In sub-Saharan Africa, red-billed oxpeckers feed on the parasites of rhinos and more than 20 other species of mammal. Now, new research suggests the birds may also serve as sentinels that help rhinos avoid humans—and potentially poachers.
Though black rhinos have a good sense of smell and good hearing, they have notoriously bad vision. If you know where one is and stay downwind of it, you can often get quite close to the animal, says Roan Plotz, now a researcher at Victoria University in Melbourne, Australia.
While Plotz was completing his doctoral thesis on black rhinos in Hluhluwe-Imfolozi Park, in eastern South Africa, he began contemplating how black rhinos might avoid humans. In recent years, poachers have decimated the ranks of the black rhino, which in adulthood can weigh a ton and a half. Today, the critically endangered species’ population is a little over 5,000, a tenfold decline since the 1970s.
While studying black rhinos, Plotz found that those close enough for him to observe generally didn't have any oxpeckers on their backs. The birds have good vision and make hissing alarm calls when certain threatening animals—such as humans—approach. Is it possible, he wondered, that oxpeckers were alerting the rhinos to his presence? Such a phenomenon is hinted at in the oxpecker's Swahili name, Askari wa kifaru—which translates to the “rhino’s guard.”
When Plotz set up an experiment to test his hypothesis, it proved true: Rhinos accompanied by the birds were more likely to detect humans, and from further away, compared to animals without oxpeckers.
Testing the hunch
For the experiment, Plotz and colleagues put radio trackers on 14 rhinos in Hluhluwe-Imfolozi, one of the oldest nature reserves in Africa. They then conducted on-the-ground surveys for both tagged and untagged rhinos on a near-daily basis over the course of 27 months. The tagged animals were easy to locate using radio telemetry. Finding the untagged rhinos was more difficult, and required the researchers to roam the landscape looking for the animals.
Of the hundred untagged rhinos the research team saw, only 17 had oxpeckers riding on them. “That seemed quite low,” Plotz says, and likely not representative of the true number of rhinos with oxpecker tagalongs. As a control for the experiment, the team located one of the 14 tagged animals on a hundred different occasions over the study period. Of those sightings, the majority—56—recorded oxpeckers perched on the animal’s hide.
The discrepancy strongly suggested that the oxpeckers on the untagged rhinos were warning their hosts about the scientists’ presence.
To better understand the rhino-bird relationship, Plotz and colleagues did further tests. They conducted 86 trials in which they approached tagged rhinos that were and were not accompanied by oxpeckers.
Every time that a tagged rhino had one or more oxpeckers on its body, it showed physical signs that it had detected a human: standing bold upright, facing downwind, and preparing to run. But when the rhinos were not accompanied by the birds, they showed signs of recognizing the human’s approach only 23 percent of the time.
For detecting humans, having more birds on board seemed to be an advantage to the rhino. In the trials, each additional bird perched on an animal was associated with sensing the human 30 feet further away, on average.
When Plotz and colleagues did a mathematical analysis of all their research, they concluded that the oxpeckers reduced human’s likelihood of a rhino sighting by 40 to 50 percent.
The research findings show that the oxpecker-rhino relationship is more complex and potentially mutually beneficial than previously thought, Plotz says. They also could have implications for conservation, suggesting that introducing the birds into areas where they’ve been depleted might help avert poaching.
Confirming traditional knowledge
Taken together, the results show that “black rhinos are able to eavesdrop on oxpecker alarm calls and by doing so, detect approaching humans at substantially greater distances,” says Daniel Blumstein, an ecologist at the University of California Los Angeles, who wasn’t involved the paper, in an email interview.
Blumstein was also fascinated by what he called the “dose-response”—the fact that each additional oxpecker on a rhino increased the distance at which it detected an approaching human.
The paper, published Thursday in Current Biology, is “a genuinely astonishing study,” said Darryl Jones, a researcher at Griffith University. But Jones acknowledged that “the local peoples of these regions would probably be less impressed, having 'known' about this relationship for millennia.
Indeed, that is one of this study's great achievements: confirming using contemporary methods an important example of traditional knowledge.”
A way to combat poaching?
As for the oxpeckers' motivations, Plotz says, it's unclear if the birds know they're alerting the rhinos. It’s possible they are merely alerting fellow oxpeckers to the presence of a potential predator.
Amanda Ridley, a biologist at the University of Western Australia, takes issue with framing this as the oxpeckers “warning” the rhinos about humans.
“Rhinos may have responded to the oxpecker alarm call, but there is no evidence that oxpeckers intentionally alerted them to it,” she says.
Judith Bronstein, a professor at the University of Arizona, agrees—but says that doesn’t diminish the importance of the paper.
“Eavesdropping on other species’ signals is a very well-documented adaptive behavior, but its distribution in nature is not well understood,” she says. “The example here is carefully worked out and involves large solitary mammals, which is new.”
Whatever the intended purpose of the oxpeckers’ calls, they could help reduce poaching by alerting rhinos to humans’ presence at a greater distance where the accuracy of firearms would be reduced, Bronstein adds.
Plotz says that introducing the birds into areas where both oxpeckers and rhinos have been depleted could potentially reduce poaching, and that potential intervention should be further studied. Red-billed oxpeckers have been extirpated from many areas; when pesticides are applied to livestock to kill parasites, the treatment also unintentionally kills the birds that feed upon those parasites.
In Plotz’s view, “There definitely could be a benefit to introducing oxpeckers back into black rhino populations.”