Mountain gorillas are found in just two isolated habitats: the Virunga mountains (spanning Rwanda, Uganda, and the Democratic Republic of the Congo/DRC), and the Bwindi Impenetrable Forest–Sarambwe Reserve region of Uganda and the DRC. Given the mountain gorillas’ small population and many conservation pressures, it is important to have accurate, up-to-date information on their numbers and demographics.
Toward this goal, gorilla population surveys – also known as censuses – have been conducted for decades, with many of the earliest ones being organized by Dian Fossey herself. The Fossey Fund has played a leading role in every Virunga census, including the most recent one in 2016. In the Bwindi forest, the first full count took place in 1997 and the Fossey Fund now assists with these censuses as well.
After participating in planning and training efforts, members of our team are currently on the ground in Uganda, contributing to the 2025 Bwindi-Sarambwe mountain gorilla census, as it began its first phase – called a “sweep” – this month. This census will be done in two sweeps to reduce the chance of missing any gorillas in the landscape, with the next sweep scheduled for October.
A collaborative conservation effort for gorillas
The 2025 Bwindi-Sarambwe census is led by the International Gorilla Conservation Programme – a coalition of Conservation International, Fauna & Flora and WWF – and carried out collaboratively by scientists and national park authorities under the framework of the Greater Virunga Transboundary Collaboration.
In addition to the gorilla numbers, the new census will also provide information on demographics (ages and genders) within the Bwindi-Sarambwe gorilla population, parasite levels carried by the gorillas, the location of groups in relation to each other within the ecosystem, and illegal activities in the forest. The Bwindi-Sarambwe census will also include the first-ever count of chimpanzees in this forest, running parallel to the gorilla census.
How the gorilla census is done
Six teams from the participating institutions – including the Fossey Fund – are now systematically moving on foot through Bwindi from south to north, following reconnaissance trails, looking for signs of mountain gorillas, recording mountain gorilla fresh nest sites, counting the nests in each nest site and collecting fecal samples to be used for genetic analyses.
The Fossey Fund has provided five field staff members (Thadee Muhire, Olivier Dusenge, Theogene Bimenyimana, Eric Kabeja and Fidele Ntawumenyumunsi) to work in this in this census, as well as expertise from our gorilla program – Dr. Winnie Eckardt, our senior manager of primate research, and Jean-Pierre Samedi Mucyo, our gorilla program manager – who are providing scientific input through all phases of the census, from planning to data analysis and report writing, as well as helping train members of the census team.
The benefits of a gorilla census
“The gorilla census process helps us to answer critical questions on the dynamics of the mountain gorilla population, such as whether the population is increasing or decreasing, and helps us to evaluate how our conservation interventions are helping this vulnerable species,” says Mucyo.
“In addition, the census strengthens the collaboration between different institutions and countries that are working together to conserve mountain gorillas. For the first time in the history of the gorilla census, all genetic analysis will be done locally and this is a huge achievement,” Mucyo adds.
“Each time, it is a great honor to participate as scientific advisor and trainer in such a huge team effort, involving dedicated conservationists from the three neighboring countries – it is a massive task,” says Dr. Eckardt.
The 2018 Bwindi-Sarambwe census resulted in a minimum count of 459 individuals in 36 social groups and 16 solitary mountain gorillas. The last census of the Virunga mountain gorilla population, showed a minimum population total of 604 gorillas in that area. Together, they have confirmed the mountain gorilla’s status as the only wild ape population whose numbers are known to be increasing, thanks to the daily protection they receive.
In addition to numbers, “these repeated counts provide critical insights into overall population trends, confirm that intensive protection efforts are working, and provide all partners with a basis for good conservation planning,” says Felix Ndagijimana, the Fossey Fund’s director of Rwanda programs.
The 2025 census is funded by generous contributions from several partners including WWF, Great Virunga Transboundary Collaboration and Partners in Conservation, range states and local stakeholders through in-kind support. Other institutions supporting and participating in the census include Max Planck Institute for Evolutionary Anthropology (MPI), Dian Fossey Gorilla Fund, Institute of Tropical Forest Conservation (ITFC), Gorilla Doctors/Mountain Gorilla Veterinary Project (MGVP), Conservation Through Public Health(CTPH), Wildlife Conservation Society (WCS), WWF Uganda Country Office, Bwindi Mgahinga Conservation Trust (BMCT), and Primate Expertise (PEx).
