sealPurdue News

March 2000

Study suggests that logging may affect monkey health

WEST LAFAYETTE, Ind. – Secondary forests, areas that have been logged and allowed to regenerate, may provide second-rate habitat for primates, even decades after the forests have been allowed to regenerate, according to a study of monkeys living in African forests.

While studying the movements of gray-cheeked mangabeys in the Kibale National Park of Uganda, Purdue University doctoral student William Olupot found some unexpected differences in weight between animals living in primary forest – areas that have never been logged – and those living nearby in a forest that was logged in the 1960s and '70s.

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Of the 31 male mangabeys studied in the project, monkeys living within the secondary forests weighed on average 15 percent less than males living in relatively untouched primary forests.

"The lower body weights were not related to the skeletal measurements or the age of the animals, which means that these differences might be attributed to different nutritional conditions and habitat quality," Olupot says. "These are only preliminary data, but they suggest that additional studies may be needed to compare the weight of animals living in logged forests to those living in primary forests."

Olupot, a citizen of Uganda, found the differences in weight while tagging animals for a three-year study designed to analyze the dispersal rates of mangabey males. The study, funded by the National Geographic Foundation, the Wildlife Conservation Society and the Rockefeller Foundation, uses a radio-tracking device to follow the monkeys' movements within their own geographic boundaries and social group, and to track how frequently they migrate into other social groups.

The study, which was part of Olupot's doctoral thesis, will appear in an upcoming issue of the journal Conservation Biology. Olupot received his doctorate in December and is now a senior research scientist at the Bwindi-Impenetrable Forest Gorilla Reserve in Uganda.

Body weight is a key indicator of good health for animals in the wild, says Peter Waser, professor of biology at Purdue who worked with Olupot on the study.

"A decline in body weight is significant for these animals, which weigh 15 to 20 pounds and are about the size of a small dog," Waser says. "For example, lower body weight could lead to an increased mortality in infants and juveniles and lower fertility in females."

Though previous studies have shown that animals decline in numbers after logging, this is the first time a researcher has documented a decline in the weight of animals living within a logged forest, Waser says.

"It is especially significant when you consider that the logging was done 20 to 30 years ago," he says.

Though further study would be required to see what, if any, direct effect the lower body weights have on mangabeys, Olupot says his study did find evidence that living in areas that have been logged may affect the social behavior of the monkeys.

"My study shows that the groups living in the logged forests are less stable, and male mangabeys in these groups are much more likely to strike out on their own," he says.

Like most Old World monkeys, mangabeys spend most of their lives in groups made up of a stable core of females and several males, Olupot says. "However, the data show that mangabeys in secondary forests tend to break into smaller groups, with a higher rate of turnover in males entering and leaving the groups."

The data also show more sightings of solitary males in logged forests, and that, coupled with fewer trees and more open spaces, makes the male mangabeys more vulnerable to attacks from predators, Olupot says.

"Males are killed by predators, such as the crowned-hawk eagle, at a much higher rate when they are alone than when they are in a group," he says. "We also found a higher rate of juvenile mortality in logged forests."

Though body weight data were secondary to the study, Olupot's findings point to the need to further evaluate the effects of logging on primates, says Melissa Remis, assistant professor of anthropology at Purdue who studies the impacts of human activities on gorillas and other wildlife in central Africa.

"This is the largest radio-tracking study of African forest monkeys to date, and it's significant because it's the first opportunity scientists have had to weigh and measure these animals and gather information directly rather than just watching them at a distance," she says.

Remis says the data suggest that logged forests, with fewer trees and resources to provide shelter and food, may create a more stressful environment for the primates. The findings may also have important implications for conservation efforts, she says.

"In addition to logging, primates face threats from hunters and agriculture," Remis says. "These threats increase when logging efforts result in new roads and increased access to remote areas."

Sources: Peter Waser, (765) 494-8129,

Melissa Remis, (765) 496-1514,

Writer: Susan Gaidos, (765) 494-2081;

Purdue News Service: (765) 494-2096;

A gray-cheeked mangabey peers out at Purdue researcher William Olupot after being tagged with a radio device designed to track its location in the 776-square-mile Kibale National Park of Uganda. The monkey is part of an ongoing study focused on the behavior and biology of male mangabeys in African forests. (Photo supplied by William Olupot)

A publication-quality photograph is available at the News Service Web site and at the ftp site. Photo ID: Waser.forests

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