Pesky parasites that thrive in fragmented forests are taking a toll on already stressed primates in Ugandan forests, suggests a new study.

The findings, detailed in a recent issue of the American Journal of Primatology, could explain why populations of black-and-white colobus monkeys are stable, while their cousins, the red colobus monkeys, are in decline.

The forests of Uganda, where the monkeys live, currently represent less than 5 percent of the original forest cover. As tropical forests are logged and converted into agricultural land at a fast rate, according to the World Resources Institute, the remaining forest comprises just small tracts hemmed in by pastures and croplands.

Thomas Gillespie of the University of Illinois at Urbana-Champaign and his colleagues followed the monkeys for four years, finding the red colobus populations in forest fragments declined 20 percent, while the black-and-white colobus remained relatively stable. Both species maintained stable populations in the undisturbed forest.

The fragmented regions also supported much higher densities of parasites than did undisturbed regions. While feeding on leaves, the monkeys ingest the larval forms of these parasitic worms, which mature in the monkey's intestines. The adult worms migrate through blood vessels, causing inflammation, organ damage and, sometimes, death for the infected monkeys.

Differences between the two monkey species affect their vulnerability to parasitic infection. Compared with the black-and-white monkeys, the red colobus monkeys eat a much more varied diet, and so they must travel farther to meet their dietary needs. The lengthy journeys would bring them into contact with more parasites.

“The red colobus typically eat 40 to 50 species of plants, but in these forest fragments we might only have 12 tree species," Gillespie said. "So there’s a dramatic reduction in what we typically would see them feed on." He added that their compromised nutritional status also weakens them, giving parasites the edge.

The black-and-white colobus munch on whatever's around at the time, allowing them to eat well under a variety of circumstances. That enhances their ability to withstand parasitic infections, Gillespie said.