Dominance style in captive, fission-fusioned bonobos (Pan paniscus): Dominance hierarchy, aggressive patterns, and conflict management
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The topic of my dissertation was dominance style and its covariates, in which I examined the question of whether bonobos ( Pan paniscus ) exhibit extreme egalitarian dominance style. I studied a group of 12 to 17 captive, fission-fusioned bonobos at the Columbus Zoo and Aquarium, Ohio. I observed the bonobos during two time periods: March, 2001 to May, 2002 and February, 2004 to October, 2006. I employed all-occurrences, scan, focal animal, and post-conflict/matched control sampling. I tested predictions concerning dominance hierarchies, aggressive patterns, and conflict management. I examined submissive and aggressive behavior patterns, post-conflict affiliative behaviors between former opponents and between victims and third-parties, and behaviors exhibited before and during periods of predictable tension (i.e. feeding). First, my prediction for the bonobo dominance hierarchy was that it could be highly variable. Prior research indicated that bonobo dominance hierarchies can be expressed in a variety of ways, in which the alpha is always female but the hierarchy could range from sexual codominance to complete female dominance. Also, according to predictions for egalitarian species, the hierarchy should be weakly linear and shallow. There was evidence for complete female dominance, in which all females outranked all males in the hierarchy. The hierarchy was highly linear (determined by the expression of submissive behaviors) and shallow (determined by wins and losses in aggressive bouts). I then examined patterns of aggression, in which I predicted that aggression rates should be low, that high-intensity aggression should be rare, and that counter-aggression would be the most common response to aggression. Aggression was infrequent and intense aggression was rare. However, the most common response to aggression was fleeing and/or submission. Second, I predicted that, according to expectations for egalitarian dominance style, bonobos engage in post-conflict affiliation via reconciliation and third-party post-conflict affiliation. Using the post-conflict/matched control methodology, I found that there were significant overall tendencies for bonobos to reconcile and engage in affiliation with third parties after the cessation of aggression. The bonobos also appeared to use specialized behaviors in reconciliation (gestures), victim-initiated third-party affiliation (socio-sexual behavior) and consolation (gestures). Finally, I predicted that bonobos would actively engage in conflict prevention during predictably tense periods (i.e. anticipation of feeding) by determining whether there was a negative correlation between affiliative behaviors during anticipation periods (before feeding) and aggressive behaviors during feeding. There was no evidence for conflict prevention in this group. However, there was a positive correlation between affiliation during the anticipation period and tolerance in the form of co-feeding during the feeding period. There was also evidence that high rates of socio-sexual behavior during anticipation correlated with decreases in socio-sexual behavior between anticipation and feeding, thus socio-sexual behavior functioned as an excitement reducer when performed during the anticipation period. The dominance style concept predicts that the traits associated with dominance style should co-vary (i.e. measures of behavior patterns should be positively or negatively correlated with one another) across species and/or relationships. Co-variation of egalitarian traits, compared to other species, did not appear to occur in this group. The low aggression rates, low rates of high-intensity aggression, shallow dominance gradient, high rates of bidirectional aggression, high rates of reconciliation and consolation, and tolerance during feeding were indicative of an egalitarian dominance style. However, the bonobos also exhibited some characteristics of a more despotic dominance style, such as a highly linear dominance hierarchy, fleeing and/or submission as a response to aggression, and no evidence for conflict prevention. I presented two alternative hypotheses for the patterns I observed. First, the lack of covariation of traits associated with dominance style may have been more apparent than real due to the unique social structure of bonobos. Second, biological market theory may be a better alternative explanation than dominance style covariation for the patterns I observed.