Social Organization
Mountain gorillas live in cohesive groups called troops or bands that are composed of one mature alpha male, also called a silverback, with a harem of females and their offspring (Harcourt 1979). This is a unique structure among the apes because there are more females than males in a group (Maple and Michael, 1982; Hamburg & McCown, 1979). A group size usually ranges from 2 to 35 individuals, with a mean group size of about 17 individuals (Maple & Hoff, 1982). The dominant silverback plays an obvious role within the group by protecting the other members, leading them in search of food, and stopping intragroup fights. The younger, blackback males spend most of their time separated from other members of the group. As the blackbacks mature, they become more dominant over the females in agonistic interactions (Hamburg & McCown, 1979). They will often mate with adolescent females, but never with the mature females (Hamburg, 1979). Adult females interact minimally with each other and immature males. Female juveniles are drawn to the silverback of the group probably for protective purposes.
Mountain gorillas are nonterritoral creatures, but conflicts may occur with other groups when the come into contact with each other. Interaction with other groups varied from friendly to aggressive (Hamburg & McCown, 1979). However, mountain gorillas have been observed to be mild, unaggressive creatures (Harcourt, 1979; Watts, 1985) that only attack if they are directly threatened. According to Schaller (as cited in Maple & Hoff, 1982), they tend to avoid other species. As noted by Sabater Pi (as cited in Maple & Hoff, 1982), mountain gorillas exhibit more aggression in the presence of young. There are five basic modes of aggression presented by Schaller (as cited in Maple & Hoff, 1982): a fixed stare, a jerk or snap of the head towards the antagonist, a forward lunge, bluffing, however, this was a rare behavior, and physical contact.
Grooming occurs frequently between males and females or females and females (Lindsley 2001). Mother and daughters have an affinitive relationship, which was seen by frequent grooming. However, it is usually the daughter that grooms the mother and the mother rarely reciprocates (Harcourt, 1979). |
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Harcourt (1979) has shown that females tend to leave their natal group around the age of 8 and travel between different groups. Males typically leave their natal group at the age of 11 and live solitary until they gain females and are able to start a group of their own. If the lone males are unable to attract females, they will attach themselves to another group but will occupy a peripheral position (Maple & Hoff, 1982). Since the females change groups at a constant rate, the majority of them are not related and affinitive interactions between them are rare. Since there is no evidence of bonds between the females, group cohesion is most likely due to the relationship between the silverback and the females. There does not seem to be an obvious hierarchy among females. Sade (1972), along with other observers, suggests that females exist in a linear hierarchy. Due to all these reasons and the fact that there exists very little competition over food, there is no reciprocal altruism. Scott and Lockard (1999) provide a contrasting opinion, they believe that there may exists an age-based dominance hierarchy, where the older females are the most dominant. This is suggested as a possibility, since age-based dominance is a general pattern in other female-transfer species. Harcourt (as cited in Hamburg & McCown, 1979) also suggests that there may be a dominance hierarchy in agonistic interactions between females. However, he states that it is a rare because of mutual avoidance.
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Sicotte (1993) discusses how females may transfer to several groups throughout their lifetime. Transfers only occur during inter-group encounters, which are usually aggressive. Transfers always took place voluntarily (Hamburg & McCown, 1979). Dominant males will use this opportunity to attract more females to his group. Females tend to transfer to groups that have been recently formed rather than well-established groups. This is due to the fact that there will be less competition between females for protection from the dominant male. There is also evidence that the number of female transfers is significantly higher in groups with more than one silverback. According to Scott and Lockard (1999), female transfer decisions depend on the quality of the dominant silverback since they provide protection against infanticide by other males. Other silverback males will kill offspring known not to be theirs in order to eliminate competition. Harcourt (as cited in Hamburg & McCown, 1979) observed that the transfer of females to other groups caused changes in the patterning interactions in the original group and the group to which they moved. Watts' (1994) data supports the fact that female tranfer does not have a large ecological cost and that the social dispersion of mountain gorillas could be due to male mating competition and male-female mutualism. |
Harcourt (1997) observed that females spent more time near the dominant male than with other females. This shows that the social bonds between the mature males and the females are strong (Watts, 1985). Females tend to compete with each other in order to spend more time with the dominant male (Scott and Lockhard, 1999). Harcourt also observed that the presence of infants sparked interest among other members in the group and caused substantial differences in the interactions between the female gorillas. Females with younger offspring spent the more time with the dominant male (Scott and Lockard, 1999). This is most likely because the dominant male wants to protect his young offspring to guarantee the survival of his genes. From his study of wild gorillas, Schaller (as cited in Harcourt, 1979) has suggested that females with infants may be slightly dominant over females with older offspring.
According to Scott and Lockard (2004), females will supplant other females over food and nesting sites.
Image courtesy of David Lawson
Newborn gorillas must be cared for constantly during the first years of life and investment during this time is very high. During these years, an infant gorilla is dependent on its mother for her milk, shelter, and nest. They are mainly cared for by their mothers and protected by the dominant male. Harcourt (as cited in Hamburg, 1979) noted that the relationship between a mother and her offspring continued after other offspring were born. As the offspring matured, they become less dependent on their mothers.