Black rhinos are mostly loners, but not always: why this matters for saving the species

Black rhinos are notoriously aggressive, not just to humans, but also to their fellow black rhinos. Aggression is so prevalent that injuries sustained after fighting with another black rhino are a common cause of death. Because of this aggression and their solitary nature, black rhino social behaviour has been largely unexplored. Yet it may be key to conservation of this critically endangered African icon.

According to the International Union for Conservation of Nature, there were an estimated 6,788 black rhinos in the whole of Africa in 2024. That year, 516 rhinos were killed by poachers, 81% of them in South Africa.

One of the most important tools in conservation of the species is translocation, where managers move individuals from established populations either into existing populations that would benefit from the introduction of new genetics, or into unoccupied regions to establish new populations. But aggression between local black rhinos and newcomers, or between individuals introduced together, can make translocation risky.

If managers know more about aggression and how to avoid it, translocation could become more successful.

One project that has successfully translocated black rhinos is the Black Rhino Range Expansion Project, a partnership between the World Wide Fund for Nature South Africa, the provincial conservation agencies Ezemvelo KZN Wildlife and Eastern Cape Parks and Tourism, and numerous reserves. For more than 20 years, the project has worked to establish new breeding populations across southern Africa. These efforts also benefit the populations that provided the animals, because black rhinos survive and breed more effectively when their populations are less dense.

When I arrived at the University of Pretoria in November 2024 to work with herbivore ecologist Adrian Shrader, studying this project’s black rhino populations, the animals were described to me as being solitary and aggressive. Local South Africans, scientists and students at the university, rhino project managers and the scientific literature all agreed.

But as I read and spoke to more people, I noticed something unexpected. Many people, and many scientific papers, described seeing black rhinos in apparently peaceful groups. In a 1967 paper, biologist John Goddard described a group of 13 individuals in Tanzania that, after two hours together, quietly separated into smaller groups or moved away individually without fighting or being aggressive towards each other.

These seemingly conflicting accounts made me interested in exploring black rhino social behaviour, aggression, and their effects on the survival of individuals. This led to the publication of a paper on associations and their effects on survival.

Using data from 69,920 sightings of 542 black rhinos in 14 reserves, we found that black rhinos are indeed very solitary. But 8.1% of sightings were of groups of 3-7 animals. Our analyses showed that when they do interact with other rhinos, it’s usually with individuals they already know and have previously associated with. They also avoid adult males.

Our findings suggest that when managers select individuals to move, picking animals with existing relationships may be important for decreasing the risk of aggression after release. It’s also important to consider the sex of the individuals being translocated.

Loners and female associates

A majority (69.2%) of the black rhino sightings documented by staff at the 14 project-affiliated reserves in the study were of only single individuals. Sightings of two animals (mostly mothers with young) made up 22.7%. The other 8.1% of sightings were of groups.

Adult females had the most associates – they were seen with more unique individuals than any other age or sex class. Females were seen more often with other females, and males somewhat more often with other males. This trend was particularly noticeable in calves (young animals that had not left their mother).

We ran the data through a series of statistical models and the results may help to explain the apparent conflict between black rhino aggression and the fact that they can also be seen in groups. Yes, black rhinos tend to spend most of their time on their own. But when they do interact with other rhinos, it’s mostly with individuals they know.

However, interacting too often with unrelated individuals decreases the long-term survival of calves. This is most likely due to the increased stress of repeatedly interacting with potentially aggressive individuals.

When adults do join groups, they tend to avoid adult males, likely because they are more aggressive. Indeed, we found that adults that associate with a higher proportion of males had relatively lower survival.

Together, these results point to a nuanced story. The solitary nature of black rhinos is maintained, at least in part, by aggression. Yet, black rhinos do periodically interact and may lessen the risk of aggression by selecting individuals they know. Similarly, they probably also reduce risk by avoiding adult males, which are more likely to be aggressive as they defend their breeding ranges.

Choosing who to move

Our findings might be used to improve one of the most important management tools for black rhino conservation: translocation. Moving animals that have existing social relationships may reduce later aggression. Because adult males may pose a greater risk to their fellows than other sex or age classes, introduction of too many males could increase aggression and fighting within the new population.

Our findings add to the growing body of research showing that social behaviour in solitary species, including carnivores, primates, rodents, and even invertebrates such as scorpions, is more nuanced than previously thought. Many solitary species are threatened or endangered, but their social behaviours are often overlooked. Our findings illustrate how understanding that behaviour can be incorporated into management strategies.

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