Bonobos vs Chimpanzees

Comparison of Chimpanzees (Pan troglodytes) and Bonobos (Pan paniscus):
Aggression Within and Between Groups, and Tension Relieving Behaviour

Introduction

Due to morphological and cognitive similarities to modern humans, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus) are often used as models for early hominid behaviour (Stanford, 1998:400). As this model, an examination of their behavioural differences and similarities may assist in determining how aggressive early hominids (in particular Australopithecus) may have been, as well as how they may have dealt with tension in their communities. Typically, chimpanzees live in male-dominated communities, and are portrayed as being occasionally aggressive, with inter and intragroup conflict being fairly commonplace. Bonobos are often portrayed as the exact opposite,  being non-aggressive, female-dominant, and highly focused on sexual behaviour, often in a non-reproductive context (Stanford, 1998:399). While there are behavioural differences between chimpanzees and bonobos, there are also quite a few similarities, in that both show aggressive tendencies (both within and between groups), and both seek to resolve tensions within the group, seeking different methods to do so.

Intragroup Aggression

Intragroup aggression has typically been observed more amongst chimpanzee groups (both in captivity and in the wild); however, this may be partially due to the fact that long-term studies of  bonobos are restricted to two site, both of which have relatively small populations (Hohmann and Fruth, 2003:568). Aggressive encounters in chimpanzee communities tend to involve very loud noises and a little biting, rarely escalating to the point of physical damage (Waal, 1989:39). The occasional exception to this rule tends to be seen during power struggles among adult males, which may take several months before being resolved, involving many intimidation displays and aggressive encounters, as well as a few physical attacks (Waal, 1989:44). This increase in aggression during these power struggles is not surprising, considering that this would be the time when the alpha-male would have to either prove his dominance or give up his power. During these aggressive encounters chimpanzees may help their favourite and form a coalition of 2-10 individuals against an opponent regardless of rank; however, the victim of this coalition may also find supporters, leading to large-scale confrontations. They also actively try to recruit support, making loud noises to draw attention to the confrontation, and will beg for help from friends and bystanders, or hide behind a protector (Waal, 1989:49). These aggressive encounters are not necessarily just amongst other adults or during these occasional power struggles. Chimpanzee males also tend to show greater aggressive behaviour towards infants and, in particular, juveniles than bonobos. This aggression is also associated with a much higher mortality rate in infant and juvenile chimpanzees (Lathouwers and Elsacker, 2004:420). Aggressiveness towards younger chimpanzees may also help them in learning these behaviours themselves, as studies have shown that, like human children, juvenile chimpanzees learn through observation and imitation (Waal, 1989:76).
Physical aggression among bonobos rarely lasts for more than a second, unlike the prolonged fights occasionally seen among chimpanzees (Waal, 1989:220). Although a large amount physical aggression has never actually been observed, close examination of a group of free-living bonobos showed that approximately two-thirds of males and one-third of females lacked fingers, toes, or entire hands. While this could be caused by a number of factors, the higher incidence among males suggests a link with intraspecific aggression. This seems more likely when we consider that, until recently, chimpanzees were also considered to be a rather gentle species (Waal, 1989:221). At feeding sights, bonobo males occasionally displaced subordinate males, however these aggressive behaviours usually ended in threatening or charging, and rarely ended in a physical attack (Furuichi and Ihobe, 1994:219-20). Instead of having the prolonged power-struggles seen in chimpanzee communities, bonobos instead seem to use play as a way of establishing dominance. This tendency to use play, which is far rougher than the play seen amongst chimpanzees, may help prevent competition from escalating into an aggressive encounter (Palagi, 2006:422,424). What aggressive conflicts there are in bonobos usually remain unresolved, with no clear signs of submission or dominance (Palagi, 2006:421). Also, because female bonobos tend to remain in estrus longer than chimpanzee females, there is less competition for fertile mates, which may help to account for the lower level of aggression between bonobo males (Stanford, 1998:404). When it comes to aggressive behaviour by adults towards infants, there is currently no evidence of infanticide by adult bonobos (Lathouwers and Elsacker, 2004:412).

Intergroup Aggression

Along with the aggression seen between chimpanzees of the same community, observations in the wild have been made of violent physical contact between males of different communities (Waal, 1989:71). Chimpanzees tend to actively avoid border-areas of their home-range, which may overlap with that of another community. On the occasions when a chimpanzee must venture into this territory, they appear to be tense or cautious, often seeking to travel to these areas in groups with large numbers of males (Wilson and Wrangham, 2003:369). For the most part, interactions with members of different communities are rare, often limited to only auditory contact. Upon hearing the other group, the reaction tends to be one of fear or excitement (Wilson and Wrangham, 2003:370). Groups of male chimpanzees sometimes patrol the borders of their territory, attacking any members of a different community who get too close, except for females during estrus, who they instead try to recruit (Stanford, 1998:401). When attacking female strangers, male chimpanzees can be exceedingly brutal. While those females in estrus appear to be exempt from most of this brutality (unless they attempt to run), mothers with infants are especially targeted; this situation occasionally ends in the males killing and eating the infant (Wilson and Wrangham, 2003:371). Between males, the most common types of physical encounters are either battles, where both sides have a large number of males, or gang attacks, where a group of males attacks an individual. While battles do not usually have very severe outcomes, gang attacks may result in severe injury or the death of the individual (Wilson and Wrangham, 2003:372). There is some danger of becoming wounded during an attack. To reduce the danger for participants, attacks usually only occur when one partly vastly outnumbers the other, with one or more members of the attacking party immobilizing the victim (Watts and Mitani, 2001:301).
Like chimpanzees, bonobos do protect their territory, however there is no evidence of lethal encounters. Occasionally, friendly groups of bonobos have even been seen lingering together for hours at a time, sometimes grooming and copulating with no traces of aggression (Wilson and Wrangham, 2003:365). Intergroup aggression tends to be infrequent and, when it does occur, less severe than that seen amongst chimpanzees (Furuichi and Ihobe, 1994:224). Bonobo foraging parties have a tendency to include females as well as males, and copulation between groups is also seen in these encounters, which rarely end in physical aggression (Stanford, 1998:402). The reason for this may be the smaller degree of sexual dimorphism in bonobos, which reduces male control over females, and produces a less territorial nature than that seen in chimpanzees (Furuichi and Ihobe, 1994:225). Once again, it is important to remember that there is a lack of data on the interactions between bonobo groups. While there is a growing amount of research into this species, the amount of data available is still only a fraction of what has been accumulated from studies of chimpanzees.

Tension Reduction

The threat of aggression leads to tension among both chimpanzees and bonobos. Each species has evolved its own ways of dealing with these tensions, many of which revolve around reconciliation after a conflict has occurred. For chimpanzees, grooming appears to play a key role in maintaining social balance. Mutual grooming is often seen amongst both rivals and allies, and grooming occurs approximately 33% more often than what is seen in bonobo communities (Furuichi and Ihobe, 1994:218-19). Chimpanzees also seem to have a larger range of behaviours, from grooming to hugging and kissing, which they use in more varied contexts to prevent and resolve conflicts. While this may reflect a greater amount of tension in chimpanzee communities, it also shows use of specific behaviours as tools to reduce tension and increase group cohesion (Furuichi and Ihobe, 1994:224). Usually, within approximately two minutes after a conflict has occurred, the conflicting parties will make friendly physical contact with either each other or a neutral third party (Waal and Roosmalen, 1979:58). Occasionally, females will assist a reconciliation between two males by having one follow her towards the other, allowing both to groom her for a period of time, and then walking away, at which point the males continue to groom each other (Waal and Roosmalen, 1979:62). Grooming, as well as meat sharing, is especially important in order to facilitate co-operation between males, which is necessary for hunting, competition against rival communities, and for patrolling the borders of the community home-range (Watts, 2002:346). This need for cohesion within the group helps to explain the amount of different techniques used to reduce tension between group members.
Despite the general description of bonobos as being far less aggressive than chimpanzees, tensions still exist in communities, and bonobos have developed a different way of dealing with these situations. Sex tends to play a cohesive role in their society, allowing bonobos to relieve tension and avoid conflict (usually regarding food) through sexual contact and intercourse. Females are almost continually receptive, and will engage in contact and intercourse with both males and other females (Waal, 1989:180). More aggressive behaviour is seen around feeding time amongst bonobos in captivity, as well as more sexual contact, likely related to tensions over food (Waal, 1989:212). Along with relieving tensions, sexual contact is used for reconciliation. After an outbreak of aggression, an immediate increase in sexual contact, embracing and friendly touching is seen, although there is a decrease in grooming (Waal, 1989:215). Also, genital-genital rubbing amongst female bonobos appears to ease tensions, especially when there is competition over food (Stanford, 1998:404). Sexual contact in order to reduce tensions is not seen amongst chimpanzees, who instead use sex entirely for reproductive purposes (Blount, 1990:706). In bonobos, co-operative hunting is rare (Hohmann and Fruth, 2003:567). This lack of co-operative hunting may be associated with the smaller amount of variation in tension and conflict resolution behaviours than what is seen in chimpanzees.

Discussion

Looking at the information, it is easy to see the differences between the two species. Chimpanzees show far more intragroup aggression, which seems to correlate to the strong social hierarchy that their societies are built upon (Waal, 1989:52-53). Most aggressive behaviours take place between adult males, focusing on establishing dominance. Bonobos, who seem to have a more egalitarian society with less displays of dominance, also have less aggression within their groups. They also use play to resolve dominance issues instead of the more confrontational methods seen in chimpanzees. When a conflict actually does escalate, physical aggression tends to be exceedingly brief when compared to chimpanzee fights. When looking an intergroup aggression, again chimpanzees seem to have the monopoly on aggressive behaviour. The prevalence of territorial defence, occasionally taken to the extremity of killing members of different communities, includes parties of males hunting individuals that stray into the borders of their territories. The fact that this includes infanticide and attacks on females, who are weaker than their male counterparts, makes it seem more a cruelty than a necessity. In bonobos, once again, we see very opposite behaviour, with rival communities interacting peacefully, and never seeing these interactions end in severe or fatal injuries. Bonobos also do not go out to actively hunt the way chimpanzees do, and tend to include females more when going to the borders of their home range. Finally, when looking at reconciliation behaviours one again sees differences. Chimpanzees seem to rely on grooming to maintain group cohesion, while this is far less frequent in bonobos. Also, in bonobos one can observe the use of sex as a tool of tension relief and reconciliation. This use of sex is never seen in chimpanzees, who use sex entirely for reproductive purposes. These differences may be explained by the reduction of sexual dimorphism in bonobos, as well as a simple lack of information on bonobo populations.
Of equal interest is the similarities between the two species. When looking at intragroup aggression, a decrease in these behaviours in bonobos does not mean that they are non-existent. Both species show conflict and tensions, and actual physical aggression is very rare in both chimpanzees and bonobos. Also, the seemingly small amount of aggression amongst bonobos may simply be due to a lack of data on wild populations. As mentioned earlier, in wild populations many individuals are missing extremities such as fingers and toes, a pattern also seen as a result of chimpanzee conflicts (Waal, 1989:61). When looking at intergroup aggression, it is easy to point to chimpanzees as being far more violent; however, tensions are there when rival groups of bonobos are seen to meet. This tension may even help to explain the sexual intercourse that occurs during these meetings, as sex is used to prevent conflict in bonobo society. As previously mentioned, when two groups of chimpanzees are aware of each other, the contact tends to be auditory only, and actual violence is rare. Chimpanzee species were once thought to be as peaceful with other groups as bonobos are now considered to be, and this assumption of greater violence between chimpanzee communities may change as more data is collected on meetings between groups of bonobos. Finally, when looking at tension relief and reconciliation, in both species physical contact appears to be a key feature. While the methods are different, both chimpanzees and bonobos look for friendly physical contact soon after an aggressive conflict occurs. It is also interesting to note that several studies have shown an equal amount of intelligence in both species (Hohmann and Fruth, 2003:567).
When looking at chimpanzees and bonobos as models for early hominids, the similarities suggest that some of these features come from an earlier ancestor, possibly the one from which the Australopithecus branch split off. One could extrapolate that aggression between and within groups of modern humans' early ancestors was relatively common, and that when tensions and conflicts did occur within groups, physical contact was key in resolving these issues and in reconciliation. Upon reflection, this is hardly a surprising conclusion to reach. Human history is full of examples of aggression between tribes, states, and nations of all sizes, many concerning territorial claims or attempts at gaining resources. There are also many examples of conflict within groups, seen in everything from domestic disputes to national rebellions. Finally, the need for physical contact in reconciliation is seen in the common saying, “Kiss and make up.” Comforting friends and family often occurs in the form of hugs, and most people show a need for physical contact when upset. These similarities between humans and their closest relatives, the chimpanzees and bonobos, make it probable that these features were common throughout our recent evolutionary history.

Conclusion

Through an examination of aggression between and within groups, as well as how both bonobos and chimpanzees reconcile and relieve tension, it is easy to see both the differences and similarities between these species. Chimpanzees appear to be more aggressive overall, but much more data is needed on wild bonobo populations in order to make this any more than speculation based on preliminary results. It is important to keep in mind the disparity between the amount of data available for chimpanzees and that available for bonobos, and that bonobos were not recognized as a separate species until fairly recently. The similarities seen in the two species, as well as the physical and intellectual similarities between bonobos, chimpanzees, and modern humans, help to support their use as a model for early hominid behaviour. As more information is collected on the social behaviour of both chimpanzees and bonobos, greater insight into the human past may be attained, leading, in the end, to a greater understanding of modern human social behaviour.



Bibliography

Blount, Ben G.
  1990  Issues in bonobo (Pan paniscus) sexual behavior. American Anthropologist 92(3):702-714.

Furuichi, Takeshi and Hiroshi Ihobe
  1994  Variation in male relationships in bonobos and chimpanzees. Behaviour 130(3/4):211-228.

Hohmann, Gottfried and Barbara Fruth
  2003  Culture in bonobos? Between-species and within-species variation in behavior. Current Anthropology 44(4):563-571.

Lathouwers, Mieke De and Linda Van Elsacker
  2004  Comparing maternal styles in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes). American Journal of Primatology 64(4):411-423.

Palagi, Elisabetta
  2006  Social play in bonobos (Pan paniscus) and chimpanzees (Pan troglodytes): Implications for natural social systems and interindividual relationships. American Journal of Physical Anthropology 129(3):418-426.

Stanford, Craig B.
  1998  The social behavior of chimpanzees and bonobos: Empirical evidence and shifting assumptions. Current Anthropology 39(4):399-420.

Waal, Frans B.M. de
  1989  Peacemaking Among Primates. Cambridge: Harvard University Press.

Waal, Frans B. M. de and Angeline van Roosmalen
  1979  Reconciliation and consolation among chimpanzees. Behavioral Ecology and Sociobiology 5(1):55-66.

Watts, David P.
  2002  Reciprocity and interchange in the social relationships of wild male chimpanzees. Behaviour 139(2/3):343-370.

Watts, David P. Watts and John C. Mitani
  2001  Boundary patrols and intergroup encounters in wild chimpanzees. Behaviour 138(3):299-327.

Wilson, Michael L., and Richard W. Wrangham
  2003  Intergroup relations in chimpanzees. Annual Review of Anthropology 32:363-392.