Dehorning may affect how rhinos interact and establish territory

Black rhinoceroses whose horns have been removed to protect them from poaching seem to have a smaller home range and fewer interactions with other rhinos, but the practice wasn’t found to increase deaths from natural causes.

The critically endangered black rhinoceros (Diceros bicornis) is often hunted for its horn. The animals are largely solitary, but social interactions are important for finding mates and establishing territories. Some research suggests that those with larger horns usually dominate in encounters between males.

Since 2017, many reserves in South Africa and elsewhere have routinely sawn off their rhinos’ horns to dissuade poachers from killing them. This has succeeded in reducing poaching, but researchers were unsure how it might affect the rhinos’ behaviour.

“One of the reserves had noticed that the dehorned rhinos, especially the bulls, would engage in inconsequential fights. These rhinos would just lie exhausted,” says Vanessa Duthé at the University of Neuchâtel in Switzerland. “So, that got me wondering if dehorning is affecting their territory.”

Duthé and her colleagues studied black rhinos from 10 reserves in South Africa. They analysed monitoring data including 24,760 observations of 368 black rhinos from 2005 to 2020. They also used GPS data to measure the home ranges of 68 rhinos two years before and after dehorning, and compared this with 120 rhinos that had never been dehorned.

The analysis revealed that the dehorning campaign coincided with a drastic fall in deaths from poaching. However, other factors may have contributed to this, including improved security, lower economic incentives for poachers and covid-19 lockdown regulations.

The study also found that dehorned females reduced their home ranges by 53 per cent and dehorned males by 38 per cent on average, and they had 37 per cent fewer social interactions after dehorning. Meanwhile, horned rhinos increased their ranges by over 50 per cent.

Duthé doesn’t think these behavioural changes have affected rhino populations overall, as numbers have remained stable in the reserves they studied. She says dehorning might have reduced the mating success of formerly dominant bulls, but opened the door for younger bulls, thus keeping the overall population stable.

Raoul du Toit at Lowveld Rhino Trust in Harare, Zimbabwe, says it is normal for the home ranges of males to shrink as they get older, and other long-term studies in Zimbabwe haven’t found that dehorning is linked to a reduced range in females. He also says that social interactions in black rhinos mostly occur at night. “Sample sizes of around 20 rhinos in three areas, with presumably daytime observations, are relatively limited to draw behavioural conclusions that could apply to both sexes,” he says.