Cooperation is easier with communication, and an African bird species and the humans that it works with know that.
The greater honeyguide, Indicator indicator, is known for guiding people to beehives in the African bush in the hope that the human honey-hunters will leave beeswax behind for the birds to snack on. But if someone wants to be led to the buzzing hive dripping with golden, sticky honey, they might need to know a special password, according to a study published Thursday in the journal Science.
"Honey-hunters in Mozambique use special calls to signal to honeyguides that they're eager to follow," says study lead author Claire Spottiswoode, an evolutionary biologist at the University of Cambridge and the University of Cape Town. And, in Dr. Spottiswoode's experiments, the birds were twice as likely to guide the humans if they used this particular summoning call rather than alerting the birds to their presence in another, more generic way.
Honey-hunters of the Yao people make their way through the wilds of the Niassa National Reserve in northern Mozambique, calling out to the birds as they go as if to say, "I'm here to find a beehive with you." Their voices ring out in a sort of loud trill, capped by a grunting noise: "brrrr-hm!"
Then, a honeyguide flies to a tree near the humans, chattering at them distinctly as if to say, "follow me."
The honey-hunters follow the bird as it flits from tree to tree, continuing to chatter away. The humans periodically repeat the same call to maintain contact. "Brrrr-hm," now seems to mean 'I'm still with you,' and the bird's chattering helps the humans follow along.
This reciprocal communication is unique to the honeyguide-human cooperation. Spottiswoode knows of no other wild animal that both uses and understands cross-species vocal signaling like this to facilitate mutualistic foraging with humans.
Here's what the "brrrr-hm" call sounds like:
"Honeyguides are making specialized calls to humans that humans are able to interpret correctly to their own benefit – finding a bees' nest. And reciprocally, humans are providing specialized calls to honeyguides that honeyguides are able to interpret correctly in choosing a good partner," she tells The Christian Science Monitor in a phone interview.
And this whole exchange happens "while wandering through the bush, bumping into the odd elephant, the odd buffalo, and the odd lion … and sweating a lot," Spottiswoode says.
The wild partnership can stretch as much as half a mile before the bird arrives at its buzzing destination. When it does, the bird quiets down and settles on a tree. Sometimes that tree contains a beehive and other times it sits nearby its target, waiting for its human partners to spot the hive.
Once the humans find the hive, they get to work. They expertly smoke out the bees before reaching into retrieve their sticky gold or felling the tree for easier access.
Meanwhile, the honeyguides wait, in the hopes that the humans will leave behind some beeswax for them.
Spottiswoode and her colleagues wanted to find out whether the "brrrr-hm" call was communicating specific information to the birds or whether it was simply one way of making a human presence known, and any vocalization would do. So she herself went out tromping around with local Yao honey-hunters.
"You can imagine it was great fun walking hundreds of miles through beautiful landscapes and occasionally bumping into elephants," Spottiswoode says. "It was really a wonderful privilege to work in the Niassa National Reserve."
Before she went out, Spottiswoode recorded different honey-hunters making the "brrrr-hm" call, then she had the same individuals say other words in the local language. For a nonhuman control, she also made recordings of the local common dove calls.
Then the experiments commenced. For each test, "two brilliantly sharp-eyed honey-hunters and I walked together in a line through the bush for 15 minutes," Spottiswoode says. "While they stayed alert for the calls and the sight of honeyguides, I played back one of those three types of sounds through a speaker at a consistent volume."
During these experiments, honeyguides came to guide the trio 66.7 percent of the times that the "brrrr-hm" call was played, but just 25 percent of the time for the other human calls and 33.3 percent for the dove sounds. Furthermore, the bird often lost interest in the control calls before making it to a hive, suggesting that the continued use of the "brrrr-hm" call also helped maintain the partnership.
Overall, the team was led to a bees nest 54.2 percent of the time that the Yao honey-hunting call was played on a 15 minute search, whereas they made it just 16.7 percent of the time with the control sounds.
"What that suggests is that honeyguides do attach meaning and respond appropriately to the signal that advertises the people's willingness to cooperate," Spottiswoode says. "So it really does seem to be a two-way conversation between our own species and the honeyguides."
"This study provides clear and direct evidence that honeyguides respond to specialized human signals that recruit the birds to cooperative hunting and that the birds associate those signals with potential benefits to themselves," John Thompson, an evolutionary biologist at the University of California, Santa Cruz, tells the Monitor in an email.
It's not surprising to see inter-species communication, Gisela Kaplan, an adjunct professor in animal behavior at the University of New England in Australia who was not involved in the study, tells the Monitor in an email. "Communication between species has been observed regularly and has been known for a long time – mainly in protection against predators," she says. But "symbiotic relationships in food finding and mutual tolerance are occasionally reported in animals or even humans (wolves and crows/ dolphins and gannets or even humans but this is less common.)"
This study "experimentally verifies complex probabilities that are mutually understood: 'I help you if you promise to help me'— a bartering system not so much based on trust but on mutual confirmation and reinforcement."
Humans are known to cooperate with other animals to find food. Domestic dogs are used for hunting, and some birds are trained to help with fishing. But in this case, says Spottiswoode, "it's a cooperation between humans and free-living wild animals for mutual benefit."
The honeyguides haven't been domesticated, coerced, or explicitly taught to lead humans to beehives. Instead, the relationship is a good example of pure mutualism, when two organisms have a relationship that is beneficial to both. Dolphins also engage in mutualism for foraging, collaborating with humans fishing in Brazil.
"Honeyguides love eating wax and know where the bees nests are located, but they're not much good at doing battles with the bees and getting into the bees nests to get ahold of it. Whereas humans, by contrast, love eating honey and they're experts at getting into bees nests with the help of two crucial skills: The use of fire to subdue the bees and the use of tools to chop down the tree and open up the bees nest," Spottiswoode says. "So there's an exchange of information for skills."
And that exchange is facilitated by their mutual understanding of each other's vocalizations.
It "makes good evolutionary sense" that the honeyguides would want to select a human partner who knows the lingo because they are most likely to provide them with a waxy reward for their guiding efforts, Spottiswoode says. If a bird tries to lead a person who isn't equipped to take on a beehive, they will have expended energy for naught and alerted predators to their presence by making a racket.
The fact that "the honeyguide literally understands what the human is saying … suggests that the honeyguide and human behavior have coevolved in response to each other," Stuart West, an evolutionary biologist at the University of Oxford in the United Kingdom, told Science Magazine.
Spottiswoode agrees that the ability to guide humans is likely an inherited ability, selected for over generations through natural selection. But she says it's likely that the birds learn the language of the honey-hunters at a young age, rather than being born understanding the "brrrr-hm" calls.
First of all, "honeyguides are not raised by their own parents," she says. Much like cuckoos, honeyguides lay their eggs in other birds' nests.
Furthermore, honeyguides are known to lead honey-hunters to beehives across many parts of Africa. And those honey-hunters use different calls. For example, the Hadza people in northern Tanzania use a melodious whistle to communicate with their avian guides.