San Jose.– Biologists and veterinarians are baffled by the death of more than 3,000 monkeys of various species at a remote national park in southwestern Costa Rica.

The deaths of the spider, white-faced capuchin, howler and squirrel monkeys occurred over several weeks late last year in Corcovado National Park, prompting Costa Rican and U.S. scientists to launch an investigation.

Eduardo Carrillo, a scientist at the National University of Costa Rica, told the press that between 30 and 40 percent of the monkey population at the park - home to some 10,000 primates - had died, probably from a disease, though the nature of the illness remains a mystery.

He said the deaths of the monkeys "are a warning signal for all of us, so we take into account that some protected areas are isolated," adding that it was necessary to increase economic support and bolster research efforts in those areas.

Carrillo and Grace Wong, another researcher, last week collected blood samples from monkeys in different sections of the park, which will be sent on to the University of Texas for analysis by colleagues there.

The most accepted hypothesis is that an illness that entered the country from outside is responsible for the deaths, although it will take a couple of weeks for the results of the blood tests to be ready.

For his part, Corcovado National Park director Eliecer Arce said that another hypothesis being considered is that lack of food brought on by excessive rainfall during the 2005 rainy season caused the deaths.

The staple of the monkeys' diet is wild fruit, tender leaves, flowers, insects and small vertebrates, food sources that may have been in scarce supply in Corcovado due to the copious rainfall.

However, Carrillo said that, although that possibility is still under consideration, it is regarded as less likely than the disease hypothesis.