Chimpanzees living around Lake Tanganyika have been surprising human beings with revelations about the behavior of both species since Jane Goodall first began to study free-ranging chimps in the area almost 40 years ago. Chimps make tools, grieve each other's deaths, kiss, play, and socialize, she observed. Yet they also spend time alone or, occasionally, form themselves into groups that go on "hunting binges" that raise disturbing questions about human beings as well as chimpanzees, their closest genetic equivalent.

At Mahale Mountains National Park, researchers from Japan continue to make startling discoveries about the behavior of chimps in the wild, using the same low-impact methods of observation and minimal interference that Goodall pioneered at nearby Gombe Stream National Park during the early 1960's. Toshisada Nishida and his team from Kyoto University are working in the Mahale Mountains to learn more about chimp society. They have discovered that males form strong coalitions, while females may transfer from one social group to another. In addition, they have discovered that chimps use plants for medicinal purposes and that hunting and the distribution of meat have social implications quite apart from nutrition.

Tools
The tool that is famous for proving chimpanzees have a distinct quality of intelligence, unmatched by other non-human species, was a grass stem plucked from the ground by David Graybeard. (Graybeard was the first chimp to accept Goodall and walk into her camp.) She saw him sitting next to a red earth mound that housed a termite colony. He poked the grass stem through a hole, withdrew it covered with termites, ate them, and threw the tool away. He immediately made history again by grabbing a vine, stripping off its leaves, and pushing the resulting bare stalk through the hole in the ground. This showed Goodall that a chimp could make a tool as well as use one. David Graybeard had modified a natural object to serve a particular purpose.

Language
If a thick growth of rainforest flora dampens the sound of the "pants," you may still hear the rising tones of the "hoots" as chimpanzees shriek to each other across the ridges of Mahale Mountains National Park in Tanzania. Dubbed the "pant-hoot" by primatologists, the most common call made by groups of chimpanzees may be marked by individual as well as regional differences. Researchers even refer to "accents" that distinguish one troupe of pant-hooters from another. Other sounds in the natural language of chimps include barks that mean "I found food!"; a series of tense screams to say "I'm being attacked!"; and a long wail that sounds like "wraa" and means "I found something strange and scary." Chimps have learned to communicate with American Sign Language at the Chimpanzee and Human Communication Institute in Ellensburg, Washington. The five chimpanzees living at the institute manage vocabularies of 30 to 300 words and use them to communicate not only with people, but also with each other.

Hunting
The only higher primate besides the human who eats meat regularly is the chimpanzee. When Jane Goodall discovered this in the 1960s, some old-school biologists were disgusted. They preferred to idealize chimpanzees as cheerful and mischievous children, who might tease each other mercilessly but would never eat anything more sentient than a blade of grass crawling with termites. Research has shown that chimpanzees are hunters and, even worse, killers who use meat strategically as a currency to buy allegiance from males and favors from females. They do not need meat to survive any more than people do, so the urge to hunt satisfies appetites that go beyond nutrition. Sometimes chimps go on "hunting binges" that last several days. A hunting party of 10 males, several females, and assorted juveniles will abruptly stop foraging and move into the forest to bag red colubus monkeys. The impetus for a binge is often a mystery, but the presence of an estrous female seems more likely than anything else to trigger one. Hunting offers a competitive arena for males, and when the males distribute meat at the end of a hunt, they feed their male allies and receptive females but refuse to feed their male enemies. A lone hunter may capture a colobus only 30 percent of the time, but a hunting party of 10 or more is nearly always successful. Red colobus account for about 80 percent of the chimpanzee meat diet, which also includes 24 other vertebrate species such as birds and eggs, bushbabies, even wild pigs and small antelopes.

Medicinal Plants
Chimpanzees have learned to cure some of their own diseases and discomforts by using plants in two different ways. To get the forest-dwellers version of castor oil, they peel the bark off young shoots of the plant vernonia amygdalina and chew on the pith, which is extremely bitter. A chemical in the pith attacks nematodes, thread-like parasites in the chimps' digestive systems that can cause discomfort and weight loss. Japanese researchers at Mahale Mountains National Park have observed that chimps recover their strength and appetites within 24 hours of using this plant. Humans living in the area have been known to use the plant for the same purpose. Another way to dislodge a parasite is to swallow a leaf whole. Chimps pull rough, hairy leaves from the aspilia mossambicensis plant and fold them into little accordions with their tongues. They swallow the leaves whole and later expel them along with worms that have latched on to the leaves.