II. African Apes
Chimpanzees live in fission-fusion polygynous societies in which members of a community form temporary foraging associations (parties) of varying sizes (Goodall, 1986). This flexible grouping pattern is thought to be a social adaptation to a reliance on patchily distributed fruit trees (Wrangham, 1977), although the energy value of particular fruits varies widely and may play an important role (Conklin-Brittain et al., 2006). Chimpanzee party size and composition varies widely among study sites (Boesch and Boesch-Achermann, 2000; Pruetz, 2006). Party size is thought to correlate with the size and distribution of fruit patches and with the presence of females with sexual swellings (Chapman et al., 1994; Anderson et al., 2005, 2006), but empirical tests that separate these influences are lacking and the relative influences may vary from site to site (te Bockhorst and Hogeweg, 1994). Chimpanzee communities vary in size from 20 to over 100, depending on the site (Nishida, 1979; Mitani and Watts, 1999). Females spend most of their time with their offspring, rarely joining large foraging parties (Goodall, 1986). Estrous females provide an exception to this pattern, being both highly sociable and strongly attractive to males. The chimpanzee diet is mainly ripe fruit (70% of the diet), but their diet includes leaves, shoots, buds, blossoms, seeds, nuts, bark, invertebrates, birds, eggs, honey and a number of mammal species (Wrangham, 1977; Goodall, 1986; Stanford, 1998). The Kasakela chimpanzees of Gombe National Park use at least 141 species of trees and plants (Wrangham, 1977). However, 95% of feeding time is spent on half of these food types, and foods are selected in proportion to their availability (Wrangham, 1977).
Gorillas occur across a wide range of habitat types, and their ecology varies accordingly (e.g, Robbins et al., 2006). Mountain gorillas (Gorilla gorilla beringei) from the Parc d’Volcans in the Virunga mountains of Rwanda feed primarily on perennially available foliage and other non seasonal foods (Watts, 1984). Fruit comprises a large percentage of western lowland gorilla diets. In Equatorial Guinea, 40% of the diet was composed of fruit (Jones and Sabater-Pi, 1971; Sabater-Pi and Groves, 1972; Sabater-Pi, 1977). In Cameroon, evidence of fruit was found in 50% of all fecal samples (Calvert, 1985). Studies in Gabon (Tutin and Fernandez, 1985, 1993; Rogers et al., 1988; Williamson, 1988; Rogers, Maisels et al., 1990; Williamson et al., 1990), the Central African Republic (Remis, 1997; Goldsmith, 1999) and the Republic of Congo (Nishihara, 1992, 1995) indicate a large proportion of fruit in the diet. Although large amounts of fruit are consumed during certain times of the year more than 90% of the fecal samples contain fiber and leaf fragments (Rogers and Williamson, 1987; Williamson, 1988) and in one study herbaceous material was eaten in equal amounts throughout the year (Goldsmith, 1999).
Gorilla ranging is strongly influenced by habitat and food availability. The day range of Karisoke gorillas is short; over a period of seventeen months a group traveled between 190 and 3,300 m per day (mean = 570 m; Watts, 1991). Watts found that the effect of group size on time spent feeding is small, which suggests that the costs of social foraging are low for mountain gorillas. Lowland gorillas travel much farther per day. Tutin (1996) found an overall mean day range in Lopé of 1.1 km/day, while Remis (1994) found a mean at Bai Hokou, Central African Republic, of 2.3 km/day. Differences between sites may be related to habitat and group size differences.
Lowland gorillas travel significantly farther during periods of fruit availability. Research in Lopé (Tutin, 1996) and at Bai Hokou (Remis, 1997; Goldsmith, 1999) demonstrates that daily ranging behaviour is influenced by the degree of frugivory. Tutin et al. (1992) suggest that due to their reliance on terrestrial herbaceous vegetation, western lowland gorilla groups do not experience high levels of within-group feeding competition. As a result, they do not need to modify their group size, explaining why their grouping pattern resembled that of mountain gorillas. At Bai Hokou, however, there was a significant positive relationship between group size and daily path length during all seasons, suggesting high levels of within-group feeding competition. In addition, groups were found to form temporary subgroups that fed and slept separately from one another, perhaps as a way of reducing feeding competition (Remis, 1994; Goldsmith, 1999).
Mountain gorilla group size does not seem to influence day range either due to the widespread, abundant foliage on which they feed (Watts, 1996). Mountain gorillas live in relatively stable groups, and a variable number of offspring. Both male and female mountain gorillas tend to emigrate from their natal groups (Harcourt, 1978). Emigrating males either join all-male bands or travel by themselves; females either join a new breeding group or take up with a solitary male (Stewart and Harcourt, 1987). Harcourt et al. (1981) reported that 60% of studied groups at Karisoke had only one adult male. Approximately 10% of the groups were all-male bands (Stewart and Harcourt, 1987). An emerging picture of lowland gorilla social organization is of less cohesive groups that are more likely to contain multiple silverbacks (Tutin et al., 1992; Remis, 1994; Olejniczak, 1996; Goldsmith, 1999). Evidence from Lopé (Tutin et al., 1992), the Ndoki (Olejniczak, 1996), and Bai Hokou (Goldsmith, 1999) suggest a mean group size of about 9.5 individuals, with groups not exceeding 18 individuals. Average group size is larger in eastern lowland gorillas (G. g. graueri) (10.8; Yamagiwa and Basabose, 2006).
There are a small but growing number of detailed ecological studies of sympatric chimpanzee and gorilla populations (Table 2b). Jones and Sabater-Pi (1971) identified several means of ecological separation between the two species in Equatorial Guinea. During the wet season, gorillas ranged in fairly open areas of regenerating vegetation, while chimpanzees utilized the upper strata of primary forest. During the dry season, gorillas were found in dense vegetation at the edge of forests and occasionally in primary forest adjacent to areas of regenerating vegetation, while chimpanzees ranged mainly in the lower strata and on the ground in primary forest. The gorillas in Jones’ and Sabater-Pi’s study were reported to feed almost completely terrestrially, whereas chimpanzees were mostly arboreal feeders.
More detailed sympatric ecological studies have been conducted in the Lopé Reserve in Gabon, where chimpanzees and gorillas live at similar population densities (Tutin and Fernandez, 1985, 1993). Lowland gorilla diet at Lopé more closely resembles that of chimpanzees than that of mountain gorillas living in the Virungas (Rogers et al., 1990; Tutin and Fernandez, 1993). It appears that Lopé gorillas satisfy a substantial part of their energy needs from fruit, relying on leaves to provide protein (Rogers et al., 1990). Most gorilla plant foods (69%) are harvested arboreally (Tutin and Fernandez, 1993). Lopé chimpanzees consist of at least 174 food items in their diet, including 111 species of fruit (Tutin and Fernandez, 1993). Approximately 76% of Lopé chimpanzee plant foods are harvested arboreally (Tutin and Fernandez, 1993). There is great overlap in the diets of chimpanzees and gorillas at Lopé with approximately 60-80% of foods being eaten by both species (Williamson et al., 1990; Tutin and Fernandez, 1993). Gorillas are more likely to feed on terrestrial herbaceous vegetation than chimpanzees and are more ready than chimpanzees to concentrate on this vegetation when fruit is scarce. Chimpanzee and gorilla diets diverge most when fruit is not abundant, although it is mainly gorillas that shift foraging strategies while chimpanzees continue to forage extensively for ripe fruit even in periods of low fruit availability (Williamson et al., 1990; Remis, 1997). Direct interspecific interference competition has never been observed.
Research on sympatric gorillas and chimpanzees in the Nouabalé-Ndoki forest of the Congo and Central African Republic have revealed similar patterns of resource use. In Nouabalé-Ndoki, gorillas are more highly frugivorous than any other studied population (Kuroda, 1992; Nishihara, 1995). Their diet consists of over 63% fruit, which is consumed seasonally. Ndoki gorillas make extensive year-round use of swamp forest (Nishihara, 1995) and feed in fig trees in proximity to chimpanzees during times of fruit scarcity (Suzuki and Nishihara, 1992). They also feed extensively on aquatic herbaceous vegetation, perhaps as a fallback food analogous to the use of terrestrial herbaceous vegetation (Magliocca and Querouil, 1997).
Eastern lowland gorillas and chimpanzees are sympatric in Kahuzi-Biega National Park in eastern Democratic Republic of Congo (Yamagiwa et al., 1994; Yamagiwa et al., 1996). Gorillas occur there at a much higher density than chimpanzees. The higher population density of gorillas may have been related to the chimpanzee frugivorous diet in a mountainous area of low fruit diversity. Yamagiwa et al. (1996) found that gorillas ate a more diverse diet than chimpanzees did. Both species ate fruit over the entire annual cycle, though not necessarily the same species at the same time. They shared at least four important fruit species in their diets and both apes sometimes fed together in the same tree crown. Gorillas found at lower elevations in Kahuzi-Biega ate more fruit than those at higher elevations, apparently related to fruit availability differences (Yamagiwa et al., 1994). Ecologically, this population appears to be intermediate between western lowland and mountain gorilla populations in the degree of frugivory and the plant species diversity in the diet.
Results of the Bwindi Impenetrable Great Ape Project include the first detailed study of Bwindi gorilla feeding and ranging ecology (Nkurunungi, 2005) and preliminary information on Bwindi chimpanzee behavioural ecology (Stanford, 1999; Stanford and Nkurunungi, 2003). Bwindi gorilla diet is seasonally high in fruit; in some months more than 50% of gorilla dung samples contain seeds (Nkurunungi, 2005). In other months, however, the gorilla diet contains no fruit and is similar to the diet of gorillas in the Virungas (Watts, 1984; Stewart and Harcourt, 1987). Gorilla and chimpanzee diets are thus similar in some months, and chimpanzees range farther when fruit is scarce to find fruit. Day range is positively correlated with the percentage of fruit in the diet, although only slightly so. Bwindi gorillas are much more likely to construct nests in trees than their Virungas counterparts, who nest entirely on the ground (Nkurunungi, 2005).