three pairs of lovers with space

ALL-MALE GROUP OF MOUNTAIN GORILLAS

 

Juichi Yamagiwa (1952-) was a research fellow with the Japan Monkey Center from 1983 to 1988, and an assistant professor with the Primate Research Institute of Kyoto University from 1989 to 1997. He served as President of Kyoto University from 2014 to 2020.

Yamagiwa’s paper was published as “Intra- and inter-group interactions of an all-male group of virunga mountain gorillas (Gorilla gorilla beringei)” in Primates 28, 1-30 (1987).

The black-and-white photographs (Figs 1, 2, and 3) were supplied by Yamagiwa. Text coloured blue indicates a synthesis provided by this website, in contrast to Yamagiwa's own words in black. Inline references and some statistical data have not been included.

 

Introduction

Long-term studies on mountain gorillas (Gorilla gorilla beringei) at the Karisoke Research Centre have provided evidence of individual migrations within the Virunga gorilla population.  Both male and female gorillas commonly emigrate from their natal groups, but only females immigrate into other groups. Males travel alone for a long period after emigration, and eventually form their own groups by taking one or more young females from other groups. […]

This study describes the prolonged associations of six unrelated males who formed an all-male group and the immigration of a male into this group. The animals formed a cohesive group for at least three years and ranged in the same manner as bisexual groups during the study period. Their spatial distribution, agonistic and non-agonistic interactions, ranging patterns, and responses to other social units are described. Factors influencing their stable and prolonged associations, and the advantages to maturing males of forming an all-male group within a gorilla population are discussed.

 

Study Group

The present study was carried out on an all-male group (Group Pn) of wild mountain gorillas in the Virunga volcanoes. The study area lies within the Parc des Volcans of Rwanda and the Parc des Virungas of Zaire. […]

Virunga Volcanoe Mountain Region

In October 1981, when the present study was initiated, the group consisted of six males. In December 1982, a subadult male (Hatari) joined the group as the youngest member.

The seven animals studied were:

Peanuts (~21yo)..... A mature silverback
Beatsme................... A silverback, younger than Peanuts

Siri (~12yo)............ A blackback
Ahab (~10yo)......... A blackback

Patti (~8yo)............ A subadult
Titus (~7yo)........... A subadult
Hatari (~6yo)......... A subadult. Joined the group in the final month of observations.

 

Methods

Data were collected during an 11-month field study from December 1980 to December 1982. However, the data employed here were limited to those collected during the two periods from November 1981 to January 1982 (52 days: 280 hrs 20 min) and from July to December 1982 (92 days: 523 hrs 40 min). […]

 

Results

PROXIMITY

All the members of Group Pn spent about 90% of their day time feeding and resting. Their daily activity cycles were composed of two travel-feed phases in the early morning and the afternoon, and a mid-day rest phase as reported in other studies. The average proportion of time which each individual spent on the same activity as the others was 83.8%. This indicated that each individual’s activity was highly synchronized with the others within the group.

 

AGONISTIC INTERACTIONS

Dominance Rank and Aggressive Behavior

Two types of agonistic interactions were observed: 1,208 were described as “approach-retreat interactions during supplanting behavior”; and 144 were described as “aggressive interactions with physical contacts”.

The direction of these interactions was highly predictable between different age classes, i.e., the silverbacks [Peanuts and Beatsme] always supplanted the blackbacks [Siri and Ahab], who often supplanted the subadults [Patti and Titus]. Therefore, among the three age classes (silver-back, blackback, and subadult), the older class may usually dominate the younger class. On the other hand, the direction was less predictable between individuals of the same age class. The two silverbacks rarely stayed in close proximity of each other, and supplanted each other rarely but equally. Interactions between the subadults were not intensive but rather playful. Only within the blackback stage did the older and larger male dominate the other.

The older animals tended to be more aggressive, and the younger ones received more aggression. Within the same age classes, aggressive behaviors were rarely observed except between the blackbacks. The occurrence and intensity of aggressive contacts varied with the participants. The most intensive aggression was observed between the silverbacks. They grappled and bit each other, and were usually wounded. Peanuts initiated the fights in three cases and Beatsme in five cases. The winner was unclear in each case. Neither Peanuts nor Beatsme showed submissiveness during or after the fights. On the other hand, aggressive interactions including blackbacks or subadults were always accompanied by screaming, crouching , or avoidance by the subordinate animals, and lasted for shorter periods than those of the silverbacks. Intensive aggressions rarely occurred in the interactions involving subadults.

Fig. 1 An example of eye-to-eye contact. Siri and Titus approached Peanuts and looked into his eyes after Peanuts threatened Ahab.

Although the males of the study group sometimes embraced or mounted each other in non-agonistic interactions, they never did so after being attacked. However, it seems probable that they use eye-to-eye contacts in appeasement. During or after agonistic interactions, the aggressee or a third participant often stood in front of the aggressor and looked into his eyes. The aggressor also gazed back at them or turned his face away, and then they calmly approached him while staring into the aggressor’s eyes. This was often accompanied by belch vocalizations from either or both sides. Such eye-to-eye contact frequently occurred also during food-begging, and may function in decreasing the social tension between the animals involved (Fig. 1)

 

Loser-support and Mediating Behavior

Dyadic interactions in antagonistic contexts usually attracted other animals and often elicited their supporting behaviors, which included chasing with vocalization (cough-grunting or screaming), charging with chest-beating and physical contacts as observed in aggressive behaviors. They occurred in more than 50% of aggressive interactions. During the study period, the third party usually supported aggressees (loser-suport) who received the initial aggression. In cases involving more than three animals, the fourth or fifth animals usually supported the third one. […]

The blackbacks showed and the subadults received [third party] support most frequently. The silverbacks never supported each other, but supported most of the younger animals. Even the younger animals supported their elder aggressees. The more frequent aggressors tended to receive more attacks from the other animals. The third party’s attacks were mainly directed at the silverbacks and Siri. Beatsme was attacked most frequently, and the subadults were never attacked during the study period. Some of the loser-supports against the dominant aggressor were reciprocal (mutually between Ahab and Titus), and the others were not.

The silverbacks were never observed to receive any support from the other animals. However, Siri, Ahab, and Titus showed mediating behavior when the silverbacks fought with each other. Three such episodes were recorded. The circumstances of one of them may be summarized as follows:

On 8th January 1982, in the feeding phase when all the members were scattered on the ground, Ahab rushed at Titus playfully. Titus avoided Ahab and moved to Patti. Then Ahab rushed at Titus again. Titus and Siri grunted to Ahab. After 2 min, Siri, who was feeding 10 m from Ahab, rushed at Ahab, cough-grunting. Ahab screamed, and Titus and Patti grunted to them. Peanuts and Beatsme ran to Siri from opposite directions. Penauts stood 2 m and Beatsme 5 m from Siri while looking at each other. When Beatsme gave a double-belch bark. Peanuts ran fast between Siri and Beatsme. Beatsme rushed to Peanuts at his back, and they grappled with each other. Siri rushed to them, screaming, and grabbed Beatsme’s back. Ahab and Titus also jumped on them with a scream, and pulled both fighters’ backs. The five animals then rolled down together along a ravine to its bottom.

Fig. 2 An example of mediating behavior. Siri attempted to discourage an aggressive contact between Peanuts and Beatsme. He approached both of them with eye-to-eye contact.

In all cases, the younger animals did not perform cooperative attacks, but intervened in fights to discourage them (Fig. 2). They were not injured during this kind of mediating behavior.

 

NON-AGONISTIC INTERACTIONS

Non-agonistic contacts were rarely observed in the group, and most of them occurred as grooming, play, and homosexual behaviors in the resting phase. The author observed 32 allo-grooming episodes which lasted for at least 2 min. All of them occurred in the resting phase and ended within 5 min. Titus embraced Siri and Ahab several times. The embracing was not a greeting but was caused by Titus’s frustration; he always gave infantile distress calls before and during embracing. They sometimes reached out with their hands to touch others, mostly for initiation of play or for soliciting homosexual interactions.

The younger animals initiated non-agonistic contacts with the elder animals when they were resting within 2 m or when the younger ones approached the elder ones. Non-agonistic contacts were never observed in two particular pairs, Peanuts-Beatsme and Beatsme-Ahab. Titus was the most frequent to touch the others in a friendly manner, while the two silverbacks did so least frequently.

The author observed 364 social play interactions with physical contacts which were more violent than those of juveniles or infants, including chasing, chest-beating, ground-thumping, strut walking, wrestling, slapping, bluff-charging, and bluff-biting, etc. […] Most play episodes occurred in the resting phase and involved subadults (93.1%). Silverbacks were involved in 37 cases (10.2%) and blackbacks in 246 cases (67.6%). Solo play was observed only in Titus. The chuckle vocalization was also heard only from Titus during social play. […]

Titus was the most frequent participant and initiator of play. The blackbacks frequently intruded upon play episodes which involved subadults, but the silverbacks never did so. Siri disturbed Ahab’s play by charging and slapping Ahab, while Ahab disturbed Siri’s play with aggressive vocalizations and by chasing Siri’s subadult partners. Both of them intruded on plays between the subadults by charging Patti, and played with Titus after their intrusion. These findings suggest that competition over playmates existed among the younger members. Titus was most preferred by the blackbacks as a playmate.

 

HOMOSEXUAL BEHAVIOR

Approaches within 2 m between blackbacks and subadults which caused non-agonistic responses usually led them to play with each other. On the other hand, more than half the approaches within 2 m towards silverbacks were in homosexual contexts. Some of them resulted in mounting with pelvic thrust in the same manner as observed in copulation between males and females of the bisexual groups.

The author observed 98 male-male mountings in eight different pairs in the group…

Mounter        Mountee       No. Observed

Peanuts.......... Titus.............. 21

Beatsme.......... Siri................. 13
Beatsme.......... Patti...............
6

Beatsme.......... Titus.............. 2

Siri................. Titus.............. 9

Ahab.............. Patti............... 1
Ahab.............. Titus..............
26

Patti............... Ahab.............. 1
Patti............... Titus..............
3

Titus.............. Ahab.............. 5
Titus.............. Patti...............
10

The elder ones usually mounted the younger ones. Neither silverback was ever observed to be mounted by the other animals. Siri mounted Titus and was mounted by Beatsme. Ahab, Patti, and Titus mounted one another, but Ahab mounted Titus more often than did Titus.

Fig.3a: Siri mounts Titus in the dorso-ventral position. 3b: Beatsme mounts Patti in the ventro-ventro position.

Homosexual courtship was usually initiated by the mounter’s intensive approach with copulatory pants or by the mountee’s slow and hesitant soliciting approach. Mounting occurred in both the ventro-ventral and dorso-ventral positions, and was followed by a variable interval for adjusting movements, or no movement, before thrusting began (Fig. 3).

Occurrence of the ventro-ventral position may have reflected body size differences between the partners. Fourteen out of 16 cases observed with ventro-ventral mounting occurred between the silverbacks and subadults, and the other two cases were between a blackback and a subadult before switching to the dorso-ventral position. One mounting position usually lasted a few minutes with intermittent thrusting, and ended with a pause by the mounter or by an abrupt break by the mountee. The average duration of one mounting position was 2.1 min. The silverbacks tended to maintain mounting longer than the younger animals. Either or both of the partners often emitted copulatory vocalizations or growls during the mounts. The mounters tended to emit copulatory vocalizations (85%) and the mountee’s tended to growl (95%). Ahab and Titus simultaneously emitted copulatory vocalizations. The mountee’s usually broke the mounts and left the mounters. Sometimes the mounters emitted a deep sigh at dismounting as if they had ejaculated. Signs of ejaculation (semen left on the participants’ body after separation) were observed once for Peanuts, who had mounted Titus, and once for Beatsme, who had mounted Siri. Such homosexual courtship and mounting resembled the heterosexual interaction observed in bisexual groups.

 

Preference of Partners

Titus was the most popular partner, being involved in courting and mounting with all other males (except the late-arriving Hatari). While Peanuts courted both Ahab and Patti (and Hatari when he arrived in the final month), he only mounted Titus.

Courtship behaviors were most often directed to the subadults. In November 1981, Beatsme sometimes followed Siri with copulatory vocalizations, and twice initiated mounting. Siri also approached Beatsme in a soliciting manner. However, Beatsme ceased to make homosexual approaches towards Siri after Beatsme abruptly began approaching Patti with copulatory vocalizations and chest- beating on December 22, 1981. From this time, mounting between Beatsme and Siri occurred only when Siri approached and presented to Beatsme. Within a week after Beatsme began following Patti, the other members showed courtship behavior towards Patti. Some of them mounted Patti, but their courtship or mounting was less intensive than Beatsme’s. On the other hand, in July 1982, the group members were attracted to Titus. In contrast with Patti, who never approached others in soliciting manners, Titus frequently solicited Peanuts. Peanuts ignored or rejected some of Titus’s approaches, but sometimes responded actively. The blackbacks and Patti often approached Titus with copulatory vocalizations, but Beatsme infrequently approached him. Intensive approaches to Titus and mounts of him were observed until November 1982, and Beatsme persistently followed Patti until December 1982. However, for two weeks after Hatari had joined the group, both silverbacks were attracted to Hatari and competed for priority of access over him.

[On copulatory vocalizations:] Until Beatsme began persistently following Patti in mid-December 1981, all vocalizations by Beatsme were emitted towards Siri. Subsequently, the frequency of Beatsme’s copulatory vocalizations rose abruptly and remained high until January 1982. Peanuts also emitted copulatory vocalizations towards Patti, but their frequency was low. In July and August 1982, Peanuts increased his copulatory vocalization towards Titus, while Beatsme reduced his vocalizations towards Patti during this period. The vocalizations of both silverbacks then decreased gradually until December 1982. However, again in mid-December 1982, both emitted them during this period. These findings suggest that both silverbacks were attracted to and directed their courtship to different partners (Peanuts to Titus, and Beatsme to Siri or Patti) until they acquired Hatari, who attracted both of them.

 

Competition over Homosexual Partners

The intensity of competition over homosexual partners varied with dominance rank and with the partner preference of the mounters. A third animal usually ignored the courtship or mounts of homosexual pairs, and appeared to avoid contacts with the pairs. Active interference was mostly observed between the blackbacks. They seemed to compete with each other over priority of access to subadult partners, especially for Titus, and their intrusions were in accordance with their dominance rank. In contrast, both silverbacks rarely intruded upon the homosexual interactions of younger males. This suggests that both silverbacks recognized the priority over homosexual partners to each other.

On the other hand, strong competition between the silverbacks was sometimes induced by the responses of the partners whom they preferred. Although Peanuts mostly ignored Beatsme’s courtships towards Patti, Patti’s frequent following of Peanuts to avoid Beatsme’s persistent approach forced both silverbacks to face each other within 2 m. This situation stimulated Beatsme’s frequents displays and led to high social tension between the silverbacks. It produced overt aggression and violent fights between the silverbacks (see AGONISTIC INTERACTIONS).

[On the frequency of chest-beating displays by the two silverbacks, including ground-thumping and branch-breaking accompanied by hoot series:] Peanuts average frequency for the entire period was 0.19 times/hr, and that of Beatsme was 1.5 times/hr, suggesting that Beatsme was more frequently in situations of high social tension. Both silverbacks never simultaneously performed such displays at the inter-unit encounters but frequently did so during homosexual interactions. Beatsme abruptly increased his frequency of displays after starting to follow Patti in December 1981. Peanuts sometimes displayed in response to Beatsme’s display, but did not notably increase his display frequency until December 1982, when Hatari joined the group. Both silverbacks then frequently displayed towards Hatari, and both silverbacks were found wounded just after Hatari joined the group. They often intervened with each other in pairing with Hatari and cough-grunted to others who stayed near Hatari. These observations suggest that the homosexual interactions with the newly-immigrated Hatari brought about a higher level of social tension between the two silverbacks.

In contrast, when either Siri or Titus showed soliciting behavior to the silverbacks, they did not increase their frequency of displays. One possible explanation for this difference is that the silverbacks might not have established a clear priority over Patti and Hatari. Neither Patti nor Hatari showed soliciting behavior to the silverbacks, and frequently avoided or ignored their courtship. These responses of Patti and Hatari might in part be responsible for the stronger competition between the silverbacks. The absence of soliciting behavior by subordinate partners probably prevented the silverbacks from establishing and recognizing a priority over homosexual partners. This resulted in more mountings or ejaculation by the silverbacks with Siri or Titus than with the other younger animals, and led to violent fights between the silverbacks around Patti and Hatari.

 

 

SOCIAL CHANGES IN THE GROUP

The Inter-individual relationships changed before and after several episodes which occurred in the group during the study period. Among them, three main episodes were probably responsible for these changes, i.e., the homosexual interactions involving Patti and Titus, and the immigration of Hatari. The author therefore divided the study period into four periods, and analyzed the proximity among the members and their agonistic or non-agonistic interactions during each period.

Period I (November 1 – December 21, 1981): Homosexual interactions were observed only between Beatsme and Siri. The blackbacks and subadults usually stayed away from both silverbacks. Peanuts spent most of the day time alone. Beatsme often made his beds more than 30 m away from the others. Aggressive behaviors were rare, and social play occurred in many pairs.

Period II (December 22, 1981 – January 21, 1982): The homosexual interactions involved primarily Patti, and Beatsme persistently followed him with copulatory vocalizations and displays. Peanuts notably increased the time spent with the others, especially with the subadults. Beatsme also spent more time with the subadults, but less with the blackbacks. This suggested that Beatsme’s persistent approaches towards Patti stimulated either the subadults or blackbacks to stay near Peanuts, and led the blackbacks to avoid staying near Beatsme. Aggressive contacts clearly increased between the silverbacks, and between Beatsme and the younger animals. Beatsme’s aggression was focused on the subadults, but they were usually supported by the blackbacks. No social play was observed between the silverbacks and the younger animals.

Period III (July 4 – December 7, 1982): The homosexual interactions involved primarily Titus. Higher proximity of the blackbacks with Peanuts and their lower proximity with Beatsme were also prominent during this period. However, Patti spent less time with Peanuts, and Titus spent less time with Beatsme. Both silverbacks tended to stay farther away from one another. Homosexual interactions were usually initiated by Titus’s soliciting approaches, and the other animals were not crowded around Titus. Since this situation did not increase social tension among the group members, agonistic interactions decreased and social play again increased between the silverbacks and the younger animals. Titus was supported by all the other member and supported Ahab during aggressive interactions, while Siri and Patti were never supported when they were attacked.

Period IV (December 8—28, 1982): Hatari immigrated into the group, and both silverbacks frequently followed him with copulatory vocalizations and displays. Both silverbacks notably increased the time spent near each other and Titus, but spent less time with Patti. This reflected Hatari’s responses to the silverbacks. He usually stayed within 10 m or 5 m, but avoided staying within 2 m of the silverbacks and chose Titus as his nearest partner in both the feeding and resting phases. The blackbacks and Patti tended to avoid these four animals during the feeding phase. The incidence of interactions was similar to that of Period II. Aggression increased and social play decreased between the silverbacks and the younger animals. Titus was often attacked and supported by others. Siri was attacked most frequently and was wounded, but he was never supported by others. This suggested that during this period both silverbacks came to regard Siri, whose back gradually changed to silver, as a competitor within the group.

 

RANGING AND INTERACTIONS WITH NEIGHBORING SOCIAL UNITS

 

Responses to Neighboring Social Units

In 1981 and 1982, Group Pn was observed to enter the auditory range (within 500 m) or visual range (within 100 m) of two lone males and five different groups. […]

The most frequent and prolonged encounters of Group Pn were observed with three social units in the northern part when and after Hatari joined Group Pn. The social units were encountered auditory, visually, or physically several times, and both Peanuts and Beatsme were found wounded after these encounters. However, in spite of the high risk of becoming wounded, for two weeks Group Pn did not move away from the encounter site and engaged in several more encounters with other social units. These encounters clearly differed from the previous ones during which Group Pn did not make physical contact with but immediately parted from [other groups]. Hatari’s movements were probably responsible for their prolonged stay around these unfamiliar areas. Since both silverbacks never moved far from Hatari, his movements had a great effect on the group’s movements. Therefore, as long as Hatari stayed in the area, Group Pn could not return to its usual range which may have been strange to Hatari.

 

Discussion

Male gorillas rarely joined bisexual groups after emigration from their natal groups. The reason for this is possibly related to the stronger inter-male competition for females between groups than that within groups. Harcourt et al. (1976) and Fossey (1981, 1983) also presumed that the males who did stay in their natal groups after maturity were the sons of leading males. These interpretations suggest that unrelated male gorillas constitute tough rivals who will compete over female mates and might not associate with each other. However, the present study demonstrated that unrelated (both mature and maturing) male gorillas formed a cohesive group while maintaining a stable composition for at least three years. The daily activities of the six males involved were highly synchronized, and they spent a considerable part of their day time within 10 m of one another during both feeding and resting phases. The durations spent by each male within 5 m and 2 m of other males were close to or even exceeded those among females, or between females and the leading male, reported in bisexual groups. The social factors contributing to such high male cohesiveness as observed in the study group may be ascribed to: (1) the homosexual attractiveness of the subadults; (2) loser-support and mediating behavior; and (3) their responses to other social units.

 

HOMOSEXUALITY CHARACTERIZING THE ASSOCIATION PATTERNS

Harcourt (1979) suggested that most female gorillas are attracted to the leading male rather than to one another. By contrast, in the all-male group, the maturing male gorillas were more attracted to one another than to the silverbacks, and their favorite companions (with whom they spent most time in close proximity) were not the silverbacks but the subadults. The cohesiveness of the all-male group was basically maintained by the tendency of the younger males to remain close to one another or to stay within 10 m of the silverbacks. On the other hand, agonistic and non-agonistic interactions were often caused by the elder males’ approaches towards the younger males who stayed within 5 m of them. The attractiveness of the subadult males was reflected in the homosexual interactions. The elder males competed over priority of access to subadult males. Mounting was induced either by the elder male’s approach with copulatory vocalizations or by the younger male’s approach in a soliciting manner.

Male-male mounting has been described in many primate species. The behavior is usually seen in the context of dominance rank and high social tension or stress rather than a sexual context. In the great apes, it has also been regarded as a submissive behavior of the mountee or a reassurance behavior of the mounter. The male-male mounting observed in the present study was not caused by high social tension or stress among the males. It occurred in both the resting and feeding phases, and did not occur in the situation of high social tension such as inter-unit encounters. These observations suggest that the male-male mounting is probably not used to decrease the high social tension among male gorillas. On the contrary, such mountings in a sexual context reduced the inter-individual distances and increased the social tension between the silverbacks. This resulted in frequent aggression and sometimes in violent fights between them.

During his 2-year study period, Schaller (1963) never observed homosexual behavior. Harcourt et al. (1981) recorded ten instances of homosexual behavior between females which were purely sexual in context and three instances between males in an agonistic context during a 40-month study period. They suggested that the homosexual behavior between males was very similar to the notifying behavior of hamadryas baboons described by Kummer (1968). They also reported 22 cases in which blackbacks mounted immatures, but mostly during play. Nadler (1986) reported 28 genital stimulation episodes involving immature wild mountain gorillas of two bisexual groups, and suggested that these episodes among immatures might be more common among wild gorillas than previously reported. In captivity, Hess (1973) described several cases of homosexual interactions among males. Both dorso-ventral and ventro-ventral positions and thrusting were observed as in the normal copulation between male and female. However, Hess also regarded these interactions as “play copulation.” In contrast, since the courtship and soliciting behavior observed in the present study parallel those of adult males and estrous females reported both in the wild and in captivity, their mountings could be purely sexual rather than playful. Genital stimulation episodes which mostly occur in play among immatures are part of the species-typical behavioral development of gorillas, and may acquire more intensity in courtship and stronger sexual motivation in context among males during maturation.

The female gorilla becomes sexually active only for 1 to 3 days in each 25 to 40-day sexual cycle. Although bisexual groups usually include more than one fertile female, the average number of females per group is 3.4. Females with young infants also have a long period (about three years) of lactational amenorrhea, when they become sexually inactive. Thus, male gorillas can be expected to participate infrequently in sexual activity. Moreover, males who travel alone or live in an all-male group have virtually no chance to do so. The present study showed that male gorillas increased their sexual activities and copulated even with ejaculation in the homosexual context. This aspect of their sexual activities clearly differs from those of other primate species.

Another important aspect of their homosexual interactions is that the silverbacks of the group retained the “ownership” of the homosexual partners as observed in heterosexual relationships. They frequently displayed around the subadult partners who avoided or ignored their courtship, but they did not display when the partners approached them in a soliciting manner. The strongest competition observed between the silverbacks on the newly acquired male implies that the silverbacks expect “ownership” even of homosexual partners as well as of heterosexual partners, and that the relationship between silverbacks may be on a similar basis to that in bisexual units of gorillas.

 

DOMINANCE RANK AND SUPPORTING FORMATION

A linear hierarchy between different age classes was observed. However, unlike the blackback class, the dominance relationships were not clear within both the silverback and subadult classes. The observed numbers and directions of agonistic interactions between the two silverbacks make it difficult to determine which silverback was dominant. The silverbacks did not always retreat from the other’s approach and never showed submissiveness, but fought violently with each other several times. This suggests that male gorillas grow up rapidly to attain physical power at the black back stage, and that they may not clearly recognize the dominant/subordinate relations with one another after reaching maturity.

Intensive aggression of silverbacks is rarely expressed within bisexual groups but is frequent in inter-unit interactions. Maturing males ultimately leave their natal groups so that fully mature males may not coexist within groups. Adult males usually live in different groups or travel alone and have fewer chances to interact with one another. The question arises as to how the maturing males and the mature males were able to coexist for a long period in the study group. This may be explained by loser-support among males, especially the intervention by maturing males in fights involving the silverbacks. Agonistic interactions were always directed from the elder to the younger age classes, while supporters’ attacks occurred in the opposite direction. Moreover, the blackbacks and subadults showed mediating behavior in violent fights between the silverbacks. Both silverbacks were attracted to the younger males through homosexual interactions, which increased the social tension among males. However, loser-support and mediating behaviors possibly prevented them from engaging in severe fights and from ultimately separating from each other. These behaviors are clearly in contrast with those of macaques and baboons, where males tend to support attackers, and generally interfere in fights only if they are dominant to the target animal. The male gorillas of the study group supported only the losers, and subordinate males frequently intervened in fights between dominant animals. This would be risky in other primate species, but is probably not so in male gorillas. These finding suggest that the coexistence of adult male gorillas within the group may not be based on dominant/subordinate relationships between them but may be dependent on the positive mediating and appeasement behaviors of the younger animals in the group.

De Waal (1978) reported that most chimpanzees in Arnhem Zoo showed loser-support, and that unrelated adult males temporarily formed alliances among them to raise or maintain their dominance rank. Goodall (1965) and Nishida (1970) suggested that the rank relationships among male chimpanzees were directly correlated with age but were different from the rigid rank system seen in macaques or baboons. Nishida (1970) called this “situation-specific” dominance; for example, a particular male showing explosive display is dominant over all others at that instant. Male chimpanzees probably need a long-term coalition with other males to maintain or raise their dominance ranks. On the other hand, the frequency of a male gorilla’s displays is unlikely to contribute to raising his rank. Beatsme did not outrank Peanuts or elicit Peanut’s submissiveness after his frequent displays. No male gorilla raised his rank by supporting or associating with others. This difference is probably derived from the different social structures: chimpanzees have flexible groups based on a promiscuous mating system, while gorillas have cohesive groups based on a one-male mating system.

 

SOCIAL FACTORS INFLUENCING THEIR RANGING PATTERNS AND THE POSITION OF AN ALL-MALE GROUP WITHIN A GORILLA POPULATION

[…]

The formation of all-male groups may be a common phenomenon for primate species in which maturing males tend to leave their natal groups. The present study clearly shows that gorilla males may also form this kind of group. However, such formation is not common in gorilla populations, and was possibly related to the recent changes of the Virunga population in the present case.

[…] aggression is the main tactic employed by a resident male against others to prevent their association with his females, [so] lone travel has become more risky for maturing males in recent situations. Males who have attempted to gain female mates have faced severe counter-attacks from their silverbacks. Human disturbance (mainly poaching) has prevented these males from traveling widely in the Virunga forest. Such changes have probably forced maturing males to form or to join all-male groups instead of traveling alone. Although the formation of an all-male group may prevent maturing males from positively participating in reproduction, it may protect them from hazards within bisexual groups and in their lone travels, and contributes to the preservation of fertile males for future group formation.

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