Seasonal feeding on bark by gorillas : An unexpected keystone food ?

There are a number of reports in the literature of' primates feeding on the bark of trees, but bark has only occasionally been considered as a major food to be studied in its own right (e.g., Waser, 1977; Beeson, 1987; Nonis, 1988). All the great apes feed on bark at certain times, and clearly have preferences as to which species they choose (e.g., Schaller, 1963; Jones & Sabater Pi. 1971; Castmtr, 1975: Ntshtda, 1976; Goodall, 1977; Rodman, 1977; Sabater Pi, 1977, 1979). Evidence has been presented that bark feeding by chimpanzees (Pan troglodytes) and orangutans (Pongo pygmaeus) is a 'seasonal phenomenon related to scarcity of preferred fruits (Ntshtda, 1976; Rodman, 1977), and similar conclusions have been drawn from studies of blue monkeys (Cercopithecus mitis) living near plantations of exotic pines (Beeson, 1987; Maganga & Wrtght, 1992). Bark feeding is also well known in other mammals where, again, it often occurs seasonally (e.g., elephants, Wing & Buss, 1970; grey squirrels, Kenward & Parish, 1986). Western lowland gorillas (Gorilla g. gorilla) were recently reported to eat bark during the dry season in Gabon (Rogers et aI., 1988; Williamson, 1988; Tutin et al., 1991), but no detailed data were given. This paper presents information from the last two years of field study in 1990 and 1991, when bark from a single species of tree was an important seasonal food.