoli community.
From the ground, researchers watched as chimpanzees perched high in trees snapped off tree branches, trimmed excess branch leaves and branch ends and used their teeth to trim branch ends to create a sharp point. Grasping the tree firmly with one hand to maintain balance and holding the weapon in the other, the chimpanzees repeatedly thrust the weapon into a tree hole, where bushbabies—small, squirrel-like creatures—typically slept during the day.
""Almost every individual exhibited very similar behavior during weapon-making,"" said Pruetz.
Chimpanzees varied only in the number of steps they needed to create their ideal wooden spear.
While adult males tend to be the hunters in chimpanzee groups, the behavior observed by researchers was more prevalent among adult females and young chimpanzees.
""Typically, male chimpanzees hunt by chasing down red colobus monkeys,"" Pruetz explained. ""While male chimpanzees work in a coordinated effort, chasing down monkeys is a very high energy activity.""
Since red colobus monkeys do not live in the Fongoli region, chimpanzees are left with limited meal options. Hunting with tools provides an energy saving alternative to the hunting style of male chimpanzees.
""There is less energy involved in jabbing a bushbaby than running after monkeys,"" said Pruetz.
For mothers of young infants, hunting with weapons provides a novel way to obtain access to prey.
However, the success rate was low. Of the 22 chimpanzee hunts observed by scientists, only one led to a successful kill. Pruetz believes the low success rate observed among chimpanzee hunters may be because researchers are unable to see all hunting incidences. Additionally, the extra caution caused by the presence of human observers may have prevented more hunts from success.
Even if it proves true that the chimpanzees do have a low hunting success rate, Strier suggests there are still advantages to the low-energy hunting method.
""If [hunting] is the only way that chimpanzees can get access to this type of prey, then the payoffs may still outweigh the costs in terms of time, energy and nutritional factors,"" Strier said.
After all, Hawks said, ""Chimpanzees rarely go ape for bananas, but they always go bananas for meat.""
Based upon thousands of hours observing the behavior of adult male chimpanzees, Pruetz is confident that females and young chimpanzees are taking advantage of a niche largely ignored by adult males.
These new insights regarding the sophisticated behavior of chimpanzees act as a lens through which researchers can look back thousands of years in an attempt to hypothesize the behavior of our early ancestors. The predominately female and young Fongoli chimpanzees not only raise the bar on our understanding of chimpanzee intelligence, but also clearly indicate the importance of the role of the female in human evolution.
""Our findings support decade-old theories that state females are more likely central to the evolution of tool technology in our lineage and should be considered more than they have been in the past,"" said Pruetz.
