Biologists jokingly call them “chamber of commerce” monkeys. Raucous and wild-eyed, they swing from branch to branch, screeching, howling and playing—all for the benefit of the seemingly endless parade of basket-mouthed tourists that visit Costa Rica’s national parks each year.

It’s an idyllic scene here among the swaying palms and crashing waves of the country’s wild Pacific coast.

But appearances can be deceiving.

A nationwide study by 18 of Costa Rica’s foremost wildlife experts reveals the country’s monkey populations have fallen by an average of 50% since 1995, a decline that roughly coincides with a coastal construction boom which has carved up large areas of wildlife habitat. “The situation is much sadder, and more serious than we thought,” said Gustavo Gutiérrez, a University of Costa Rica geneticist, after a late-July conference of primate experts here.

Populations of Costa Rica’s four monkey species are becoming fragmented through habitat loss, scientists say. The much loved, but critically endangered squirrel monkey (Saimiri oerstedii) has dwindled in number from 7,300 to 4,200 in little more than a decade. Squirrel monkeys now are found mainly in Manuel Antonio National Park—one of Costa Rica’s most visited—and the more remote Corcovado National Park, on the Osa Peninsula.

Stranded troops

Scattered troops of monkeys exist between them, but not enough to permit the interbreeding needed to maintain adequate genetic variability. The result, experts say, is inbreeding and—on account of genetic homogeneity—a greater susceptibility to disease.

“As they become weaker and weaker genetically, the risk of extinction increases,” says Gutiérrez, who calls the problem an embarrassment for a country that boasts of protecting almost 25% of its land in parks. “What we have is populations stranded in parks, with nothing in between to connect them.”

The five-year study, presented at last month’s conference, included blood tests and genetic exams of 329 individuals drawn from the country’s four monkey species: the squirrel, the white-faced capuchin (Cebus capucinus), the howler (Alouatta palliata) and the spider (Ateles geoffroyi). The result, explains Ronald Sánchez, a University of Costa Rica ecologist, is a littany of ailments ranging from cataracts and dental problems to skin pigment discoloration and various viruses and parasites.

Most troubling was the discovery of a strain of a malaria parasite not seen before in Central American monkeys, says Sánchez, who has roamed Costa Rican jungles for decades.

“As humans start to live closer and closer to the monkeys, the effects on [monkeys’] health are becoming serious,” says Sánchez, who reports almost all Costa Rica’s monkeys now come into contact with humans on a regular basis—a big change from just 20 years ago.

Howlers, whose numbers fell 64% from 102,000 in 1995 to 36,000 in 2007, fared worst on their physical exams. These large, gregarious animals, whose primordial roar is often heard by tourists in rainforest hotels, digest their food high in the leafy canopy. This subjects them to increasing solar radiation as the climate warms and the ozone layer thins—possibly boosting the incidence of cataracts and changes in skin pigmentation, Sánchez says.

Corridors needed

Gutiérrez expects such problems to worsen if more isn’t done to reconnect isolated monkey populations by creating biological corridors of undeveloped land. “Our populations are increasingly unhealthy, and when combined with their weak genetics, they are at a severe disadvantage when it comes to extreme weather events or catastrophes,” he says.

The risk of such events aren’t remote, says Grace Wong, a National University researcher who is one of the country’s premier wildlife biologists. During Costa Rica’s wet season in 2005, she reports, abnormally heavy rains and low temperatures led to a shortage of the green shoots, fruits and insects monkeys depend on, decreasing Corcovado Park’s monkey population by as much as 40%, she says.

Spider monkeys experienced the biggest proportional decline—of 72%, their population shrinking from 26,000 to 7,225 since 1995. Scientists point out that compared to the more adaptable white-faced capuchin, the spider monkey is shy and picky about its habitat, which is fast being consumed by development. Says Sánchez: “Their future is in serious doubt.”

All this has scientists and environmentalists scrambling to save what is arguably Costa Rica’s trademark animal—and one of its most visible tourist attractions. “People come to our country for the wildlife. If we don’t have monkeys, then what do we have?” says Sánchez.

Last month’s conference closed with discussion of such remedial steps as biological corridors, more public education and Central America-wide research on monkey populations. It also addressed a key ingredient for such efforts—funding. Says Gutiérrez: “I can capture all the monkeys I need. What’s more elusive is the funding to make these studies happen.”