Western Lowland Gorilla (Gorilla gorilla gorilla) Fact Sheet: Behavior & Ecology

More field studies of eastern (mountain) gorillas than western species

• Western species have been difficult to habituate because they're more heavily hunted (avoid humans)
• Schaller 1959-1961 mountain gorilla study
• Karisoke Research Center established by Dian Fossey in 1967 to study Virunga mountain gorillas in Rwanda. (Fossey was murdered in 1985 but studies continue)

Diurnal - forage on ground from daybreak to nightfall (females and young may climb, feed and sleep in trees)

Morning and afternoon feeding periods with a long mid-day rest in between. (This pattern is also seen in chimpanzees and other monkeys)

• Morning feeding is intense - at least 2 hours
• Weather influences length of rest period (longer on cooler days); a nest is usually not constructed
• Afternoon feeding more prolonged but less intense

When little fruit is available, energy is conserved by decreasing range and feeding on lower quality herbs

Travel ~1-3 km / day

• Travel farther when forests have more fruit and termites.
• Larger groups must travel farther to secure sufficient food for the group
• Human hunters and leopards affect travel

Nest building begins after afternoon feeding - up to an hour before dusk. Silverback initiates process. Nest on ground or in trees - used for one night only

(Barmejo, 2004)

~7-14 sq km

No territory defense - groups overlap. Encounters involve vocal exchanges and chest-beating.

Ranging areas of neighboring gorilla groups overlap extensively

Groups frequently enter auditory range of each other but avoid contact

  General

• Most diurnal primates live in groups because of predation threats
• Group size: ~4-6 adults and young (2-32)
• Composition: Dominant male silverback, 3-4 females and young (larger groups have a higher proportion of adult females)
• Female social relationships influenced by relatedness.
  
  • Indifference/hostility toward non-kin
  • Grooming rarely observed between adult females
• Females compete with one another for social access to males; silverback intervenes to stop conflicts.
• Maturing males usually leave natal group in late adolescence; may take females with them
  
  • Remain solitary until they can establish their own group
  • Multi-male groups rare. (More common in mountain gorillas)
• Many females also transfer between groups
  
  • Voluntary transfer from natal (8-9 years of age)
  • Involuntary transfer: female joins new male after only silverback has died
  • May consecutively join different groups
• Temporary division of groups into subgroups that stay apart for several days.

       Aggression

• Little aggression among members of the same group. Silverback maintains peace.
• Includes fixed stare, thrusting head toward antagonist, bluff charges, roaring
• Silverbacks may exchange explosive displays: chest beating, hooting
  
  • Ritualized display beginning with series of soft hoots, escalating to climax
  • Facilitated by air sac within chest that resonates when inflated
  • Animal rises up to bipedal position just before climax
  • Running, slapping/tearing/throwing vegetation can occur
• Severe fights between adult males can occur
  
  • Effort to gain/retain mates and protect offspring
  • Scars and wounds common
  • No fighting over food
• Infanticide infrequent (so far, observed among Virunga gorillas, after death of protector male) (Fossey, 1984)

Nest Building (Schaller, 1963)

• All gorillas >2.5 years build nests for sleeping at night and sometimes for mid-day rest
• Usually built on level ground. Vegetation is folded around and under the body. (Some nests built in trees)
• Adult male nest = 5 x 5 feet or 5 x 4 feet (oval)
• Entire process takes ~1-3 minutes
• Sometimes sleep on bare ground
• Nest counts are often used to determine population size
• Chimpanzees and orangutans also build nests

Play

• Most commonly seen in infants beginning around 4-5 months of age
  
  • Lone play - Running, climbing, shredding leaves, slapping vegetation
  • Social play - wrestling, mock biting
• Diminishes around 6 years of age

Tactile

• Mutual grooming uncommon - Some mother-child, female-silverback

Visual communication

• Facial expressions

Vocalizations

• At least 22 distinct sounds: grunts, barks, screams, hoots
• Hoots may be contact calls to communicate forest location

Olfaction/Scent Marking

• Gorilla 'fear odor' (the scent of scared gorillas) has been detected in the air after encounters between western gorillas and leopards (Fay, 1995)

• Quadrupedal knuckle walking
• Cautious tree-climbers (mainly juveniles, females and infants)
• No brachiation
• No swimming; ford streams 1-2 ft deep but not deeper or wider waters

• Eat many of the same foods as chimpanzees and elephants - competition for resources but no agonistic encounters observed.
• Flee from leopards and humans

Doran & McNeilage, (1998)
Doran-Sheehy et al, (2004)
Tutin, (1996)
Fay, (1995)
Robbins, (2004)
Schaller (1963)
Maple, (1982)
Barmejo, (2004)
Fossey, (1984)