While it has long been known that in many parts of Africa, people and a species of wax-eating bird called the greater honeyguide work together to find wild bees' nests, new research has unraveled a novel dimension to this unique human-animal relationship.
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Experiments carried out in the Mozambican bush have shown that not only do honeyguides use calls to solicit human partners, but humans use specialized calls to recruit birds' assistance.
"What's remarkable about the honeyguide-human relationship is that it involves free-living wild animals whose interactions with humans have probably evolved through natural selection, probably over the course of hundreds of thousands of years," said one of the researchers Claire Spottiswoode from University of Cambridge.
Human cooperation is crucial to honeyguides because bees' nests are often hidden in inaccessible crevices high up in trees -- and honeybees sting ferociously.
Therefore the honeyguide waits while an expert human undertakes the dangerous tasks of subduing the bees (by smoking them out using a flaming bundle of twigs and leaves hoisted high into the tree) and extracting the honey from within, usually by felling the entire tree.
There is no competition for the prize -- the honey-hunters harvest the honey and honeyguides devour the wax combs left behind.
The new experiments revealed that by using specialised calls to communicate and cooperate with each other, people and wild birds can significantly increase their chances of locating vital sources of calorie-laden food.
In a paper published in the journal Science, the researchers revealed that honeyguides are able to respond adaptively to specialized signals given by people seeking their collaboration, resulting in two-way communication between humans and wild birds.
This reciprocal relationship plays out in the wild and occurs without any conventional kind of 'training' or coercion.
With the help of honey-hunters from the local Yao community, Spottiswoode carried out controlled experiments in Mozambique's Niassa National Reserve to test whether the birds were able to distinguish the call from other human sounds, and so to respond to it appropriately.
The 'honey-hunting call' made by honey-hunters, and passed from generation to generation, is a loud trill followed by a short grunt: 'brrr-hm'.
To discover whether honeyguides associate 'brrr-hm' with a specific meaning , Spottiswoode made recordings of this call and two kinds of 'control' sounds : arbitrary words called out by the honey-hunters and the calls of another bird species.
When these sounds were played back in the wild during experimental honey-hunting trips, birds were much more likely respond to the 'brrr-hm' call made to attract them than they were to either of the other sounds.
"The traditional 'brrr-hm' call increased the probability of being guided by a honeyguide from 33% to 66%, and the overall probability of being shown a bees' nest from 16% to 54% compared to the control sounds.
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In other words, the 'brrr-hm' call more than tripled the chances of a successful interaction, yielding honey for the humans and wax for the bird, Spottiswoode said.