Western Species (Gorilla gorilla)
The ecology and behavior of both western gorilla subspecies are poorly known compared to the well-studied mountain gorillas (Gorilla beringei beringei). Though western lowland gorillas have been the subject of much research in captivity (e.g., Lukas et al., 1999; Mallavarapu et al., 2006), field studies of these animals have been limited and began only in the 1980's for western lowland gorillas (e.g. Tutin and Fernandez 1984, 1985) and the mid 1990's for Cross River gorillas (McFarland 2007). Unlike the mountain gorillas, habituation (desensitizing them to the presence of people) of western gorillas has proven extremely difficult. As a result, much of the research on populations of these gorillas has been on unhabituated animals from viewing platforms (e.g., Remis 1999; Magliocca et al 1999; Parnell 2002), or using indirect methods such as DNA analysis (e.g., Bradley et al. 2004; Bergl and Vigilant 2007) or examination of fecal samples and feeding remains (e.g., Goldsmith 1999).
Western lowland gorillas inhabit primarily primary and secondary lowland forest, swamp forest and, in some areas, submontane forest. Many western lowland populations also frequently visit "bais": large swampy clearings in the forest where they feed on plants rich in protein and minerals. Cross River gorillas utilize a wide variety of habitats ranging from lowland rainforest at around 100m in elevation to montane rainforest and grasslands at over 2,000m. Probably due to the greater diversity of foods available in these habitats compared to those of the mountain gorilla, western gorilla diets include large amounts of fruit (Rogers et al., 2004). The Cross River gorilla has a diet particularly high in fruit that is also highly seasonal, varying considerably between the dry and rainy seasons.
Both western gorilla subspecies have larger home ranges and travel further over the course of a day than mountain gorillas (Remis 1997; Bermejo 2004; Doran et al., 2004; McFarland 2007). These larger home ranges and daily travel distances may be related to the more diverse (and seasonal in the case of the Cross River gorilla) diets of western gorillas. The large amount of fruit in their diets may also explain the generally smaller size of western gorilla social groups (Magliocca et al.,1999; Doran and McNeilage 2001). Smaller group size may help to avoid competition for food resources. Though social groups of western gorillas are generally smaller than those of mountain gorillas, groups of 20 or more individuals have occasionally been recorded for both western lowland (e.g., Carroll 1998; Magliocca et al. 1999) and Cross River (Oates et al., 2003; Bergl unpublished data) subspecies.
- Text by Richard Bergl, Ph.D.
Eastern Species (Gorilla beringei)
Most of what is known about gorilla ecology and behavior is derived from the long-term studies of the Virunga mountain gorillas conducted at the Karisoke Research Center. Established in 1967 by Dr. Dian Fossey, Karisoke has hosted scientists from around the world who have studied topics including maternal behavior, infant development, general social behavior, reproductive strategies, feeding behavior, habitat use and life history decisions (see Robbins et al., 2001; The Dian Fossey Gorilla Fund International, www.gorillafund.org). As data on other populations of gorillas have become available, it has become evident that mountain gorillas occupy an 'ecological extreme' that has considerable implications for their behavior and social organization. Essentially, mountain gorillas live in a salad bowl of highly nutritious, locally abundant vegetation. Such high food availability results in smaller home ranges, shorter day journey lengths, and larger group sizes than their western counterparts (Doran and McNeilage, 2001). It may also affect reproduction patterns and group composition. Mountain gorillas wean their infants a year earlier than western gorillas (Nowell and Fletcher, 2007) and have a much higher prevalence of multi-male groups (Weber and Vedder, 1983; Gray et al., 2005).
Bwindi mountain gorillas and Grauer's gorillas live at slightly lower altitudes than Virunga mountain gorillas. As a result, fruit is an important part of their diets, and, probably as a result, they have larger home and day ranges (Doran and McNeilage, 2001; Yamagiwa et al., 2003).
Research on mountain gorilla behavior and ecology is ongoing in both the Virunga and Bwindi populations. Visit www.gorillafund.org for information on the research activities of the Karisoke Research Center and itfc.must.ac.ug for information about research on the Bwindi population from the Institute of Tropical Forest Conservation.
Studies on Grauer's gorillas are currently focused on obtaining accurate estimates of population density and distribution. These studies are being carried out by the Union of Associations for Conservation of Gorillas and for Development in the East Democratic Republic of the Congo (UGADEC) and The Dian Fossey Gorilla Fund International.
- Text by Tara S. Stoinski, Ph.D.
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