‘Psst! The poachers are coming.’ How tiny birds tip off rhinos.

From afar, a rhino looks a bit like a living, breathing tank, oblivious to the smaller critters around it. But these great beasts may actually be paying attention.

Critically endangered due to poaching, black rhinoceroses have learned to tune into the alarm calls of little birds, known as red-billed oxpeckers, that hitchhike on their backs, according to a study published last week in the journal Current Biology.

This isn’t a completely new revelation. In fact, the Swahili name for oxpecker translates to “the rhino’s guard.” The relationship isn’t completely harmonious – more on that later – but it does highlight the interconnectedness of the natural world. And for Western science, it offers an expanded view of how eavesdropping fits into the social order of animals, and how animals communicate and interact with one another more broadly.

“Who would’ve thought that a 2-ton behemoth would pay attention to what this tiny … bird sitting on their back says?” says Erick Greene, a professor of biology at the University of Montana in Missoula. “These studies of animal behavior show us just how attuned species are to each other in weird and mysterious and wonderful ways.”

Researchers used to see animal communication in fairly simple terms, says Dr. Greene, largely individuals of the same species sending direct signals to each other. But, he says, “we’re realizing that eavesdropping is huge.”

Dr. Greene wasn’t involved in this particular study, but his own research focuses on the soundscape created by birds and small mammals in forests. “Everybody basically is listening to everybody else,” he says. 

In fact, birds and squirrels seem to communicate warnings in tandem. Birds have a specific alarm call to alert others that a predator is present or approaching, called a “seet” call for how it sounds – and Dr. Greene has observed squirrels making and responding to that same sound. “They’re basically speaking the same language,” he says. In his research, he has encountered similar interactions in forests around the world.

Some researchers had surmised that social, vocal species might be the only ones eavesdropping, and that solitary species might not have the necessary capabilities. But rhinos are neither social nor very vocal.

“It seems to be that their vulnerability to human predation” may be more of a key factor, says study author Roan Plotz, a behavioral ecologist and lecturer in environmental science at Victoria University in Melbourne, Australia. Perhaps “it’s predation threat alone and not sociality that drives the capacity to eavesdrop,” he says.

Predation certainly is a large factor in shaping many behaviors in ecosystems, says Daniel Blumstein, a professor of ecology and evolutionary biology at the University of California, Los Angeles who was also not involved in the new study. “I think we expect that any species that has any risk of predation to pay attention to what’s going on in its environment to understand changes in threats.”

The risk of predation seems to be a powerful force driving the rhinos to accept the discomfort of the oxpeckers’ presence. Although the birds munch on harmful parasites on the beasts, they also feed on open wounds in the rhinos’ flesh – a behavior that makes some other creatures try to toss the birds off. But the black rhinos can’t see very well to spot poachers, which may be why they seem to tolerate the annoyance that comes with the auditory warning. 

The rhino-oxpecker relationship doesn’t come as a surprise to black rhino experts like Mike Knight, chairman of the International Union for Conservation of Nature’s African Rhino Specialist Group. And, in an email to the Monitor, Dr. Knight cautions against overstating the results based on a small sample size. 

Dr. Plotz and colleagues focused much of their research on 14 black rhinos that they had already fitted with tracking devices for a different study. 

In the first phase of the study, the researchers counted how many rhinos with birds on them they could find without the aid of tracking devices. According to their hypothesis, they expected to primarily find rhinos without any hitchhikers because the birds would have alerted the animals to their presence. Indeed, out of 100 rhino sightings, just 17 had oxpeckers perched on them. In their control group of tracked rhinos, 56 out of 100 rhinos had birds on them.

Then they tried sneaking up on the rhinos. The beasts without birds on their backs reacted just 23% of the time when the approaching human was an average of 27 meters (88 feet) away. The rhinos with oxpeckers atop reacted every time – and the human was much farther away, at an average of 61 meters (200 feet). 

Questions remain about why the birds call when a human approaches in the first place. They themselves are not in danger from poachers, so are they warning the rhino? Or perhaps warning each other that this behemoth on which they are riding is about to make a sudden movement?

Dr. Plotz and his colleagues have suggested that their study might offer another tactic in conserving rhinos: protect and replenish the diminishing populations of oxpeckers. Dr. Knight is skeptical of that, saying he sometimes uses the alarm calls of oxpeckers to locate rhinos himself, so perhaps poachers have learned the same thing.

Furthermore, this warning system may also not have much of an actual impact in reducing poaching, Dr. Knight says. Since 1960, the black rhino population has plummeted by an estimated 97.6% largely due to poaching, according to IUCN estimates. Even when oxpeckers were more plentiful, Dr. Knight says, the rhino population was being devastated. 

However, Dr. Knight adds, anything that might reduce poaching is good, and the oxpeckers also serve an important ecological function as they pick parasites off the bodies of many species, including rhinos. So, he says, they’re worth protecting for myriad reasons.

Regardless, Dr. Blumstein says, this study highlights the interconnectedness in the animal world. 

“Nature is grand and complex and it’s studies like this that are beginning to show us the complexities of relationships between species,” he says. “When we lose species, we lose those relationships. And we can’t necessarily predict when or how systems are going to fall apart when you start pulling apart important relationships.”