In parts of Africa, people communicate with a wild bird -- the greater honeyguide -- in order to locate bee colonies and harvest their stores of honey and beeswax.
It's a rare example of cooperation between humans and wild animals, and a potential instance of cultural coevolution.
UCLA anthropologist Brian Wood and University of Cape Town ornithologist Claire Spottiswoode were lead authors on a study showing how this valuable partnership is maintained and varies across cultures. Their article, "Culturally determined interspecies communication between humans and honeyguides," was published in Science.
"Our study demonstrates the bird's ability to learn distinct vocal signals that are traditionally used by different honey-hunting communities, expanding possibilities for mutually beneficial cooperation with people," Wood said.
"Honeyguides seem to know the landscape intimately, gathering knowledge about the location of bee nests, which they then share with people, Spottiswoode said. "People are eager for the bird's help."
The honeyguides also benefit from locating the colonies: They eat the leftover honeycomb.
The study's findings build on research published in 2014 that showed the immense benefits of this relationship for the Hadza people. Honeyguides increased Hadza hunter-gatherers' rate of finding bee nests by 560% and led them to significantly higher-yielding nests than those found without honeyguides. This prior research also found that 8%-10% of the Hadza's yearly diet was acquired with the help of honeyguides.
Spottiswoode and Wood's study was done in collaboration with the Hadza in Tanzania, with whom Wood has been conducting research since 2004, and the Yao community of northern Mozambique.
Their prior work in both communities documented differences in how each culture attracts honeyguides. Among the Hadza, a honey-hunter announces a desire to partner with the bird by whistling. (Listen to the Hadza vocal signal.)
In Mozambique, Yao honey-hunters do so with a trilled "Brr! ..." followed by a guttural " ... hmm!" (Listen to the Yao vocal signal.)
Using mathematical models and audio playback experiments, the team studied these signals, their utility to people and their impacts on birds.
They experimentally exposed honeyguides in Tanzania and Mozambique to the same set of prerecorded sounds. This enabled the researchers to test whether honeyguides had learned to recognize and prefer the specialized signals that local honey-hunters used -- or were innately attracted to all such signals.
The honeyguides in Tanzania were over three times more likely to cooperate when hearing the calls of local Hadza people than the calls of 'foreign' Yao. The honeyguides in Mozambique were almost twice as likely to cooperate when hearing the local Yao call, compared to the 'foreign' Hadza whistles.
The study proposes that differences in honeyguide-attracting signals are not arbitrary, but make practical sense. While honey-hunting, both the Hadza and Yao encounter mammals, but only the Hadza hunt them, using bows and arrows. The Hadza's hunting might explain the less conspicuous whistles they use. Filmed interviews show Hadza hunters explaining that they can evade being detected by their prey because their whistles "sound like birds."
"Not just among the Hadza, but in hunting cultures around the world, people use whistles as a form of encrypted communication -- to share information while avoiding detection by prey," Wood said.
Conversely, the guttural trill-grunt signal the Yao use to communicate with the honeyguide can help scare off animals they find dangerous.
Although both humans and birds can learn new signals, the authors propose that the mutually beneficial relationship between birds and people spawns local traditions of human-bird communication that remain stable over time.
"The benefits of the honey-hunter-honeyguide relationship should produce long-lasting, 'sticky' traditions," Wood said.
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