There are three distinct subspecies: Mountain and eastern and western, lowland 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.
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.)
Gorillas have small ranges and are not very territorial.
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.
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.
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.
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).
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.
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.