An assessment of behavioural dominance in a social subterranean rodent, Cryptomys hottentotus pretoriae, using the cardinal dominance index method

Abstract

The highveld mole-rat, Cryptomys hottentotus pretoriae, is a social bathyergid that exhibits a high reproductive skew and occurs in colonies of between 5 and 14 individuals. Behavioural dominance has previously been assessed in this species using an ordinal ranking procedure and it was found to possess a non-linear dominance hierarchy. The determination of dominance using ordinal ranking is often subjective and cannot deal with ties in rank. This study adapts an objective and robust cardinal assessment method to assign dominance indices to individuals. These indices are then ordered to assign a final rank to each individual within a colony. The non-linear, exponential decay model provided the best fit to data from average cardinal dominance index versus final rank and revealed that colonies contain a single behaviourally-dominant individual. However, final rank could not be predicted on the basis of body mass, sex or assumed reproductive status and is not always statistically significant due to the inherent variation. We propose that the cardinal method of dominance assessment, with the adaptations made in this study, be used in future dominance-related assessments for other bathyergids and social organisms in general.