Suppose you’re one of your animal ancestors, swimming around in one of the warm and shallow Cambrian seas 500 million years ago. You’re a small predator and the struggle of life is as fierce as it ever was. To increase your chances of survival, would you invest your energy in larger eyes and brains so that you can discover prey quicker and more effective than your competitors? Or would it be wiser to increase your muscle power, allowing you to cover a larger area in your hunt for prey?
The example above illustrates some of the trade-offs between optimizing a sensory system and movement performance. Considering the fact that neuronal tissue consumes 20 times more energy than muscle tissue, this is not a trivial dilemma! Humans and some other animals are lucky to have stumbled upon movable heads and eyes, so we don’t have to turn our entire body if we want to look somewhere. Other species are less lucky though…
One such species is a fish with a badass name: the black ghost knifefish. They spend their nights hunting for insects in the Amazon. With such a name come cool abilities: they use an electric field around their body to find their prey. As soon as an insect enters the field, the fish detect the disturbance of the field and can locate their prey. It is this South-American knifefish that is used to illustrate the the conflict between movement and information processing, reported by MacIver and colleagues in PLoS Computational Biology.
When they search for prey, the fish hold their head and body down in a -30° angle. Such a posture does not make sense from an aerodynamic (hydrodynamic?) perspective, since it increases water resistance and makes movement more costly. An experimental model of the fish experienced more than 2 times as much drag at an angle of -45° and a speed of 15 cm/s, compared to a neutral angle.
The advantage of swimming at such an angle becomes apparent when you consider the space that the knifefish can cover and hunt in. Take a look at the figure below, and imagine the knifefish is travelling forward. The area that the electrical field (the grey blob around the fish) covers, is two times as much under the -30° angle as compared to the neutral body pitch. This effect is caused by the increased absolute height of the electrical field.
The authors were able to estimate the energy required to encounter prey using a formula I won’t pretend to understand. This formula predicted an optimal pitch of -50°, which is steeper than the observed angle of -30°. The authors think this difference arises because their model doesn’t include the effect of the angle on the propulsive power of the ribbon fin on the belly of the fish (see picture on top), which drops by 25% at an angle of -50°.
The researchers finish their paper with two thought experiments. In the first scenario, they analyzed what would happen if the knifefish used its eyes instead of it’s fancy electric powers. In the second scenario they investigated whether the knifefish would be better off decoupling its electrical sensing from its body movement (much like our eyes: we can rotate our eyes in our sockets and turn our heads, without turning our bodies). You can see the results in the graph below. As you can see, swimming at an angle doesn’t make much sense if you’re using your eyes. However, decoupling the electrical field from body movement seems like a smart thing to do! Whether this is biologically and physically possible is a whole different question altogether of course.
I don’t usual read papers on animal behaviour and biomechanics, but this one was well-written and interesting to boot. I keep thinking that this principle must also hold up in the smaller worlds of bacteria and protists, I wonder if anyone did any research on that specifically. At least it would be more simple to do controlled experiments and find traces of information/movement constraints on a genomic level. Ah, cross-pollination between different spheres of science.. A subject worthy of a blogpost on itself! Be aware of standard repetition and direct application though. As the Black Ghosts (how appropriate!) sing: repetition kills you!
MacIver, M., Patankar, N., & Shirgaonkar, A. (2010). Energy-Information Trade-Offs between Movement and Sensing PLoS Computational Biology, 6 (5) DOI: 10.1371/journal.pcbi.1000769
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