From ants and aphids to clownfish and anemones, the animal kingdom is full of interspecies mutualism. However, systems in which humans successfully cooperate with wild animals are rare.
One such relationship involves the greater honeyguide Indicator indicator , a small African bird known to lead humans to wild bees' nests. Humans open the nests to collect honey, and the honeyguides eat the newly exposed beeswax.
In a new study published in the December 9 issue of Science, researchers show that honeyguide birds understand and respond to the culturally distinct bird calls made by human hunters in different parts of Africa, suggesting cultural coevolution between species.
"The study highlights that a mosaic of interspecies communication traditions exists between people and birds across Africa, in a form of culturally dependent biodiversity," said study author Claire Spottiswoode, a researcher at the University of Cape Town, South Africa, and University of Cambridge.
Human Honey Hunters and Their Guides
Throughout nature, different species will occasionally team up in unlikely alliances to work together for each's mutual benefit. For example, woolly bats in the rainforests of Borneo are known to roost inside tropical pitcher plants. While the hollow-bodied plants provide a safe home for the tiny bats, the plant benefits by catching the guano that the animals produce. The mutual relationship helps ensure each species' survival.
However, human honey hunters and honeyguide birds have cultivated a unique relationship for thousands of years — and perhaps as early as our hominin ancestors.
Honeyguides are one of a relatively few species of bird that feed regularly on beeswax locked inside the nests of wild bees. Similarly, humans eagerly seek the nests for the honey they contain. However, locating and accessing these nests, which often rest high in the branches of trees, presents a challenge to humans and birds.
With their eyes in the sky, honeyguides naturally know the locations of bee colonies, and humans have the skills to climb or fell trees with nests, subdue the angry bees and open their nests, exposing beeswax for the honeyguides and honey for themselves. Thus, a mutualistic relationship has emerged between honeyguides and humans, where birds exchange their knowledge of bees' nest locations for humans' adept skills at accessing the resources inside.
"By birds and people partnering together, everyone gains something of value," said study author Brian Wood, an anthropologist at the University of California, Los Angeles. "This is an example of a very rare phenomenon in nature — cooperation between people and a wild animal."
Would-be honey hunters attract the birds using specialized and culturally unique calls to signal they are looking for a honeyguide partner and to maintain cooperation while following a guiding bird to a nest.
For example, honey hunters from the Yao cultural group in northern Mozambique use a loud trill followed by a grunt ("brrr-hm"), while honey hunters from the Hadza cultural group of northern Tanzania use a melodic whistle. These successful calls have been maintained in these groups for generations, often passed down by fathers or community elders.
Spottiswoode and Wood wanted to know whether honeyguides from different regions are more likely to respond to signals of their respective local human culture than those of another or are instead innately drawn to the whole range of signals humans use to announce their presence.
Assisted by Yao and Hadza honey hunters, the authors played audio recordings of different honey hunters' calls and observed how honeyguides responded in Mozambique and Tanzania.
They found that honeyguides in the Yao area were more than three times more likely to initiate a guiding response to the Yao's distinct call than the Hadza's whistle. Conversely, honeyguides in the Hadza area were more than three times as likely to respond to the Hadza's whistle than the Yao's brrr-hm.
"We now know this relationship involves communication using culturally varying signals and the learning of those signals by the wild honeyguides, something that has not been demonstrated before," said Wood.
According to the researchers, multi-species relationships are complex and there are lots of questions yet to be explored. Spottiswoode and Wood hope to keep the research going to gain greater insights into what influences the form of human-bird communication and learn more about the meanings — for both humans and birds — that are embedded in these collaborative communications systems.
One such question is why the Hadza people use whistles instead of other sounds like the Yao do. What factors drive the cultural variations in human honey hunter calls?
"There are very few studies of communication between people and wild animals and our study provides a general framework for scientific research in this area that includes insights and methods from anthropology, linguistics, behavioral ecology, and bioacoustics," said Wood.
"I think our study does an admirable job of showing how these interdisciplinary perspectives can be brought to bear on the study of multi-species cooperation and communication."
However, the human-honeyguide partnership is dwindling. According to Spottiswoode, recent changes including beekeeping, urbanization and the exclusion of people from protected wild areas threaten the viability of this unique partnership.
The honeyguides still call to humans, but in many places, humans can no longer understand or follow the birds.
"There is a risk that we could lose this likely ancient part of our humanity and our place in the natural world," said Spottiswoode.