It has long been assumed that when one brown ghost knifefish 鈥渃hirps鈥� at another knifefish by altering the electrical field it produces, it is communicating with the other fish. But it appears that these chirps may instead be a kind of probe聽that helps a fish improve its electrolocation of objects, especially other electric fish.
Weakly electric fish generate a low-power electric field around their body, which they sense with special receptors in their skin. This allows them to detect distortions in the electric field induced by nearby objects, and thus to navigate and hunt in the dark.
Brown ghost knifefish (Apteronotus leptorhynchus), which live in the Amazon river basin, can detect objects and prey up to 12 centimetres away. They can also sense other knifefish up to 40cm away due to the interference between their overlapping electrical fields.
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Individual knifefish produce an electric field that usually oscillates at a constant frequency of between 500 and 750 Hertz in females and 750 to 950Hz in males. But sometimes knifefish 鈥渃hirp鈥� by altering their usual frequency.
Because these chirps are produced mainly when other brown ghost knifefish are nearby, it has been thought for decades that they are a form of communication, used to scare away rivals or to attract mates. There are dozens of papers describing such behaviours.
But at the Humboldt University of Berlin claims that none of these studies provide definitive evidence. After analysing hundreds of recordings of tens of thousands of chirps, he and his team have come to the conclusion that the chirps are a way of improving electrosensing rather than a form of communication.
For starters, they found that simply making the environment more complex resulted in more chirping. 鈥淲e could reliably increase chirping by adding materials, shelters or [fake plants],鈥� says Oboti.
What鈥檚 more, when the team played recordings of chirps to knifefish, particular types of chirps didn鈥檛 result in any consistent change in behaviour, such as attracting a fish or making it retreat.
Instead, when two fish interact, Oboti and his colleagues found that the types of chirps produced depend on the difference in the frequencies between the individuals and also on how close they are. 鈥淎ltogether, these results make a strong case for a probing function,鈥� he says.
For instance, the fact that females chirp differently depending on whether a male or female is nearby is usually interpreted as showing that chirping is being used for sexual communication. But if Oboti is right, this is simply a result of the frequency differences between the sexes. He describes his claims as 鈥渉eresies鈥�.
鈥淸The] idea will be quite controversial, but I think it deserves careful attention,鈥� says at Trinity College in Connecticut.
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