These habitats provide refuge for endangered animals, such as the tapir and crocodile (both frequently seen at Corcovado Lagoon), ocelot, cougar, giant anteater harpy eagle (the largest bird of prey in the world), and the scarlet macaw (which has the largest population in the country).  Other protected species are howler, spider, squirrel and white-faced capuchin monkeys, great tinamou, silky anteater, poison dart frog, glass grog (with transparent skin), cayman, bushmaster snake, and leatherback, olive ridley and green turtles that nest at Llorona Beach.

The forests are tall, thick and dark. Identification has been made of 500 species of trees, 140 of mammals, 367 of birds, 40 of freshwater fish, 117 of amphibians and reptiles, and it is estimated that there are some 6,000 species of insects.

The climate at Corcovado on the coast and plains and in the valleys is generally hot and very humid.  The climate in the highlands, which reach 745 metres above sea level, is mainly cool, more as a result of the cloud cover than the height.  Average annual rainfall is quite high, 4,500-7,00 mm., with torrential rains from April to December.  It rains all year long on the peaks, providing sufficient humidity to fulfill all the needs of the forest mass.  A short dry season, which adversely affects the understorey, lasts from February to the first week in April.

The Osa Peninsula can be reached by air or by land, following the Southern Interamerican Highway. The Park Headquarters are locatel at Puerto Jiménez, 371 kms. from San José.  From this point there are dirt tracks and flagstone road that border the coast as far as the station at Sirena, 60 kms, away.