The rivers of the tropical Americas hum and crackle with electric fields generated by knifefish. The fish use electric discharges to search their murky surroundings for food and to communicate with mates. But new research suggests these electric signals may also be used to develop and maintain a sophisticated social order.
Brown ghost knifefish (Apteronotus leptorhynchus) are related to electric eels, but rather than emitting powerful jolts, the fish continuously produce weak electric fields. Still, when Till Raab, a neuroethologist at the University of Tübingen in Germany, and his colleagues went to the fish’s natural habitat in Colombia in 2016 to study their social behaviour, they came away with “more questions than answers”, says Raab.
In just a 9-square-metre area, Raab and his colleagues recorded 30 fish communicating with each other electrically. So many fish in one spot inevitably means competition for food, shelter and mates – and the knifefish exist in these high densities across South America.
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The researchers knew “there must be some kind of hierarchy to avoid constant, repetitive fighting”, says Raab, so they brought some of the fish into the lab. They paired 21 knifefish in 37 different combinations in tanks each containing a shelter made of PVC pipe. The fish competed for access to the shelters, and the researchers recorded their behaviours using infrared cameras and their electric discharges using electrodes in the tank.
The team found that fish denied access to the shelter would commonly target their competitor with electric “rises”: gradual increases in discharge frequency followed by a rapid fall to normal. Initially, Raab thought these might be submissive gestures, but what he found “was way more complex”.
They seemed to be provocations, triggering the dominant fish to chase and bite the subordinate agitator.
While this didn’t allow the losing fish to supplant its competitor’s dominant position, it appeared to provide a slight boost in social status – one that seemed to improve the odds of success in future conflict. For instance, Raab recounts one trial where a subordinate male repeatedly made rises against a dominant female, and she eventually granted access to her tube shelter.
In that way, the rises may allow the fish to improve their standing while recognising and respecting the social hierarchy, which suggests the fish are capable of surprisingly complex social manoeuvring. Such a system might keep violence to a minimum, allowing competitors to come to a mutual understanding.
“They don’t have to fight too much, but everyone gets a little bit of what they want or what they need,” says Raab.
“The fact that rises occur before attacks and not in response to them is intriguing to me,” says Rossana Perrone, a neuroethologist at the Clemente Estable Biological Research Institute in Uruguay, adding that other electric fish make submissive signals following conflicts.
Perrone cautions that since each knifefish was used multiple times in the experiments, winner and loser effects – where a win or loss makes a repeat outcome more likely in the next contest – might influence some results.
Going forward, Raab wants to see how these electrically charged encounters tweak relationships across an entire group of knifefish. It is possible that the knifefish can estimate their chances of winning a competition by watching other fish.
Raab says he and his colleagues are only just at the beginning of understanding the social convolutions of knifefish and their relatives. Brown ghost knifefish also have an entirely different set of electric “chirps”, with which they communicate.
For Raab, the findings are a reminder that the social lives of many animals are unexpectedly sophisticated, “in spite of their size or how simple they look”.
Reference: bioRxiv,
