Upon rising from their night nests, gorillas spread out to forage. The animals sit on their haunches or eat while walking. Gorilla b. beringei is terrestrial for 96% of feeding time. Adults climb for only 4% of feeding time, while immatures feed in trees 18% of the time.
Adult males eat up to 30 kg of vegetation per day. There is little competition between individuals, as food is generally abundant. Gorilla b.beringei is a picky feeder, chooses special plant parts, and frequently feeds on rare species diet is high in protein (3.5-25.38% dry weight) and low in acid and fiber and condensed tannins.
93% of food intake is herbaceous foods that are abundant and ubiquitous, mainly leaves and stems. Here >75% of the diet consists of 3 species: G. ruwenzoriense, P. linderi and C. nyassanus ,bamboo shoots. Highland populations of eastern lowland gorilla G. b.graueri also eat bigger amounts of the basal parts of the sedge Cyperus latifolius.
The lowest number of species eaten was recorded for mountain gorilla G. b.beringei in Rwanda (62 species) and the highest for the Bwindi gorilla in Buhoma (140 species). Gorilla b. graueri is intermediate, with many more species eaten in the lowlands than in the highlands of Kahuzi-Biega N. P., approx. 121 species vs. 79 species.
The consumption of fruits of G. b.beringei is minor because of lack of suitable fruit in the environment. Gorilla b.graueri feeds on more fruit species than Sympatric P. troglodytes. 25% of food species are fruits in highlands, 40% in lowlands.
The diet of Bwindi Gorillas is more similar to G. b. graueri than to G. b. beringei, due to greater overlap in food availability. Fruit remains are found in 66–82% of fecal samples, forming about 26% of species in the diet. The fruits represent approx. 25% food; the most significant species is M. holstii (seeds found in 20% of fecal samples).
Seasonality of diet increases as altitude declines. Fruits vary every year. Gorilla b.graueri and Bwindi gorillas rely on fibrous items consumed only when the fruit is scarce. All subspecies of G. beringei consume insects. Gorilla b. beringei does so rarely as per ants availability while Gorilla b.graueri in the plains frequently consumes ants and termites (ants in 37% of fecal specimens).
There are few sex or age differences in the diets of G. beringei, but in Bwindi Gorillas, smaller individuals are more insectivorous (2% of fecal samples from adults contain ants, compared to the last 13.11% of young ones).
Individuals of some groups ingest soil a few times a year. Geophagy coincides with feeding on plants containing high levels of toxins. Gorilla b. beringei also eats dung. The functions of coprophagy are unclear, but adults and juveniles sometimes compete for the dung of adults.
Gorilla beringei subspecies move 1 km each day, but differences are known. The distances traveled daily are greater in the plains than in the mountainous regions. Gorilla b.beringei is surrounded by food, so browses 570 m/day while G. b. graueri in the highlands goes 851 m in search of fruit.
Bwindi Gorilla groups travel 716 m/day beringei range is smallest as herbaceous food densities are exceptionally high (annual home-range 3.1–33.8 km2). Gorilla b. graueri in the lowlands requires a larger area than in the highland sector but size of home-range is unknown.
Estimates for the highlands vary considerably: from 23 to 31 km2. The total area used in eight years was 42.2 km2. The home ranges of Bwindi’s gorillas are comparable to those of G. b. graueri and G. g. gorilla, annual home range 16–28 km2 (mean = 22 km2 , 45.5 km2 for 6 years).
Gorilla b. beringei group home-ranges overlap 24–72%. Overlap is similarly ‘extensive’ for G. b. graueri and Bwindi Gorilla The availability of particular foods also affects where the abundant food is located. Gorilla b. beringei does not show seasonal use models of the range, except for increased time spent in the bamboo zone when shoots are present.
Gorilla b.graueri increases movement to preferred fruits to access them and increases range during the dry season. The limited distribution of bamboo causes seasonal changes in distribution. Ranging patterns are also influenced by social factors such as inter-group encounters, mate-searching, and the acquisition of group members.
Mate competition has a strong short-term effect, at times concealing the influence of ecological factors. Groups also range farther after interactions with other social units, and aggressive encounters can cause abrupt range changes.