SOCIAL ORGANIZATION AND BEHAVIOR
Current knowledge on the social organization and behavior of gorillas is dominated by results from research on mountain gorillas though there are some data for eastern and western lowland gorillas. The minimum group size for all subspecies is two individuals (usually a silverback and a female), except for males ranging alone, while maximum group size varies slightly for each subspecies (Yamagiwa et al. 2003). The maximum group size for mountain gorillas can exceed 20 individuals, while eastern and western lowland gorillas generally are not found in groups larger than 17 to 20 individuals and western lowland gorilla groups are always observed in groups smaller than 20 individuals (Yamagiwa et al. 2003).
Mountain gorillas live in age-graded groups of, on average, 9.2 individuals, with one adult male (though there may be more than one), multiple adult females, and their offspring (Watts 1996; Doran & McNeilage 1997; Robbins 2001). Natal dispersal is much more common for females than for males in mountain gorillas (Watts 1996). In general, females emigrate from their natal groups to avoid inbreeding. They do not always stay in their new groups throughout their lives and secondary transfer is common (Watts 1996; Robbins 2001). Males either remain in their natal groups or disperse. If young males remain in their natal groups, they will be subordinates to the silverback, but may have the opportunity to mate with new females or become dominant if the silverback dies. The other strategy for mountain gorilla males is to leave their natal groups and become solitary, attracting emigrating females and starting new social groups. In groups containing only one silverback male, females disperse and find new social groups upon the death of the silverback (Yamagiwa & Kahekwa 2001). This may be related to high rates of infanticide documented among mountain gorillas at Karisoke. Infants deprived of protection by an adult male are almost certain to be killed and as a tactic to protect against this, females join new groups in the absence of a silverback (Watts 1989). Until recently, infanticide had only been recorded among mountain gorillas; direct evidence now exists for eastern lowland gorillas and indirect evidence has been recorded among western lowland gorillas (Stokes et al. 2003; Stoinski pers. comm.). Finally, males can live in all-male groups, although this seems to be a strategy generally employed by young males when their group disintegrates before they reach adulthood (Stoinski pers. comm.).
Eastern lowland gorillas generally live in groups of one male, multiple females, and their offspring, though these can shift to multi-male groups for brief periods (Yamagiwa et al. 2003). The average group size is 9.8 individuals (Yamagiwa & Kahekwa 2001). Males spend a few months to a few years alone after emigrating from their natal groups at or before the age of 15, eventually attracting dispersing females and creating a new social group (Yamagiwa et al. 2003). Average age of transfer for female eastern lowland gorillas is nine (Yamagiwa & Kahekwa 2001; Yamagiwa et al. 2003). Female gorillas do not always emigrate independently; sometimes they transfer with another female to join a solitary male. Group succession, as seen in male mountain gorillas that remain in their natal groups, has not been observed in eastern lowland gorillas. If the silverback dies, the multi-female group and their offspring may continue to associate, rather than disperse, until a maturing silverback transfers into the group (Yamagiwa & Kahekwa 2001). One possible explanation for this behavior is protection against predators. Rather than roaming with just their offspring, females remain in groups of unrelated individuals to decrease chance of being attacked by leopards (Yamagiwa & Kahekwa 2001).
Difficulties with habituation and ecological constraints on observation have limited the research on social behavior of western lowland gorillas (Magliocca et al. 1999; Parnell 2002; Stokes et al. 2003). Group composition in western lowland gorillas is generally one silverback, multiple females, and their offspring with groups averaging 8.4 individuals (Parnell 2002). Groups with two silverbacks have been observed at several sites (Stoinski pers. comm.). Males generally emigrate from their natal groups and are solitary, as is seen in eastern lowland gorillas. Western lowland gorillas were not thought to live in bachelor groups btu there is now evidence from several sites that males will coexist without breeding females; group composition generally consists of a single adult male plus several younger males and nonreproductive females (Stoinski pers. comm.). Both natal and secondary transfers are observed in female western lowland gorillas and, as is seen in mountain gorillas, when the silverback dies, groups disband and females immigrate into new groups (Stokes et al. 2003). Though it has never been directly observed among western lowland gorillas, infanticide is the probable cause of this behavior. Females protect against infanticide when their resident silverbacks die by seeking the protection of other adult males in new groups (Stokes et al. 2003).
Knowledge about social relationships is most extensive for mountain gorillas. The core of social groups is the male-female bond, which is reinforced by grooming and close proximity (Watts 2003). It is important for female mountain gorillas to develop strong relationships with males because males offer such services as protection against predators, protection against infanticide by other males, and mating opportunities (Watts 2001). It is common to see aggressive behavior between males and females, though rarely is it intense or does it lead to serious injury (Watts 1996). Female-female relationships vary, but generally differentiate along lines of relatedness; maternal relatives associate closely and often affably interact. Generally, though, female gorillas have limited friendly relationships and multiple aggressive encounters (Watts 1996). These aggressive encounters often revolve around social access to males, and males intervene in contests between females (Watts 2003). Relationships between male gorillas are generally weak, especially in heterosexual groups where the dominance hierarchy is quite apparent and there is strong competition for mates. Relationships between members of all-male groups, in those subspecies where they occur, are slightly more affiliative, and they socialize through play, grooming, and close proximity (Robbins 2001).
Data from wild mountain gorillas and captive western lowland gorillas provide the majority of information about reproductive parameters of gorillas and the differences between the subspecies are few. Menarche occurs in female gorillas around age six and is followed by a period of adolescent infertility that usually lasts two years (Czekala & Robbins 2001). The estrus cycle lasts between 30 and 33 days and there are very subtle outward signs of ovulation, unlike the very obvious sexual swellings seen in chimpanzees ( Pan troglodytes) and bonobos ( P. paniscus). Gestation lasts 8.5 months. For wild mountain gorillas, the average age at first parturition is 10 years and the interbirth interval is four years (Czekala & Robbins 2001). It is more difficult to assess male reproductive maturity because they can be fertile before exhibiting secondary sexual characteristics (the silver saddle that characterizes adult males). Males between eight and 12 years are called blackbacks (Robbins 2001). In captivity, though, males can sire offspring by the age of six (Stoinski pers. comm.). Usually by age 12 or 13, males can be considered silverbacks, but most will not reach their full adult size until the age of 15 (Czekala & Robbins 2001).
There is no evidence of birth seasonality and mating occurs year-round (Watts 1991; 1998). A female will initiate copulation by pursing their lips and slowly approaching a male, establishing prolonged eye contact. If he does not respond she may reach towards him, touch him, or slap the ground in front of her to attract his attention (Sicotte 2001). In groups with multiple males, solicitation is taken as indication of female preference, though females may be coerced to mate with multiple males during the estrus period (Sicotte 2001). A male initiates copulations by approaching the female and displaying at her or touching her and giving a "train grunt" vocalization (Watts 1991).
Since mortality is as high as 38% in mountain gorillas during the infancy period (from birth to three years), caregivers, primarily females, are acutely important in the survival of their infants (Watts 1989). Because of their long period of development and dependence, gorilla mothers can expect to invest years caring for their vulnerable offspring. Although male gorillas do not take an active role in caring for infants, they play an important role in their socialization, often associating with older infants and juveniles (Stewart 2001). The relationship that develops between the silverback and infants in his group is usually supportive; he serves as an attachment figure during the difficult time of weaning and he shields young gorillas from intragroup aggression by intervening in disputes involving older, more dominant individuals (Stewart 2001).
For the first five months of life, infant mountain gorillas remain in constant contact with their mothers and females seek close proximity to their resident silverback for protection (Stewart 2001). During this period, infants are dependent on their mothers for food, suckling at least once per hour, and sleep at night in their mothers' nests (Stewart 1988). After five months, mother-infant pairs break body contact, but only for a few seconds and by 12 months, infants venture up to, but never more than, five meters (16.4 ft) away from their mothers. By 18 to 21 months, this distance between the pair is regular and increases (Fletcher 2001). Concurrent with this decrease in proximity is a decrease in nursing frequency, with infants only nursing once every two hours (Stewart 1988). By the age of 30 months, infants spend only half of their time with their mothers.
The juvenile period is from three to six years and is characterized by a decrease in maternal grooming, no longer sharing a sleeping nest with the mother, and weaning (Stewart 2001). Because of the enormity of the task of rearing infants and the stress of lactation on the mother's body, female gorillas experience lactational amenorrhea until the infant is weaned at three or four years. After the infant is weaned, the mother begins to ovulate and shortly thereafter becomes pregnant (Stewart 1988; 2001). Weaning conflict is minimized in gorillas, compared to other great apes, because of the cohesive nature of gorilla groups. The constant availability of play partners (including the silverback) may contribute to less intense conflict between mother and infant during this period (Fletcher 2001).
Vocal communication among gorillas is important in within-group interactions as well as extra-group interactions. Within-group calls, include "copulatory grunts" and "whimpers" during copulation, "whines" and "whimpers" by infants, "play chuckles" during play, "intense" and "mild cough grunts" during mild threat displays, and "close" calls that include both "syllabled" and "non-syllabled" calls such as "train-grunts" and "dog whines" (Fossey 1972; Harcourt et al. 1993). "Close" calls are commonly given within the group in situations of either potential separation or potential conflict. Extra-group calls serve to alert group members of potential predation and include "barks" or are given as long-distance threat displays upon detection of another group and include the "hoot series," which may be accompanied by chest beating (Fossey 1972; Harcourt et al. 1993). Most calling occurs within-groups during feeding times, though gorillas also call during rest periods as well (Harcourt et al. 1993).