Dominance, age and weight in American bison males (Bison bison) during non-rut in semi-natural conditions

In the wild, male American bison (Bison bison) aggregate only during the breeding season or rut with female herds to reproduce. During the non-rut, they form bachelor groups. Very little is known about dominance structures in these bachelor herds. In this report, we evaluate social dominance in bachelor groups of bison bulls kept on a commercial bison farm in Belgium, where natural seasonal grouping dynamics are imitated. Observations were carried out during five different non-rut periods over several years. Agonistic behaviour was scored and used to determine dominance rank. The relationships between dominance and the individual factors age and weight were evaluated. We found a strong and significantly linear dominance hierarchy in the male bachelor groups. There was a reversal in rank among same-aged individuals between two subsequent study periods during one non-rut period. Dominance and age were strongly correlated in every study period, as were dominance and weight. We conclude that during non-rut bison bulls can form temporary groups with a linear dominance hierarchy in which older and heavier individuals occupy the highest social positions. © 2004 Elsevier B.V. All rights reserved.status: publishe

Dominance and its behavioral measures in a captive group of bonobos (Pan paniscus)

We investigated the existence of a social dominance hierarchy in the captive group of six adult bonobos at the Planckendael Zoo. We quantified the pattern of dyadic exchange of a number of behaviors to examine to what extent each behavior fits a linear rank order model. Following de Waal (1989), we distinguish three types of dominance: agonistic dominance, competitive ability and formal dominance. Fleeing upon aggression is a good measure of agonistic dominance. The agonistic dominance hierarchy in the study group shows significant and strong linearity. The rank order was: 1. female (22 yr), 2. female (15 yr)., 3. male (23 yr.), 4. female (15 yr.), 5. male (9 yr.), 6. male (10 yr.). As in the wild, the females occupy high ranks. There is prominent but nonexclusive female agonistic dominance. Teeth-baring does not fulfil the criteria of a formal submission signal. Peering is a request for tolerance of proximity. Since its direction within dyads is consistent with that of fleeing interactions, it is a useful additional measure to determine agonistic ranks in bonobos. In competitive situations, the females acquire more food than other group members do. The rank obtained from access to food resources differs from the agonistic rank due to female intrasexual social tolerance, expressed in food sharing. We typify the dominance styles in the group as female intrasexual tolerance and male challenging of rank differences. The agonistic rank order correlates significantly with age and has a strong predictive value for other social behaviors

Dominance and its behavioral measures in a captive group of bonobos (Pan paniscus)

We investigated the existence of a social dominance hierarchy in the captive group of six adult bonobos at the Planckendael Zoo. We quantified the pattern of dyadic exchange of a number of behaviors to examine to what extent each behavior fits a linear rank order model. Following de Waal (1989), we distinguish three types of dominance: agonistic dominance, competitive ability and formal dominance. Fleeing upon aggression is a good measure of agonistic dominance. The agonistic dominance hierarchy in the study group shows significant and strong linearity. The rank order was: 1, female (22 yr), 2. female (15 yr)., 3. male (23 yr.), 4. female (15 yr.), 5. male (9 yr.), 6. male (10 yr.). As in the wild, the females occupy high ranks. There is prominent but nonexclusive female agonistic dominance. Teeth-baring does not fulfil the criteria of a formal submission signal. Peering is a request for tolerance of proximity. Since its direction within dyads is Consistent with that of fleeing interactions, it is a useful additional measure to determine agonistic ranks in bonobos. In competitive situations, the females acquire more food than other group members do. The rank obtained from access to food resources differs from the agonistic rank due to female intrasexual social tolerance, expressed in food sharing. We typify the dominance styles in the group as female intrasexual tolerance and male challenging of rank differences. The agonistic rank order correlates significantly with age and has a strong predictive value for other social behaviors.status: publishe

The pivotal role of rank in grooming and support behavior in a captive group of bonobos (Pan paniscus)

We investigated dyadic grooming relationships in a captive group of bonobos (Pan paniscus) and questioned what social function grooming fulfils in the 'market of services and favors'. Hereto we examined which of two theoretical models - grooming for support (Seyfarth, 1977, 1980) or grooming according to the similarity principle (de Waal & Luttrell, 1986) -best accounted for the observed grooming distribution. Similarity in traits did not correlate with increased grooming or close proximity among the individuals. Therefore, the similarity hypothesis was rejected. Seyfarth's model of rank-related grooming was largely confirmed. The animals distributed their grooming according to the rank of the receivers. We found an exchange between grooming and receipt of support. There was more grooming up than down the hierarchy. However, not all predictions about rank-related competition over grooming were confirmed. We found that dyadic grooming reciprocity indeed increased with decreasing tank distance. Yet, there was no increase of grooming within the dyad with decreasing rank distance and high ranking individuals were not competed over at the highest rates. The observed correlation between grooming and support received represents an important fit with Seyfarth's prediction, but does not allow for conclusions about underlying causal processes. Other causal explanations, besides the 'groom to receive support' hypothesis, that could explain a similar correlation are discussed.status: publishe

The influence of steepness of dominance hierarchies on reciprocity and interchange in captive groups of bonobos (Pan paniscus)

Biological market models explain variability in reciprocity and interchange between groups. In groups with a shallow dominance gradient, grooming will be mostly exchanged for itself (i.e. exchange will occur). In groups with steep dominance hierarchies, interchange is expected: individuals will groom higher ranking individuals to get access to limited resources or commodities such as support in conflicts, and grooming will be traded for these commodities. We examine patterns of reciprocity in grooming and support, and of interchange of grooming for support or for tolerance in six captive groups of bonobos. We test whether differences between groups in patterns of reciprocity and interchange can be attributed to differences in a measure of steepness of dominance hierarchies, which is based on dyadic agonistic interactions. We found that grooming was reciprocal in some, but not all groups. Support was highly reciprocal, but this was a side effect of dominance in most groups. Interchange between grooming and support was observed in some groups. Corroborating earlier findings, this was a side effect of individuals preferring high ranking individuals as grooming and support partners, possibly because these high-ranking individuals provide more efficient support in conflicts. There was no evidence for interchange of grooming for tolerance. Variability in grooming reciprocity was explained by differences in steepness of dominance hierarchies, as predicted by the biological market models. In groups with a shallow dominance hierarchy, grooming was more reciprocal. This was not true for reciprocity in support. There was some evidence that individuals groomed dominants more frequently in groups with a steep dominance hierarchy. The variation in interchange relations between grooming and support did not depend on the steepness of dominance hierarchies. We suggest that grooming in itself is a valuable commodity in bonobos, especially under captive conditions, which can be exchanged reciprocally. Bonobos may interchange grooming for another value equivalent, with food sharing as a very likely candidate. This interchange effects seem more dependent on potential to monopolise food than on steepness of dominance hierarchies per se

Peering is not a formal indicator of subordination in bonobos (Pan paniscus)

It has been suggested that peering behavior in bonobos is a formal signal acknowledging social dominance status. We investigated whether peering meets the published criteria for a formal signal of subordination in five captive groups of bonobos. The degree of linearity in the set of peering relationships was significantly high in all study groups, and a linear rank order was found. However, unidirectionality was low, and there was little correspondence between the peering order and the agonistic dominance rank. Therefore, peering does not satisfy the criteria of a formal subordination indicator. We also studied the relation between peering and agonistic dominance rank, age, and sex. Animals directed peering significantly more often at high-ranking animals in four of the groups. We suggest that peering is indirectly related to dominance rank by the resource-holding potential of individuals. In contexts where dominant individuals can monopolize resources, peerers may direct their attention at those high-ranking animals. When resources are distributed more evenly, high-ranking animals may peer down the hierarchy. We speculate on the reasons why a formal dominance or subordination signal appears to be absent in bonobos.status: publishe

Peering is not a formal indicator of subordination in bonobos (Pan paniscus)

It has been suggested that peering behavior in bonobos is a formal signal acknowledging social dominance status. We investigated whether peering meets the published criteria for a formal signal of subordination in five captive groups of bonobos. The degree of linearity in the set of peering relationships was significantly high in all study groups, and a linear rank order was found. However, unidirectionality was low, and there was little correspondence between the peering order and the agonistic dominance rank. Therefore, peering does not satisfy the criteria of a formal subordination indicator. We also studied the relation between peering and agonistic dominance rank, age, and sex. Animals directed peering significantly more often at high-ranking animals in four of the groups. We suggest that peering is indirectly related to dominance rank by the resource-holding potential of individuals. In contexts where dominant individuals can monopolize resources, peerers may direct their attention at those high-ranking animals. When resources are distributed more evenly, high-ranking animals may peer down the hierarchy. We speculate on the reasons why a formal dominance or subordination signal appears to be absent in bonobos. Am. J. Primatol. 65:255-267, 2005. © 2005 Wiley-Liss, Inc