Limber 512 Gorilla video notes p. 1

1 Gorilla

There are three distinct subspecies: Mountain and eastern and western, lowland gorilla.

A. Mountain gorilla

A rare endangered species, estimated population is less than 500. Fossey (1983) counted 242; less than half reported 20- years earlier by Schaller (1964). The Mountain Gorilla --naturally--lives at higher altitudes, is more terrestrial, has longer hair and expanded nostrils than the lowland varieties.

Fossey reports 29 morphological differences. These animals have been even more threatened by recent civil wars in Rwanda and nearby countries.

B. lowland gorillas

There may up to 10,000 of these animals. The nose of these animals forms a continuous heart-shaped ridge around the nostrils. Their climate is warmer; their diet may contain more fruit.

2 diet

Their diet is largely, perhaps exclusively vegetarian: leaves, bamboo shoots, bark. Males may take in over 30 kg of food each day; females about 18 kg. These foods are not easily digested and gorillas do not have the gut adaptations that other species, e.g.. cattle, that have similar diets.

Eating may take up half the day; resting and digesting the remainder. Fossey reports some fruit and occasional fungi, grubs and snails also may be eaten --these are a very small part of the normal diet. (She comments on gorillas rejection of termites, treated as delicacy by chimps.)

3 habitat

Like all primates (but humans), gorillas are constrained to environments where their huge food supply is readily available. Gorillas are primarily terrestrial, living and sleeping on the ground in "nests" although some young animals and females may nest in trees.

Gorillas have small ranges and are not very territorial.

4 behavior

Observe their typical primate features--including manual dexterity and ability to stand upright for short periods. Also note their apparently placid, gentle temperament.

A. social organization

Gorillas are found in polygamous groups from several up to 20 or more. Typically there is one older male ("silverback"), some younger males, and a number of females and young. These "family" groups travel, feed, and rest together.

Groups may have overlapping ranges and young females may transfer to another group. Males leaving a group do not join another and each must start up his own by attracting females.

There are some reports of lone males attacking groups, killing infants (see infantcide and kin selection) and "abducting" the female. Dian Fossey suggested this is brought about by a high ratio of adult males to females.

B. reproduction

Gorillas are rarely observed mating, in contrast to chimps, and only the silverback mates with adult females. Younger males may mate with younger females not yet likely to become pregnant.

It may be that gorilla sexual behavior involves less competition than in chimp "society" where competition for mates may be greater. (Hence larger testicles in the chimps?)

Reproductive rate is expectedly slow; Fossey reports females with 42 to 55 months between births. Gestation takes about 258 days. Zoos have had difficulty in breeding gorillas; females are very particular about mates and may not naturally know how to mother without appropriate social experiences.

C. aggression

Unlike the long Hollywood tradition of King Kong and Mighty Joe oung, Gorillas seems to be rather non-aggressive, though silverbacks will vigorously attack threats to their group, e.g. from a human or another gorilla.

The large canine teeth of the male are exclusively for competitive displays --and occasional combat -- with other males. I have read of male gorillas dying of infections probably resulting from other males' bites.

5 size

Gorillas are highly dimorphic, i.e. show great sexual differences in size (dimorphism). Males weigh between 140 and 180 kg; females about one half that.

6 conservation

The "Gorilla" video reveals the serious problems facing survival of gorillas; loss of habitat due to human farming needs and the dangers of poachers.

Dian Fossey, alive when this video was made, spent a many years studying gorillas and working recklessly to preserve them. She was murdered in her camp in 1985. See Fossey (1983) and Montgomery (1991).

7 psychological research--"mind?"

Gorillas were virtually unknown to European and American scientists until 1842. The famous physiologist, Sherrington, about 1900, published an account of the gorilla's brain and nervous system showing its great similarity to humans. this inspired lots of speculation on their intellect.

Compared with chimps and other primates, little research has been done with gorillas. "Goma" the first zoo-born and home-raised gorilla is discussed in the video. It also includes the most famous recent project involving Koko a female that has been "taught" some manual signs. (Patterson et al, 1987).

Claims about these communicative behaviors should be interpreted with caution; anthropomorphism--putting a human interpretation on a nonhuman behavior-- is a potential problem. Gallup (1983) discussed in our readings, discusses the absence of mirror self-recognition in gorillas but not chimps. On other learning tasks, he reports gorilla performance is "largely indistinguishable from chimpanzees and orangutans. p.481"

Koko is also shown and discussed in the Chevalier-Skolnikoff, S. (1977) reading in connection with ape socialization potential.

See Gorilla Information from Busch Gardens for more details. (Use the arrow keys to move around in that document.)

References

Chevalier-Skolnikoff, S. (1977). A Piagetian model for describing and comparing socialization in monkey, ape, and human infants. In S. Chevalier- Skolnikoff F. E. Poirier (Eds.), Primate bio-social development (pp. 159-187). New York: Garland Publishing, Inc.

Fossey, D. (1983). Gorillas in the mist. Boston: Houghton Mifflin.

Gallup, G. C. (1983). Toward a comparative psychology of mind. In R. L. Mellgren (Ed.), Animal cognition and behavior (pp. 473-510). Amsterdam: North-Holland.

Leyton, A. S. F., & Sherrington, C. S. (1917). Observations on the excitable cortex of the chimpanzee, orangutan, and gorilla. Quarterly Journal of Experimental Physiology, 11, 135-222.

Montgomery, S. (1991). Walking with the great apes. Boston: Houghton Mifflin.

Patterson, F. G., Patterson, C. H., and Brentari, D. K. (1987). Language in child, chimp, and gorilla. American Psychologist, 42, 270-273.