The highest upland watershed area contains expansive forests, which are primary in the highest elevations and secondary in the lower foothills of the Maya Mountains. These montane areas provided an impenetrable protective backdrop for the ancient Mayan settlements here, preventing invasion from any tribes resident in Honduras or Guatemala. These forests have important lumber species such as mahogany and cedar trees and a broad panoply of other broadleaf tree species as well as numerous lianas. Fauna include the jaguar, margay, tapir and venomous pit viper Fer-de-lance, "Bothrops asper".