Mountain Gorilla Nest Building — A Nightly Behaviour With Big Scientific Consequences
Every evening, as the light fades in the Virunga forest or Bwindi’s impenetrable canopy, mountain gorillas perform one of their most distinctive daily behaviours: building a fresh sleeping nest. The nest is constructed quickly — typically in under five minutes — from whatever vegetation is immediately available at the chosen sleeping site, and is used for a single night before being abandoned. By the following evening, the gorilla family has moved to a new location and built a new set of nests. This behaviour, practised by every member of the family from the dominant silverback to the oldest juvenile, is simultaneously a practical adaptation, a social communication, and the foundation of the scientific methodology used to count mountain gorilla populations.
Construction Method
A mountain gorilla nest is built by bending, folding, and pressing vegetation — typically the surrounding grass, herbs, ferns, and small branches — into a circular or oval platform approximately a metre to a metre and a half in diameter. The gorilla sits in the centre of the forming structure, reaching outward to gather and fold vegetation inward beneath itself, building up a layered mat of compressed plant material. The nest is not structurally complex — it does not involve weaving, intertwining, or the more elaborate construction of some bird nests — but it is functionally effective: the compressed vegetation provides thermal insulation against the mountain night temperatures, and the soft surface of the nest is substantially more comfortable than the hard root-complex and uneven rock of the forest floor on which the gorilla would otherwise sleep.
Infant gorillas and juveniles sleep in their mother’s nest for the first three years of life — the infant’s small size and the need for proximity during nursing makes co-sleeping the functional arrangement. From approximately three to four years of age, juveniles begin attempting their own small nest construction adjacent to the mother’s nest — a behaviour that develops through play and observation before it becomes competent enough to produce a functional nest. Adult competence in nest construction is the result of years of daily practice.
Why a New Nest Every Night
The construction of a new nest every night, rather than returning to and reusing an existing nest, is a parasitic hygiene behaviour: faecal matter, insects, and other organic material accumulate in a used nest, and the daily nest-and-move cycle ensures that the sleeping surface is always fresh. The same behaviour pattern is observed in chimpanzees, which also build new nests nightly, and is believed to have developed from the common ancestor of the great apes as a response to the ectoparasite load — ticks, lice, and biting insects — that would accumulate in a reused nest over time.
Nest Counts and the Gorilla Census
The gorilla nest’s physical durability — a nest constructed from compressed vegetation remains identifiable in the forest for several weeks to several months depending on rainfall and humidity — is the property that makes it useful as a census tool. Researchers walking systematic transects through Bwindi or the Virunga can count, measure, and characterise gorilla nest sites across the transect area, and from the spatial density of nest sites, derive statistical estimates of the gorilla population density in the surveyed area using distance sampling statistical frameworks. The nest count methodology is the primary non-contact census technique used in both the 2011 and 2018 mountain gorilla censuses, supplemented by genetic analysis of faecal samples collected from recent nest sites.
Each nest in a nest cluster represents one gorilla — adult and sub-adult gorillas each build their own nest; infants share the mother’s. A nest cluster of nine nests therefore represents nine individuals above the infant stage plus however many nursing infants are present, identifiable by size and by the location of the infant-scale addition within a larger adult nest. Experienced researchers can characterise a nest cluster’s demographic composition from the nest size distribution before any direct gorilla observation is attempted.
