After sunset in the Amazon rainforest, a war of stealth begins in the rivers and streams.
Electric eels ‘see’ the world through electronic pulses. They send out electricity and, through specialist sensory organs, detect distortions in their electric field which allow them to ‘see’ what is in their environment – including their prey, knifefish.
But knifefish share this sensory system: in order to find their own food, they also send out electronic pulses.
Because both predator and prey have specialist electricity-sensing organs, they can detect one another’s electronic pulses. This led scientists to wonder if electric eels and knifefish might ever ‘silence’ their own electric pulses in order to conceal themselves from one another.
“The idea emerged from realising how often electric eels and knifefish encountered each other in this small rainforest stream,” Lok Poon, lead author of the study, tells BBC Wildlife. “It seems intuitive: if sensing reveals your presence, perhaps it is better not to sense at all.”
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The researchers put electrodes in an Amazonian stream, which recorded the electronic soundscape of the habitat. Because each species has a distinct electronic signature, the team could tell which ‘electric species’ were present.
The team found that, in over half of encounters where eels could potentially detect knifefish electricity, eels stopped emitting their own electronic pulses. This silence often came before the eel produced a high-voltage discharge – the killer strike. Similarly, sand knifefish 'switched off’ their electricity when electric eels were in the vicinity.
This shows that both electric eels and their knifefish prey adopt a strategy of ‘electric silence’: electric eels hunt ‘blind’ so they can evade detection by knifefish and stealthily sneak up on them, while some species of knifefish, when they detect eels, 'switch off’ their sensory system so they are less visible to the eels.
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But there is a cost to switching off their electronic pulses: both the electric eel and the knifefish are less able to navigate and hunt successfully. They therefore switch between being silent and emitting electronic pulses.
In a case of convergent evolution, this is exactly what orcas do when they hunt other toothed whales. Orcas use echolocation to detect other whales, but these whales also use echolocation and can therefore ‘eavesdrop’ on the orcas. The orcas and their prey therefore adopt similar tactics, where both switch between echolocating and stealthy silence.
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Top image credit: Lok Poon
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