How Scientists Count Mountain Gorillas — The Census Methodology
The statement “there are approximately 1,063 mountain gorillas on earth” is one of the most frequently cited facts in gorilla conservation communication. Less frequently addressed is how that number was arrived at, what the methodology behind it involves, and what precision and confidence actually attach to a census of wild animals living in dense mountain forest across three countries in central Africa. Understanding the census methodology changes how the population figure is interpreted — and why the IUCN’s 2018 decision to downgrade the species’ threat classification from Critically Endangered to Endangered was based on a rigorous scientific process rather than optimistic estimation.
The Challenge of Counting Animals in Dense Forest
Counting mountain gorillas in the Virunga range and Bwindi cannot be done the way you count animals in an open savannah — aerial surveys, transect line methods, or direct visual observation from elevated positions. The dense montane forest that mountain gorillas inhabit prevents large-scale visual observation, and the animals’ ground-level ranging pattern makes aerial assessment impossible. The census methodology for mountain gorillas must therefore use indirect evidence — signs left by the animals — supplemented by direct observation of individuals in known, habituated groups.
Nest Counts
The primary census method for mountain gorillas is nest counting. Mountain gorillas, unlike chimpanzees, build new sleeping nests every night — each adult and juvenile gorilla constructs a new nest each evening from bent vegetation, and these nests are distinctive, large, and relatively durable. A team of researchers walking transects through the forest can count, measure, and record nest sites across a defined survey area, and from the distribution and characteristics of those nests, derive statistically valid estimates of the gorilla population density in the surveyed area.
Nest counts are analysed using distance sampling methodology — a statistical framework that accounts for detection probability decreasing with distance from the transect line, which produces population density estimates with calculable confidence intervals rather than single point estimates. The 2018 census of the Virunga population used a network of transect lines covering the full extent of the Virunga range across all three countries, walked by research teams from each country’s national park authority working under the coordination of the International Gorilla Conservation Programme.
Direct Observation of Habituated Groups
For the 12 or more fully habituated gorilla families at Volcanoes National Park — the families that RDB rangers monitor daily and that are the subject of continuous long-term research by the Dian Fossey Gorilla Fund and Gorilla Doctors — the census does not depend on nest counts. Every individual in every habituated family is known by sight, named, and has a documented life history. The census count for habituated families is therefore derived from direct individual-level observation rather than statistical sampling — a higher-precision count that anchors the broader statistical estimate derived from nest surveys.
The combination of direct counts from habituated families and nest-count estimates from unhabituated populations produces the overall census figure, with the confidence intervals that allow the IUCN to assess whether the population trend is positive, negative, or uncertain. The 2018 census produced a count of 604 gorillas in the Virunga range and 459 in the Bwindi-isolated population — totalling 1,063, the first time the combined mountain gorilla count had exceeded 1,000.
Genetic Sampling
DNA analysis from gorilla faeces collected during transect surveys has become an increasingly important supplementary method in mountain gorilla census work. Genetic analysis from faecal samples can identify individual gorillas without direct observation — the DNA extracted from faecal material is gorilla-specific, and individual genotyping from multiple samples can confirm whether a sample represents a new individual not previously identified or a re-sampling of a known animal. This method improves the precision of non-habituated population estimates and reduces the double-counting risk inherent in purely nest-count-based methods.
What the 1,063 Figure Means
The 1,063 figure from the 2018 census represents the population at the time of the survey — a point-in-time estimate with associated confidence intervals, not a precise count of every living individual. The true population could be somewhat higher or lower than 1,063 within the bounds of the methodology’s precision. The significance of the figure is not the exact number but the trend it represents: the Virunga population has grown at approximately 3% per year through the monitoring period from the 1980s to the present, and the Bwindi population has also grown. A species growing at 3% per year is recovering, not declining — and that recovery is the scientific basis for the IUCN reclassification from Critically Endangered to Endangered that has made the mountain gorilla conservation story one of the genuine successes of twentieth and twenty-first century species protection.
