Gorillas are the only great apes that consistently avoid crossing deep water, and this behavioural trait — which seems like a simple quirk but has significant ecological implications — reveals something interesting about the differences between the great ape species and about the specific evolutionary pressures that shaped mountain gorilla life in the forests of East and Central Africa. Understanding why gorillas avoid water, how this shapes their movement and range, and what it means for their conservation helps paint a richer picture of these animals than you get from the beautiful-photographs-in-forest version of the story.
The Evidence: Gorillas and Water
Field observations of mountain gorillas in Bwindi and the Virungas confirm that gorilla groups consistently avoid deep water crossings. They will cross shallow streams — those where the water depth is less than roughly waist-height on an adult female — without significant hesitation, using rocks or fallen logs to minimise direct water contact when these are available. But when encountering deeper streams or rivers, gorilla groups have been observed to travel considerable distances along the bank to find a shallow crossing point rather than swimming across.
This is in contrast to chimpanzees and orangutans, which, while not enthusiastic swimmers, will wade through and occasionally swim in deeper water when the incentive is sufficient. Gorillas appear to have a stronger and more consistent water avoidance that is not easily overridden by food or social incentives.
Why Gorillas Avoid Deep Water
Several explanations have been proposed, and the evidence suggests that multiple factors contribute.
Buoyancy and Body Composition
Gorillas have a body composition — high bone density, significant muscle mass, dense fur — that makes them less naturally buoyant than a human swimmer of similar size. The same physical characteristics that make a silverback extremely powerful on land make swimming less efficient. There is evidence that gorillas are physically capable of swimming if necessary, but the energy cost and the risk during the learning phase are higher than for species with lighter, more buoyant bodies.
No Evolutionary Pressure to Learn
In the montane forests of East Africa where mountain gorillas evolved, the streams and rivers are typically small enough to cross without swimming. There was no evolutionary pressure to develop swimming capability because it was rarely needed. Species that develop swimming behaviours typically do so either because their habitat requires crossing larger water bodies or because aquatic food resources make the investment worthwhile. Neither condition applies to mountain gorillas in their natural habitat.
Infant Safety
Mountain gorilla infants ride on their mothers’ backs until approximately three years of age, and the risk calculation for deep water crossing with a dependent infant is different from the calculation for a solitary individual. A female carrying an infant who falls into deep water faces a much more dangerous situation than she would alone. The consistent water avoidance of the whole group may be partly a generalised strategy that protects the most vulnerable members.
Ecological Implications
The consistent avoidance of deep water crossings has significant ecological implications for mountain gorilla range and habitat connectivity.
Natural Range Boundaries
Rivers and streams have historically served as natural range boundaries for mountain gorilla family groups. Two groups separated by a deep river may have overlapping ranges on either bank but rarely cross the boundary. This natural separation has both social benefits (reduced frequency of potentially damaging inter-group encounters) and conservation implications (a natural fragmentation of the population).
Forest Fragmentation
The water avoidance behaviour means that habitat corridors between forest fragments must be navigable on dry ground to be useful to gorillas. A corridor that requires crossing a deep stream will be unused. This is an important consideration in conservation planning for the mountain gorilla range — corridors between protected areas must be designed with gorilla movement biology in mind, not just mapped as any contiguous vegetation connecting two protected areas.
Bwindi Streams and Gorilla Crossings
Bwindi Impenetrable National Park is crossed by numerous streams, and researchers monitoring gorilla movement patterns have documented the crossing points used by different family groups. The data shows consistent use of specific shallow crossing points — particular rocks, certain log falls, sections where the stream widens and becomes shallower — by gorilla groups over years and decades. The knowledge of where to cross, and how to cross safely, is learned behaviour that juvenile gorillas acquire by watching adults.
On a gorilla trek, you will often cross streams that the gorilla family has also crossed that morning. The point where the trackers cross — typically on rocks or logs — is often the same crossing point the gorillas use. The stream that looks like an obstacle to the tourist group is a managed crossing point for the gorilla family, navigated with the ease of long familiarity.
Chimpanzees in Kibale: The Contrast
If you add chimpanzee trekking in Kibale Forest to your Uganda itinerary — which many visitors to Bwindi also do — the contrast in water behaviour between the two species is sometimes visible. Chimpanzees in Kibale have been observed wading through streams and occasionally swimming short distances. They are louder, faster, and more physically adventurous than gorillas in ways that reflect the different evolutionary pressures of lowland versus montane forest life. Watching both species on the same trip provides a comparison that makes each species’ specific characteristics more legible.
Planning Your Visit
Uganda gorilla trekking permits are $800 per person. The trek to the gorilla family involves crossing streams — your guide will lead you to the shallowest points and help where assistance is needed. Contact Uganda Gorilla Trekking to book your permit and plan your full Uganda itinerary. The streams you cross to reach the gorillas are the same streams they navigate every day. You are following their routes, with their knowledge of the water.
