The Primate Land Lubber - Gorillas are the most terrestrial of all the great apes spending approximately 90% of their time on the ground, although this does depend on the species. Western gorillas are known to be more inclined to climb trees than their eastern counter parts, particularly the mountain gorilla (due to their size, weight and diet). From field studies gorillas have been observed to spend between 5-20% of the day in trees, whereas chimpanzees spend 47-61% of the day above the ground and orang-utans almost 100%. Nevertheless, gorillas do like to climb in order to play or to harvest fruit. They almost always climb quadrupedally and only very rarely brachiate, or jump from branch to branch. Silverback males don't often move above the ground because of their great weight but even they will climb high into fruiting trees if the branches can carry them. On the ground gorillas are quadrupeds and move on all fours, knuckle walking but they can stand upright for short periods. At Mbeli Bai (Republic of Congo) certain females have been observed to use sticks to aid them wade through water, the first observed tool use amongst gorillas; a truly remarkable discovery.
All gorillas are primarily vegetarians, although they will eat some insects (termites) as well mineral soil. Like humans, gorillas have a single non-fermenting stomach which is less efficient at digesting vegetation than the multi-chambered stomachs of colobus monkeys and hoofed ruminants (hence they have a rather bulbous paunch and can be conspicuously gaseous!). Consequently gorillas are fairly sedentary in comparison to other primates and spend much of their day light hours feeding or at rest.
Relaxed Routine - Since mountain gorillas have been studied considerably longer than any other gorilla much more is known about their daily routine (the first research was initiated by George Schaller in the Late 1950s in the Virungas). The daily routine of the mountain gorillas in the Virunga Volcanoes starts when they leave their sleeping sites as the sun rises at around 6 am, except when it is cold and overcast; they then often stay longer in their nests taking a lie-in. They spend the early morning foraging and then rest in late morning and around midday. In the afternoon they forage again before resting at night.
Mountain gorillas are said to spend approximately 30% of their time feeding, 30% of their time moving and foraging and the remaining 40% of their time resting. Social contacts occur mainly during their rest periods. Therefore the midday rest period is very important for the social life of the group, as this is the time when the animals interact with their companions and when the young gorillas can play without being interrupted.
It’s usually the silverback who dictates the daily routine of his family choosing foraging routes, places to feed and in the late afternoon he will typically move again to find a suitable place to spend the night. Each gorilla will make their own nest every night, except for infants who will sleep with their mothers until they are completely weaned between the age of 3-4 years, or when their mother has a new infant.
Vegetarian Gastronomes - What gorillas eat depends on what their habitat provides and on the time of the year. At first all gorillas were assumed to be strict vegetarians; however various field studies have observed deliberate and regular eating of insects, especially in western lowland gorillas (the first evidence of regular termite feeding by western lowland gorillas, in Gabon, was only published in 1983). Gorillas seem to prefer ants and termites, which are absent, or less abundant, in mountain areas. As a result of initially studying mountain gorillas in the Virungas, they were in general thought to mainly eat leaves and stems of herbs, vines and shrubs supplemented with shoots, bark and roots. This idea of the overwhelmingly ‘folivorous’ gorilla had to be changed when lowland gorillas appeared to include more fruits in their diet, these being much more available in lowland forests. As such western lowland gorillas eat significantly more fruit than their eastern cousins; however the Bwindi mountain gorillas do live in a more fruit-rich habitat and have been observed to take full advantage of this. Wherever available, gorillas of all ages will climb trees and shrubs to harvest fruits, even heavy weight silverbacks so long as the branches will bear their load.
The food range of the western gorillas is very broad: they eat the leaves, stems, fruits and piths of about 200 plant species; they are particularly fond of plants belonging to the ginger and arrowroot families as well as aquatic herbs (all rich in minerals and proteins). In addition they are known to eat invertebrates and soil rich in minerals. Outside of the fruiting seasons, western gorillas eat more fibrous vegetation including more shoots, young leaves, seeds and bark.
In contrast, the mountain gorillas in the Virunga Volcanoes have been recorded to eat 38 different plant species, of which only a handful of species form the main stay of their diet, including a straggly vine Galium, thistles, celery and nettles. Stinging nettles, berries and bamboo are also important seasonal food sources whilst roots, bark (eucalyptus is a prized), grubs, snails, dirt and dung are taken in small amounts possibly to compensate for dietary deficiencies.
Forest Nomads or Homemakers? - As mentioned already gorillas are considered relatively sedentary and live in groups, under the ever watchful eye of an indomitable adult silverback male. Gorilla group sizes normally range between 7 and 16 individuals, most being typically between 8 and 11.
On the whole gorilla groups consist of one dominant adult male (the silverback), several females (3 or 4) and their offspring (four or five). This simple harem-like arrangement describes almost all western gorilla groups, about 90% of eastern lowland gorilla groups and 60% of mountain gorilla groups. The balance is made up of all-male groups and multiple-male groups (usually related). Gorilla groups tend to have cordial but not necessarily strong social bonds, except those between mothers and infants (particularly when compared to chimpanzees).
Maturing offspring of either sex usually leave their natal group. Mature males (8-11 years) leave, either taking females with them, spending time in an all-male group or remaining solitary until they can establish a group of their own by attracting females. Sexually mature Females (at about 6-7) will transfer between groups, sometimes more than once, joining another group or a lone male. Harems can be taken over by another male, when a silverback dies or is deposed. In males, the separation process is slow: they spend more and more time on the edge of the group until they leave altogether. In contrast, a female leaves her group only if she encounters another male.
Gorillas have no mating season. Mating and births occur throughout the year. Female gorillas have a hormone cycle (similar to humans), which is usually 26-32 days long. They will come into season after raising their last youngster which can be between 3-6 years (4 years on average) but won’t usually have their first baby until 10 years old. The gestation period is 8.5 months. In general, gorilla babies are nursed for at least 2 years. At 4-6 months they start to put plant parts into their mouth to bite on them and they can start to walk quadrupedally at this age as well. At 8 months they regularly ingest solid food. At approximately 3 years they start to become independent because their mother gives birth to the next baby. In spite of this, mother and older offspring maintain a strong relationship.
The home ranges of various gorilla groups and of lone silverback males tend to overlap, so encounters are frequent. Lone males often make a special effort to seek out harem groups, as this is their only chance to gain females. The leaders of stable harem groups avoid contact with other adult males in order to avoid losing females. If they detect a competitor, they try to drive him away by displaying with vocalisations and chest-beats or by attacking the intruder which can be quite violent.
Gorillas do not occupy discrete territories and do not defend these areas against other gorilla groups. Instead, they roam in so-called home ranges and where food sources are widely dispersed, these home ranges tend to be larger. The more members a group has, the further the group has to roam and the bigger is the home range. Among primates there is a strong relationship between diet and their foraging behaviour. Those that feed on high energy foods (such as fruit) that vary seasonally, in variety and distribution tend to have greater day ranges compared to those feeding on lower quality but more consistently available foods.
Gorilla home ranges comprise of several vegetation zones which are seasonally exploited. Western gorilla home ranges typically exceed 20km2 whereas the typical home range for the Virunga mountain gorillas is 5-11km2. Fruit is more widely available in western gorilla ranges which accounts for their great home ranges and a more mobile lifestyle. In Bwindi home ranges are between 20-40km2 which also reflects a higher availability of seasonal fruit and the gorillas preference to travel for preferred foods.
As for how much they travel each day, well this does vary. Western gorillas travel about 3 km per day in fruiting months and 2 km per day when they are feeding on foliage etc. On the other hand mountain gorillas on average move less than 1 km per day and rarely more than 2 km.
