(Tsunehiko Kohashi)

These Fish Speak With Electricity, But They Talk Just Like Us

8 JUNE 2021

Constant chatter isn't the best way to get your message across. Taking a breath before saying something important can make people listen more closely to what you have to say. 

 

And that's not just a trick we humans have figured out. 

Frogs and birds have also been found to take purposeful pauses in their speech, and as it turns out, mormyrid fish, or elephantfish, time their silence in similar ways. 

Underwater, these freshwater fish communicate through weak pulses of electricity, and new research has found the information they send to one another is also marked by well-timed pauses.

When placed in pairs, mormyrids (Brienomyrus brachyistius) actively generate pauses in their electric signals before sending out a burst of electric pulses. When isolated, they do this far less often, which suggests it's a feature of mormyrid communication.

Such well-timed moments of silence are thought to stop fish nearby from becoming so used to the constant stream of electric signals that they no longer really listen to what's being said.

Instead, a pause of about one second may prime a fellow mormyrid to receive future messages.

"Our finding that burst displays tend to occur immediately after pauses in mormyrids is similar to the finding that human speakers tend to place pauses prior to words with high information content," the authors write.

 

This isn't the first time mormyrids have been found to take pregnant pauses during electric communication. In fact, other electric fish, called gymnotiformes have also been found to do this during courtship.  

That said, this new study is the first to propose a cellular model that might explain these pauses.

Using intracellular recording, researchers found the brains of mormyrids are more easily stimulated after a short bout of silence. 

By artificially inserting pauses into the electric signals of one fish in a pair, the authors were able to show an increase in the brain activity of the other fish.

"Interestingly," the team notes, "the relevant timescales for pauses in human speech are roughly similar to those in the electric communication of mormyrids, occurring in the range of hundreds of milliseconds to seconds."

This suggests a similar cellular process is occurring in both fish and human brains, and the authors think they know what it is.

When receptors in the brain are stimulated over and over, synapses are known to weaken over time, lowering the activity of sensory circuits overall.

 

This is known as synaptic depression, and it's what allows the animal brain to learn which signals are most important and therefore most needing of attention.

When researchers artificially stimulated the midbrain of electric fish with a constant signal, they noticed the fish's sensory circuits produced weaker and weaker responses to the continuous 'noise'.

A silent pause, on the other hand, gave these neurons a break, "thereby maximizing the impact of sensory inputs" when communication was resumed.

"Pauses inserted in electric speech reset the sensitivity of the listener's brain, which was depressed during the continuous part of the speech," explains neurobiologist Tsunehiko Kohashi from Washington University in St Louis. 

"Pauses seem to make the following message as clear as possible for the listener." 

Incidentally, this is when electric fish send out their most important information. Researchers found a pause in communication among this species is usually followed by a high frequency burst of electric pulses.

By taking a quick break, it seems mormyrids are ensuring they have the full attention of their peers.

Human or fish, electric pulses or speech, the best animal communication depends on silence.

The study was published in Current Biology.