By Bruce Bower
CHICAGO — Give the chimpanzees living at Uganda’s Toro-Semliki Wildlife Reserve a hand for having the mental moxie to dig water-collection holes along the edge of a river that flows only during rainy months. In fact, give them two hands, because wells dug by these chimps show no evidence of having been fashioned by either right-handers or left-handers, according to anthropologist Linda Marchant of Miami University in Oxford, Ohio.
Evidence of ambidexterity in Semliki chimps counters a suggestion from other researchers, based largely on studies of captive animals, that chimps often favor one hand over the other when performing various tasks. If it exists, chimp handedness interests researchers because it may reflect an evolutionary move toward a brain organized more like that of people — with one hemisphere dominating over the other and prompting either right- or left-handedness —than has often been assumed.
“We see no signs of handedness among the Semliki chimps, which appears to be the condition in the wild,” said anthropologist and study coauthor William McGrew of the University of Cambridge in England.
Marchant presented her team’s new findings on April 3 at the American Association of Physical Anthropologists annual meeting.
Rather than excluding hand preferences altogether among wild chimps, findings at Semliki indicate that chimps use both hands equally on physically demanding jobs, such as well digging, remarked Elizabeth Lonsdorf, director of the Lester E. Fisher Center for the Study and Conservation of Apes at Lincoln Park Zoo in Chicago.
“I suspect that for activities requiring fine movements, such as termite fishing, chimps’ hand preferences will pop out on closer study,” Lonsdorf said.
Earlier work at Gombe National Park in Tanzania found that wild chimps living there tended to use their left hands when engaging in a practice known as termite fishing, she noted. In this activity, chimps poke sticks or grass blades into termite mounds and then put the insect-covered implements into their mouths for a protein-rich snack.
Digging wells at Semliki takes far more effort than collecting termites at Gombe. Chimps scoop out holes in sandy riverbeds, a process that leaves two piles of soil on opposite sides of each depression. During rainy times of the year, river water fills the holes, from which chimps drink either by mouth or by dipping and then sucking on absorbent wads of leaves.
Over two days during the dry season in 2006, Marchant’s group counted 91 chimp-dug wells along Semliki’s main river. Initial observations of the holes indicated that they had been dug by individuals using both hands.
A six-month study, conducted from May to November 2008, confirmed that finding, Marchant said. That period included both wet and dry seasons.
Researchers walked for more than 5 kilometers along the same river and identified 121 wells. A series of measurements determined that each hole was symmetrical and likely produced by two hands acting in concert. Each of the two soil mounds adjacent to any particular well weighed the same.
Wells were dug in both dry and wet seasons. Chimps didn’t dig more wells at rainy times because water continually flowed in the river in 2008, Marchant said.
Chimps dug shallower wells last year than in 2006, apparently because of heavier rainfall in 2008.
So far, Marchant and her colleagues have not directly observed any Semliki chimps in the act of digging wells.
Marchant had assumed that only Semliki chimps created water holes by rivers. But in another meeting presentation the same day, a team led by Jill Pruetz of Iowa State University in Ames reported that chimps living in an area of southeastern Senegal called Fongoli also scoop out makeshift wells on river banks.
