"The principal factor that reduces the populations is always sooner or later the deterioration of habitat," University of Costa Rica researcher Gustavo Gutierrez told Reuters.

The spider monkey, characterized by its lanky limbs, has been the most affected, its population falling to 7,000 from 26,000 a dozen years ago, said University of Costa Rica ecologist Ronald Sanchez.

Costa Rica protects the environment of about a quarter of its territory to varying degrees, with 12 percent inside national parks.

But urban development, including new tourist hotels and condominiums, is making protected areas into virtual islands, fragmenting the monkeys' habitats and inhibiting genetic diversity needed to keep them healthy, Gutierrez said.

"What we have is 25 percent of the country in islands, without genetic contact between one island and another," Gutierrez said.

To restore the populations, scientists want the government to increase protected areas and create corridors to reconnect the isolated patches, Sanchez said.

Shifts in weather patterns believed to be caused by global warming could lead to lower growth of plants eaten by the monkeys, further hurting populations, scientists said.