Clever knifefish hunts by biomechanics

NORTHWESTERN (US)—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.

The researchers found that the increased cost of movement caused by body tilting was more than counterbalanced by increased sensory performance. Past a certain angle of tilt beyond what was naturally observed, the additional cost of moving with the body tilted was greater than the energy gained by sensing more prey.

“That the fish tilts to be able to scan a larger area for prey despite the energy expense is a very interesting result,” MacIver says.

“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 is published by the journal PLoS Computational Biology.

Neelesh Patankar, associate professor of mechanical engineering, worked with MacIver to develop a hydrodynamic simulation code to calculate the drag forces of the fish when it’s hunting and when it’s cruising.

“Once we do simulations we can analyze the hydrodynamics of the fish and come up with an understanding as to why it has to spend energy in this scenario and what is the optimal situation where it can spend minimum energy, for example,” Patankar, a co-author of the study, says.

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.

“If the fish was able to swivel its region of prey sensitivity, like a vision-based animal can shift its gaze, it would save even more energy,” he explains. “This conclusion helps us understand why animals like us can move our eyes.”

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