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

Long-term Studies

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)

Activity Cycle

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

Territory Size

(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

Social Groups

  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

Communication

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)

Locomotion

  • 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

Interspecies Interactions

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

Page Citations

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

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