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Counting mountain gorillas: more than a numbers game


Survey results released this week have revealed that the number of mountain gorillas in the Virunga Massif have increased to a minimum of 604. This brings the total wild population of mountain gorillas to more than 1,000 and makes it the only great ape in the world that is believed to be increasing. But there’s so much more to this story than ‘just’ a number…

Mountain gorillas in Volcanoes National Park, Rwanda © Julia Cumes

Where in the world?

Mountain gorillas are only found in two isolated populations in Africa. One population lives in the Virunga Massif – an area comprised of three national parks, spanning the borders of Rwanda, Uganda and the Democratic Republic of the Congo (DRC). The other population lives in Bwindi Impenetrable National Park in Uganda, which is connected with DRC’s Sarambwe Nature Reserve.

Map showing the two mountain gorilla populations

The best monitored ape subspecies

There have been surveys of mountain gorilla numbers every 5-10 years since 1971. Separate surveys are conducted in the two areas where mountain gorillas live, and together the results give us a global estimate of mountain gorilla numbers. The most recent survey was of the Virunga Massif.

No mean feat

Data collection involved twelve teams of six people, plus extra guards – a total of 78 people from more than 10 different institutions. Each team is comprised of a team leader, an assistant leader, two gorilla trackers, a ranger, and a cook. Importantly, each team included representation from each of the three countries involved, which is important for strengthening relationships and ensuring collective ownership. Throughout the survey, at any one time, half of the teams were in the forest, whilst the over half would be resting and restocking equipment and supplies. Every two weeks they would swap over. Together, the teams systematically covered more than 2,000 km of difficult forested terrain in the Virunga Massif. Check out this interview with Joseph Arinaitwe who shares his experiences as a census team leader:

A census, but not as we know it

Counting mountain gorillas doesn’t actually involve counting gorillas. Some mountain gorillas have been habituated to the presence of people for the purpose of tourism or research. Those gorillas are monitored on a pretty much daily basis and it would be possible to literally count them. But crucially, the census also tells us information about unhabituated gorillas – those that aren’t regularly monitored – and it would be impossible to directly count those gorillas. The survey teams instead search for signs of mountain gorillas, such as trails and footprints, and use these to locate gorilla nest sites, where they can collect faecal (poo!) samples for genetic analysis.

What’s in a number?

So, we know now there are 604 mountain gorillas in the Virunga Massif – right? Well, sort of. What we’re certain of is the number of known individuals based on the DNA samples that were collected – that’s 604. But even with the enormous effort that was put in, it’s very unlikely that we got faecal samples from every gorilla in the landscape. That means that the ‘number of known individuals’ is a minimum count and a conservative estimate of the population. But there are ways to calculate a more representative estimate and we’re working on those.

The latest survey of the gorillas in the Virunga Massif involved two entire ‘sweeps’ of the landscape – in other words, the teams covered the entire area and then a few months later they covered it again. During each sweep, faecal samples were collected and, through DNA analysis in a laboratory, individual gorillas were identified. By comparing those individuals that were identified in the first sweep with those that were identified in the second sweep and applying some clever statistics, we can get a sense of the amount of gorillas we missed in the survey. We use that information to come up with a more accurate overall estimate, which will be a range of values within which we are sure the true value lies. We’ll release those estimates later in the year.

It’s complicated

Undertaking two sweeps during the survey is part of technique known as “capture-mark-recapture”. One of the fundamental assumptions of the capture-mark-recapture method is that each individual in a population has an equal chance of being ‘detected’ – in the case of the mountain gorilla survey, by ‘detected’ we mean their poo is collected. For mountain gorillas, that assumption isn’t entirely valid; gorillas live in social groups so if you ‘detect’ (find the poo of) one individual in a group you’re more likely to ‘detect’ (find the poo of) the other members of that group too because they will be nearby. Going forward, we’re going to try and apply some more clever statistics to address this challenge.

Mountain gorilla in Volcanoes National Park, Rwanda © Perry White


We’re getting better and better at estimating mountain gorilla population numbers. That’s absolutely a good thing, but it complicates our ability to compare the most recent survey results with previous counts. The last census of the Virunga Massif was in 2010 and that identified an estimated gorilla population of 480. But that estimate was based on only one sweep of the landscape, and included an estimated correction for unknown infants. And so we can’t directly compare the results without acknowledging that the different methodologies may have had some impact.

Only (approximately) half of the story

As these results from the Virunga Massif survey are released, teams in Uganda are recovering from the first sweep of the survey of the gorilla population in the Bwindi-Sarambwe landscape. Sweep two of that landscape will be carried out later in the year and then, once the genetic analysis is done, we’ll be able to update the estimate for that population. Fingers crossed we see an increase there too!

Together we can do so much

The positive survey results are testament to the extraordinary efforts made by rangers, communities, governments and NGOs. People are, quite literally, risking their lives to protect mountain gorillas. Their dedication cannot be overstated. Today is a day for celebration, undoubtedly. But the work to protect this incredible species and its habitat isn’t yet done. The threats to mountain gorillas haven’t disappeared – in many cases they are increasing.

We’re committed to doing everything we can to build on today’s survey results and secure a future for the mountain gorilla. Are you with us?

We work on mountain gorilla conservation as part of the International Gorilla Conservation Programme (IGCP).

The Virunga Massif mountain gorilla census was conducted by the Protected Area Authorities in the DRC, Rwanda, and Uganda (l’Institut Congolais pour la Conservation de la Nature, the Rwanda Development Board and the Uganda Wildlife Authority) under the transboundary framework of the Greater Virunga Transboundary Collaboration.

The census was supported by the International Gorilla Conservation Programme (a coalition of Fauna & Flora International and WWF), Max Planck Institute for Evolutionary Anthropology, Dian Fossey Gorilla Fund International, Institute of Tropical Forest Conservation, Gorilla Doctors, and North Carolina Zoo.

 The census was funded by Fauna & Flora International, WWF, and Partners in Conservation at the Columbus Zoo & Aquarium. Additional financial support to the census science committee was provided by Berggorilla & Regenwald Direkthilfe.

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