Small-brained elephantnose fish can think big like humans

The elephantnosed fish has shown an almost mammalian level of sensory discrimination(Credit: Proceedings of the Zoological Society of London )

Fish may be smarter than we thought. Not only can some recognize human faces, but others can use their senses in a way that it was believed only humans and other mammals could manage. A team of zoologists at the University of Bonn has discovered that, despite lacking a complex brain, the African elephantnose fish can swap between its electrical and visual senses in the same way a person can switch between sight and touch.

The ability to recognize objects across different is senses is regarded as an evolutionary plus because it provides the animal with greater sensory flexibility in different environments. However, it's been thought that this ability was confined to certain highly developed mammals, including monkeys, dolphins, rats, and humans, because the information processing was thought to require a cerebral cortex. That is, until the elephantnose fish came along.

Peters' elephantnose fish (Gnathonemus petersii) is common to the slower rivers of West Africa, where it hunts for insect larvae just before dawn and after sunset. It's also one of a number of fish equipped with an electrical organ that allows it to generate a weak electrical field that allows it to sense its immediate environment like a living motion detector. It also has eyes that it uses to navigate as well as to track and catch its supper.

What the Bonn team found was that the elephantnose can switch between these two senses despite having a tiny brain.

People and animals can navigate a darkened room by touch, hearing, or even smell, and then switch immediately to sight when the lights come on without having to reassess their surroundings. This was thought to require the cerebral cortex of the mammalian brain, but the elephantnose fish doesn't have a cerebral cortex and seems to manage it like a ZX-80 running Windows 10.

To study this phenomenon, the zoologists took 10 fish and taught them how to recognize objects using both their visual and electrical senses. They did this using an aquarium separated into two chambers that held a sphere and a cuboid object. Each fish was introduced to the aquarium and taught to select one or the other object with rewards of larvae.

What the researchers found is that the fish could recognize objects electrically that they had only seen visually, In addition, they favored whichever sense was most reliable at the time with the electrical sense dominating when the object was close up, while vision dominated with distant objects.

The fish were started in the experiments with the lights on, where they could use both senses, then in the dark. With the lights off, the team could see that the fish found their goal, but only at short range – indicating that it was using its electrical sense. The scientists then made the objects electrically invisible, so the fish could use only sight and repeated the experiment.

According to Bonn, statistical analyses showed identical performance as they swapped between vision and electrical impulses. Also, they did it with a consistency that indicates that all elephantnose fish share this talent.

As to why the elephantnose can do this, the Bonn team hasn't speculated, but it could be that the fish uses a specialized algorithm or processing system, the job of sense-swapping is less complex than believed, or fish are just really clever.

The research was published in PNAS.

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