What are the greatest threats facing these species?
Well, at the risk of sounding shamelessly self-important, I would argue that one of the biggest threats to these animals is a lack of knowledge about them and their habitat. Monkeys are fairly conspicuous and their populations can be monitored, but many of the nocturnal species are rarely seen and thus we know very little about their numbers and how they respond to development and human activities in the forest. Habitat loss due to development and deforestation is a huge concern for all animals, but with arboreal species we don’t even know yet what makes for good or bad habitat in the treetops.
Assessments of habitat quality are typically done through ground-based methods. And while they may be somewhat related, no one knows whether high-quality habitat on the ground means there is high-quality habitat in the canopy. We think in great detail about where reserves are placed on the landscape two-dimensionally, but in order to ensure that the largest possible suite of species can be protected within a given area, you need to have an understanding of how the entire forest is used by its inhabitants in three dimensions.