Does the resource dispersion hypothesis predict grouping behaviour in a solitary megaherbivore?

The resource dispersion hypothesis (RDH) predicts that when resources are abundant and heterogeneously distributed, multiple individuals may satisfy their resource needs in a shared area without imposing large costs on one another. Under such conditions, groups can form even in the absence of other benefits, and other life history traits may not entirely explain group composition. For example, groups forming under conditions associated with the RDH may not display a biased sex ratio even if otherwise expected. Solitary herbivores, which are unlikely to gain benefits from grouping, may be strongly subject to the RDH, especially during growing seasons when there is an abundance of resources. However, to the best of our knowledge, no study has evaluated whether grouping behaviours in any solitary herbivore follows RDH predictions. To explore this, we used 48,678 sightings of black rhinos (Diceros bicornis) from 13 reserves in southern Africa to assess whether grouping behaviour in this solitary megaherbivore aligned with predictions of the RDH, or reflected predictions informed by the species ecology. We found that group formation and size largely followed RDH predictions where groups were more likely to form when resources were heterogenous and of high quality. Additionally, groups were larger during the growing season and when resources were heterogeneous. In contrast, as resource quality increased, group size decreased. Similarly, two of the four group composition results followed RDH predictions, where increasing group size was associated with decreasing within-group familiarity and an unbiased sex ratio in opposition to usual expectations. The other two followed species ecology predictions wherein increasing group size was associated with an increasing proportion of subadults and all large groups included at least one cow-calf pair. Ultimately, our results show that, in solitary herbivores, the RDH may be useful in predicting group formation and size and may explain instances when group composition diverges from species-specific expectations. Understanding grouping behaviour in endangered solitary species such as black rhino may improve management by allowing managers to identify conditions under which individuals are more tolerant of conspecifics.