Researchers set up cameras on the ground, in the understory and even climbed very tall trees to attach cameras in the canopy.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/new-population-rare-dryas-monkey-videotaped-first-time
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/mongabay
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/wildlife
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/monkey
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/biodiversity
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/congo
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/africa