Changes in social dominance in a group of subadult white rhinoceroses (Ceratotherium simum) after dehorning

Abstract

In many social species physical attributes correlate with dominance rankings and influence the outcomes of dyadic interactions. We investigated the processes which affect white rhinoceros social behaviour in response to a reduction in horn size asymmetries within a group of subadult individuals. We monitored agonistic social interactions and the orderliness of social rankings between six free ranging rhinoceroses before and after they underwent a second dehorning procedure. We used a modified version of Landau's h to measure linearity, a score of steepness to measure power asymmetry, and a measure of triangle transitivity to assess relationships in the presence of null dyads. Agonistic social interactions were significantly greater after the monitored dehorning procedure. Hierarchies possessed significant steepness and transitivity prior to the procedure but not after. Linearity was nonsignificant and rank order did not correspond to changes in horn size or age. Our results provide the first evidence of a dominance hierarchy among free-ranging white rhinoceroses outside of reproductive competition but indicate that physical attributes alone do not explain social rankings. Rhinoceroses transitioned to a more egalitarian dominance structure than a despotic one after the procedure, but dominance ranks were only weakly differentiated within the group. Although a reduction in horn asymmetries may increase agonistic behaviours through psychosocial or behavioural changes, drier climatic conditions cannot be ruled out as the causative factor and as the subadult group stayed together rather than dispersing, any increased fitness costs are likely to be minimal and outweighed by the benefits of group membership.

Dehorning impacts white rhinoceros behaviour less than social events: evidence from Botswana

“…We observed only one fight, which occurred before dehorning. Thus, our results neither confirm a decrease (Patton et al, 2018) nor an increase (Penny et al, 2022) of agonistic social interactions after dehorning, but the low number of aggressive interactions observed supports the classification of white rhinos as the most social rhino species (Shrader & Owen-Smith, 2002). Since the dehorning in the study by Patton et al (2018) was undertaken to reduce the number of fights, effects of dehorning on aggressive behaviour might have only been observable due to an atypically high rate of aggressive interactions before dehorning.…”

“…There are limited opportunities to observe wild rhinos around dehorning events; therefore, our results are based on a small number of individuals. However, with nine individuals, our sample size is comparable to other studies (Patton et al., 2018: n = 6, Penny et al., 2022: n = 6). For logistical reasons, our observation times were relatively short and rare events such as a fight after dehorning might therefore not have been observed due to a sampling effect.…”

“…We observed only one fight, which occurred before dehorning. Thus, our results neither confirm a decrease (Patton et al., 2018) nor an increase (Penny et al., 2022) of agonistic social interactions after dehorning, but the low number of aggressive interactions observed supports the classification of white rhinos as the most social rhino species (Shrader & Owen‐Smith, 2002). Since the dehorning in the study by Patton et al.…”

“…Furthermore, it is not clear what amount of horn is needed to show horn-related behaviours. However, rhinos are able to adapt to the strong change in their facial structure as horns occasionally break off and wear down naturally; such adaptation has been shown in previous dehorning studies (Kock & Atkinson, 1993;Patton, 2021;Penny et al, 2022). Studies analysing the behaviour after natural horn loss would be helpful to disentangle the effects of the dehorning procedure and the horn loss itself.…”

“…Females use their horns in socio‐negative interactions to deter approaching males or other females and calves from their feeding places. Horn growth is faster in males than in females (Rachlow & Berger, 1997), and although horn size is not the deciding factor for female mate choice (Kretzschmar et al., 2020), it can affect the rhino's dominance status (Penny et al., 2022). Thus, we expect that costly horn‐related aggressive behaviours will be reduced after dehorning because of the reduced effectiveness and risk of injuries with a shortened horn.…”