Mandrills represent their own dominance hierarchy on a cardinal, not ordinal, scale

Attempts to measure dominance relationships using cardinal, rather than ordinal ranks have a long history. Nevertheless, it is still unclear if cardinal dominance ranks have an impact on the life of animals. In particular, no information is available on how individual group living animals represent their own dominance hierarchy. This can be investigated testing whether cardinal rank differences affect how animals interact with different group mates. In this study, we evaluated how mandrills (Mandrillus sphinx) interacted with group mates in relation to differences in cardinal ranks while controlling for differences in ordinal ranks. Mandrills were more likely both to avoid an approaching group mate and to direct their grooming to a group mate when differences in cardinal ranks were larger (controlling for differences in ordinal ranks). These results suggest mandrills represent their own dominance hierarchy as based on a cardinal, not an ordinal, scale.