High-quality, meaning: meat. The shift toward eating more meat may have played an important role in the evolution of a larger, more sophisticated brain. Here's how the thinking goes. Brains are, to use terminology coined by researchers Leslie Aiello and Peter Wheeler, "expensive tissue." To keep a bigger brain functioning, some other organ or system needed to become more streamlined. A chimp doesn't have to eat nearly as much of an energy-rich food like meat as he would of low-nutrient plant matter. Expending less energy on digestion means you can afford to apply it elsewhere, perhaps to power an expanded brain.
As if on cue, a female named Tia appears in our sight lines 20 feet ahead, sitting on a boulder pulling raw flesh off a limb like a picnicker with a comically huge drumstick. Pruetz raises her binoculars, then lowers them again. "Holy crap! It's a bushbuck." She can tell from the white markings on the hide, a long strip of which hangs from the leg. "That's the biggest animal I've seen them eat." She surmises it was a fawn. Gombe chimps have occasionally killed bushbuck fawns as well. They are the largest prey on record for a chimpanzee.
Hunting at Fongoli coincides with the rainy season, and Pruetz has some theories about why this is. As water holes fill and shoots and other greenery become more plentiful with the rain, the land provides enough sustenance to support a sizable group of chimps on the move. There are advantages to traveling in a large group. A single chimp or small group that heads out on its own can easily lose track of the community for days at a time. For a chimp, sociability is important. Pruetz points to an estrous female named Sissy, her pink swelling bobbing behind her like a bustle. "Otherwise you miss out on that." She means, of course, the chance to mate, to pass along your genetic material.
Right now, two rains into the rainy season, there's enough water and food for the group to travel together, but just barely. Pruetz believes it is this scenario—large crowd competing for limited resources—that has pushed certain members of the community to try their hand at novel things.
Things like sharpening sticks to spear bush babies. It is a different kind of hunting than the organized colobus monkey raids documented at other sites. A chimp who comes across a dead, hollow tree limb—promising real estate for day-sleeping bush babies—will sometimes break off a branch from a nearby tree, remove the leaves and the flimsy ends, and then use its teeth to whittle one end to a point. This tool is then stabbed into an opening in the tree limb until the animal inside is out of commission. Whereupon it is eaten, head first, Pruetz says, "like a Popsicle."
Adult female and juvenile chimps—the low rankers—have been seen hunting bush babies most often. This makes sense. Dominant males are not generous with food they find, and no one can force them to share. Fongoli females appear to have taken matters into their own hands.
Now here comes Farafa, her baby Fanta on her back and a bushbuck haunch in her jaws. It's a complicated, messy piece of anatomy, with sinew and hide hanging off one end. Tia sees her and stands up to move away. My last glimpse of Tia is with her now bare bone brandished above her head, standing erect, as though reenacting the "dawn of man" scene from 2001: A Space Odyssey.
Fongoli chimps have a flair for the dramatic.