Black rhinoceroses are the junkyard dogs of African rhinoceroses. They are not the largest species on the continent, but they are known for aggressive patrolling and defending their territory, and they are quick to attack any person, vehicle, or other rhino they consider an intruder.
One of the keys to that behavior, it turns out, seems to be their horns.
Research published Monday shows that black rhinoceroses that have been dehorned in an effort to thwart poachers have significantly less interaction with other rhinoceroses and are shrinking their habitat.
“It definitely disrupts their social networks,” said Vanessa Duthé, a doctoral candidate in conservation biology at the University of Neuchâtel in Switzerland, and lead author of the findings, which appeared in the Proceedings of the National Academy of Sciences.
“It’s like putting a muzzle on a dog,” Mrs. Duthé said. “They are not so sure of themselves anymore. They have lost their main defense and their confidence.”
Rhinoceroses stripped of their main weapon, their horn, seem to feel more vulnerable, Ms Duthé said. This vulnerability is reflected in their reduced appetite for exploration and entering into conflict with other rhinoceroses.
The study does not address whether black rhinos’ “very strong response” to dehorning has an overall positive or negative effect on the species, Ms Duthé added, such as whether it will result in over time genetic changes by shifting reproductive dynamics, or the number of animals a given landscape can support.
Dehorning has become increasingly common in southern Africa over the past decade as a means of deterring poachers from killing rhinoceroses for their horns, which can be worth more than diamonds or gold on the black market in Southeast Asia.
Dehorning is a painless procedure in which vets first anesthetize a rhino. They blindfold the animal and put in ear plugs to prevent sensory overload. They then use a chainsaw to cut off the top of his horn, but only the portion that does not contain nerves. The base of the horn is then sanded. The whole process takes no more than 20 minutes. Like fingernails, rhino horns grow back over time and animals are usually dehorned once every 18 months.
Despite the prevalence of this practice, until now researchers were unaware of the effects dehorning had on rhino behavior and survival.
More orner than white rhinoceroses, their larger and more populous cousins, black rhinoceroses are a critically endangered species: Only 5,500 to 6,000 individuals remain, with 36 percent in South Africa. Ms. Duthé and her colleagues analyzed 15 years of data on the movements of 368 of these animals in 10 South African nature reserves. Prior to 2013, none of the black rhinos included in the study had been dehorned, but by 2020 it was 63 percent.
The researchers found that dehorning does not increase the chance that a rhino will die from causes other than poaching. However, the habitat of dehorned animals shrank by an average of 45.5 percent, with some rhinoceroses losing up to 80 percent of their territory. Dehorned individuals were also 37 percent less likely to engage in social interactions, especially those between males.
“The study is robust and well scientific, with long-term data and a large number of observations,” said Sam Ferreira, a large mammal ecologist with the International Union for Conservation of Nature’s African Rhino Specialist Group, who was not involved in the study. “The results highlight important unintended consequences when addressing indirect approaches such as dehorning to address societal pressures on rhinos,” in this case poaching.
Rhino poaching has declined somewhat since its peak in 2015, when 1,349 animals were killed out of a total African white and black rhino population of about 22,100. But today’s situation is still “very critical and urgent,” Ms Duthé said, with more than 548 rhinos poached across Africa last year.
While the rise in dehorning has been linked to a decline in rhino kills, rhino poaching is also influenced by a complex mix of economic, social and security factors. In terms of dehorning, “No one has yet come to a conclusion whether it is effective or not,” Ms Duthé said.
But even with all the unknowns, and with the new results pointing to implications for rhino behavior, dehorning still appears to be a valuable conservation tool that “is necessary in some cases,” Ms Duthé said. This is especially the case in sanctuaries that cannot afford to increase other security measures for their animals.
Michael ‘t Sas-Rolfes, a conservation economist at Stellenbosch University in South Africa who was not involved in the study, said dehorning is not ideal, but “a somewhat desperate measure”.
“It’s all very well to advocate ideal solutions, but we need to be pragmatic in the short term to ensure that rhinos survive the constant attacks of poachers,” he said. “The fact that dehorning is now so widespread shows how serious the poaching problem remains.”