But Dr. Pruetz could not simply settle down right away and watch the chimpanzees. At first, the sight of her frightened them off. So Dr. Pruetz and her colleagues let the apes grow accustomed to their company. That alone took four years.

At last, in 2004, Dr. Pruetz and her colleagues could follow the chimpanzees from dawn to dusk. “You just have to drink water all day,” said Dr. Pruetz, now a professor at Texas State University.

The team gradually built up a catalog of strange behaviors — ones rarely if ever seen in others. Forest chimpanzees get enough water from the fruit in their diet so they need less drinking water and can wander in search of food. The Fongoli chimps, by contrast, required daily drinking water and anchored themselves to reliable water sources in the arid landscape.

And while forest chimpanzees are active throughout the day, Dr. Pruetz found that the savanna chimpanzees rest for five to seven hours. Dr. Pruetz could often find them lurking in small caves in the dry season, and when the rainy season arrived, the chimpanzees would slip into newly formed ponds and bob there for hours.

Forest chimpanzees typically spend all night in nests they build in trees. But at Fongoli, the research team noticed that the chimpanzees often made a late-night racket.

Staying up all night to watch them, Dr. Pruetz discovered that they spent hours after sundown searching for food. “It might as well have been a daytime scene,” she said.