Seeing the forest, from the trees

5 min read

Researcher gets a chimp’s-eye view of life

It was Valentine’s Day 2000 and Alain Houle was not quite sure what to do. He was alone in a fruit tree and the chimps were coming back.

“I thought I’d be killed,” Houle said later. “They climbed up, looked at me, barked at me, and then settled down to eat.”

After Houle climbed down that day, he returned to the research station in Uganda’s Kibale National Park and met Richard Wrangham, Ruth Moore Professor of Biological Anthropology, who has studied the park’s chimpanzees since 1987.

Though Houle was in the park studying the diets of monkeys for his doctoral work at the University of Quebec at Montreal, Wrangham expressed interest in Houle’s experience and said that chimpanzees had never been studied at eye-level in the treetops before.

Houle received his doctorate in 2004 and immediately came to Harvard to work with Wrangham. He is using his tree-top technique to understand how a female chimpanzee’s social status affects her breeding success. Houle believes the key is her diet.

Previous research has shown both that higher-status female chimpanzees have better breeding success than lower-status females and that higher-status females feed higher in the forest canopy than lower-status females.

Houle thinks the link between those two findings is the fruit. Higher-status chimpanzees, he said, likely feed on more, higher-quality fruit — found higher up in the tree — than lower-status chimpanzees, which leads to the chimps being in better physical shape and greater breeding success.

To explore that question, Houle returned to Uganda. Five or six days a week for a year, he rose at 4 a.m. and hiked through the forest to a likely chimpanzee feeding tree and, using ropes and a harness, climbed in the dark.

“My objective was always to be in the trees before them. I would stay there until they came,” Houle said.

Because tropical forest trees don’t fruit at the same time and fruiting trees are typically far apart, it was relatively easy to predict where the chimps would feed each day, Houle said.

The trees ranged from 75 to 165 feet high, giving Houle a perspective of the whole forest that was unique — and unforgettable.

Houle said he would sit and observe the chimpanzees’ behavior in the trees, watching carefully which fruits they selected, how many they ate, and at what height. After a while, the chimpanzees would climb down and go on their way, but Houle would remain as the next wave of hungry visitors — monkeys, birds, and other creatures — followed the chimps. Some days the chimpanzees would return to the same tree as afternoon wore on, to feed again before nightfall.

“I could stay 12 hours in the tree,” Houle said. “From up there, I could see they’d pick the ripest fruit first, then pick the part-ripe fruit, but not the green ones. … It was the best time of my life.”

Day after day, Houle repeated the ritual, eventually collecting 440 samples of 17 different fruits, and even breakfasting on the same fruits the chimpanzees were eating. He said he never ate at the same time as the chimps, though, to avoid any confrontation with them.

“I tried all of their fruit,” Houle said.

The figs were the best, while many others had an astringent flavor. Some, however, were just plain disgusting, he said.

Houle’s fieldwork ended in 2005 and he’s been busy running nutritional tests on fruit samples and entering and analyzing data since.

He figures there’s three possible ways the fruit could prove the link between female chimpanzee social status and reproductive success. The first is that the fruit at the top of the tree could be higher quality, providing them more nutrients than fruits eaten lower in the tree by lower-status females. The second is that there could be a greater quantity of fruit, so higher-status chimps just eat more. The third is that they could grow more closely together, meaning the higher-status females have to burn less energy to eat the same amount, leaving more energy for reproduction.

“I don’t know which hypothesis will fit,” Houle said.

Though the analysis is incomplete, Houle said preliminary results show that fruits from the tops of the trees versus lower in the canopy have between 3 percent and 10 percent more sugar and are 10 percent to 50 percent bigger. In addition, he found that the fruit density in the top of the canopy is between two and 140 times higher.

Houle, who expects to begin publishing the results of his work this coming December, said he first began climbing trees for research in 1999, while working on his doctoral dissertation. The first time he climbed, he said, he got 150 feet up in a tree. Between nerves and inexperience, he failed to organize his ropes properly and fell 15 feet before a safety device — installed on the advice of a mountaineering friend — stopped him.

Despite that inauspicious start, Houle continued climbing. Once his work on chimpanzees is concluded, he said, he’s interested in pursuing similar research on bonobos — a chimpanzee cousin — in the Democratic Republic of the Congo.