Clever knifefish hunts by biomechanics

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"The hunting strategy of a slender fish from the Amazon is offering insight into how to balance the metabolic cost of information with the metabolic cost of moving around to get it.

Malcolm MacIver, assistant professor of mechanical engineering and of biomedical engineering at Northwestern University, led a team that analyzed the hunting behavior of the weakly electric black ghost knifefish.

The fish, which has become the fruit fly of studies on how animals process sensory information, hunts at night using a self-generated electric field to sense its surroundings, like a bat uses sonar, MacIver says.

The fish hunts while its body is tilted downward, which, much like standing up on the pedals of a bicycle while going downhill, causes more than twice as much resistance to movement than if the fish were swimming with no tilt. The posture allows

the fish to scan a wider area of fresh water and encounter more prey."

“To better understand the way animals are the way they are, we need to not look only at neurological function or only at sensory function—we have to look at mechanics. We need to think of the intelligence of the body as a central component to our overall intelligence and think of energy saving as cleverness.”

"The study also suggest that hunting at a drag-inducing position could be the basis for the fish’s unusual, elongated body.

MacIver says the findings give insight into certain patterns in animal evolution, such as why we and most other animals have moveable sensory systems like eyes, fingers and arms"

Although there is much more drag on the body when the electric black ghost knifefish hunts while tilted (blue), its electric sense scans so much more water for prey that it is a net energetic benefit. When it isn't hunting, it swims straight to minimize drag (red). (Credit: Northwestern U.)

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