Immatures and the Dominant Male

Like females, immature mountain gorillas are strongly attracted to the dominant male. In fact, the only other adult with whom immatures have an equal or stronger relationship is their mother (Stewart and Harcourt, 1987). The mother is the one who stays bonded to the dominant male not only for her own protection, but for the protection of the offspring as well. However, she is not the one doing the actual protecting. An attraction to the silverback becomes apparent between the second and third year of life, when the immatures regularly begin to seek the silverback’s proximity, often leaving their mother to do so (Stewart and Harcourt, 1987). As with the females, the most important relationship is with the protector, the dominant male. This attraction is especially important for infants and young juveniles that have lost their mothers due to emigration. Indeed, orphans treat the silverback as their “caretaker” even when older siblings are present (Stewart and Harcourt, 1987).

 

On the whole, the immatures are responsible for maintaining proximity with the silverback, and he does not actively seek their contact or carry them. However, Ymke and Williamson (2001) described a previously unreported behavior in their studies of mountain gorillas. Two of 12 infants in one group were carried eight times by one silverback, Pablo, in front of another silverback, Cantsbee. This behavior is so remarkable because normally the silverback’s behavior is simply one of tolerance. The researchers proposed two explanations for such behavior. If Pablo perceived that a situation could become aggressive due to the presence of Cantsbee, he may have picked up an infant to deter a potential attack by Cantsbee or to protect the infant (Ymke and Williamson, 2001). Was the silverback acting to protect himself or the infant?

Stewart and Harcourt (1987) have also noticed that the silverback becomes extremely protective during threatening situations. This applies to threats from both outside predators (mainly humans) and within the group. For example, they report that silverbacks intervene in fights between group members (by cough-grunting and running at the participants) three times more often than the median adult. When silverbacks supported any one combatant, it is almost always the younger (89% of 33 incidents). Thus, Stewart and Harcourt (1987) would conclude that infant carrying behavior observed by Ymke and Williamson (2001) by the dominant male was to protect the immatures from possible aggression. Therefore, Pablo expended energy in carrying infants to increase their survival and, consequently, reproductive success. This leads into to my final topic, social cooperation.

Why Polygyny?

The gorilla mating system of the single silverback social groups (as described in the discussion of gorilla social spacing, 60% of all groups are under one male (Stewart and Harcourt, 1987)) can be classified specifically as female defense polygyny. This type of mating system occurs when females live in social groups, often for protection, but not in a range that can be defended by the male. Males may instead follow the female groups and defend them. There is also resource defense polygyny, but, as described in the discussion of gorilla social spacing, mountain gorillas eat a primarily folivorous diet. Since their resource is abundant and evenly distributed, resource defense polygyny would not be logical.

To determine if the young of gorillas are born at certain times of the year, Schaller (1963) estimated the probable months of birth of all infants of the ten mountain gorillas groups he studied in the same area. He discovered births occurring throughout the year; thus evidence points to a lack of preferential breeding season in gorillas. This makes sense because no data has shown that silverback males come into contact with each other more often during one season than another. Therefore, if they display no preference on when they inherit a female, copulation must not be limited to a certain season. As described in the discussion of gorilla social spacing, gorillas live in overlapping home ranges, so contact with competitive adult males is inevitable at all the time. Males acquire females by attracting them away from other silverbacks during these encounters with bisexual groups.

Rather than having short-lived harems in which the male follows females for just one breeding season, gorillas display permanent harems. The bond between males and females is what keeps the females around. One major factor prevents females from straying from males: protection. As described in the discussion of gorilla social spacing, due to their massive size, male gorillas have potential to inflict great harm on females and offspring. One cause of the vulnerability of gorilla populations is the female’s long reproduction time (Stewart and Harcourt, 1987). Stewart and Harcourt (1987) presented information on several reproductive parameters, with all wild data coming from the mountain gorilla population in the Virugna Volcanoes. Their data has shown that there is a period of “adolescent sterility” in females between first estrus at around 7.5 years and first conception at around 10. Males are unlikely to start breeding before about age 15 due to competition they receive from older animals. It is not until this age that males develop their complete secondary sexual characteristics, in particular the silver saddle of hair on the back, that has earned fully adult males the name “silverback” (Stewart and Harcourt, 1987).

Across gorilla populations in East Africa, reproductive output for females is about one surviving offspring every eight years, where surviving is defined as reaching breeding age. Thus, given a reproductive life span of approximately 25 years, a female gorilla may expect to produce about three surviving offspring in her life (Stewart and Harcourt, 1987).