Consistency of dominance rank order: a comparison of David's Scores with I&SI and Bayesian methods in macaques.

In nonhuman primate social groups, dominance ranks are usually assigned to individuals based on outcomes of dyadic agonistic encounters. Multiple approaches have been used, but currently there is no consensus. One approach, David's Scores (DS), offers dual advantages of yielding cardinal scores that may in turn be used to compute hierarchical steepness. Here we correlate rank orders yielded by DS with those yielded by both the traditionally used I&SI approach and the recently proposed parametric Bayesian approach. We use six datasets for female macaques (three despotic and three tolerant groups), and 90 artificially generated datasets modeling macaque groups. We also use the artificial datasets to determine the impact of three characteristics (group size, interaction frequency, and directional asymmetry of aggression) on the magnitude of correlation coefficients, and assess the relative utility of two indices used to compute DS: Dij versus Pij. DS-based rank orders were strongly positively correlated with those yielded by the other two approaches for five out of the six macaque datasets, and for the majority of artificial datasets. Magnitudes of correlation coefficients were unrelated to group size or interaction frequency, but increased with directional asymmetry, suggesting methodological inconsistencies were more likely when dyads had more frequent reversals in directions of aggression. Finally, rank orders calculated using the Dij and Pij indices were similarly consistent with orders from other methods. We conclude that DS offers consistent estimates of rank orders, except perhaps in groups with very low levels of aggression asymmetry. In such "tolerant" groups, we suggest that the relatively greater methodological variability in rank orders may reflect behavioral characteristics of tolerant groups rather than computational inconsistencies between methods. We hypothesize that this quality may be quantified using posterior probability scores of Bayesian rank orders and may also index macaque social styles.

Using biological markets principles to examine patterns of grooming exchange in Macaca thibetana.

Biological markets principles offer testable hypotheses to explain variation in grooming exchange patterns among nonhuman primates. They predict that when within-group contest competition (WGC) is high and dominance hierarchies steep, grooming interchange with other "commodity" behaviors (such as agonistic support) should prevail. In contrast, when WGC is low and gradients shallow, market theory predicts that grooming reciprocity should prevail. We tested these predictions in a wild, provisioned Tibetan macaque (Macaca thibetana) group across six time periods during which the group had been subjected to varying degrees of range restriction. Data on female-female aggression, grooming, and support were collected using all-occurrences and focal animal sampling techniques, and analyzed using ANCOVA methods and correlation analyses. We found that hierarchical steepness varied significantly across periods, but did not correlate with two indirect indicators of WGC (group size and range restriction) in predicted directions. Contrary to expectations, we found a negative correlation between steepness and group size, perhaps because the responses of group members to external risks (i.e. prolonged and unavoidable exposure to humans) may have overshadowed the effects of WGC. As predicted, grooming reciprocity was significant in each period and negatively correlated with steepness, even after we controlled group size, kinship, rank differences, and proximity. In contrast, there was no evidence for grooming interchange with agonistic support or for a positive relationship between interchange and steepness. We hypothesize that stressful conditions and/or the presence of stable hierarchies during each period may have led to a greater market demand for grooming than support. We suggest that future studies testing these predictions consider more direct measures of WGC and commodities in addition to support, such as feeding tolerance and access to infants.