Although it's not so hard for us to understand how a bat navigates by echolocation, imagining how a shark `feels' an electric field is probably beyond most of us: but not Brandon Brown! He has a model that explains how hundreds of electric sense organs buried in the fish's head generate nerve signals that the shark interprets as it homes in on its helpless prey(p. 999).
But how did Brown get interested in sharks when he usually works on the physics of superconductivity? He explains that a colleague put him onto shark's electrosensory perception by telling him about the strange gel that can be extracted from a shark's head. This gel, which packs the shark's electric sense organs, intrigued Brown. From then on, he wanted to know how sharks feel their electric world, so being a physicist, he decided to build a mathematical model to see how these strange sensory organs pick up electric fields around them.
Each electroreceptor is shaped like a deep pore, sometimes 20 cm long, with an electrosensitive bulb at the end, called an ampulla. Some species have several hundred of these pores aligned at various angles through the sharks skin and head. Brown's first challenge was to find out how a single receptor sensed another fish's electric field.
From his calculations, he realised that a single receptor behaves a bit like an antennae, but much slower, so that the sharks sense voltage fluctuations. Brown explains that they don't measure absolute voltages like volt meters because they `aren't grounded'.
Once he knew how an electric field `felt' to a single receptor, Brown modelled how the array of receptors in the marine shark's head reacted to an electric field as the shark swam through it, to see if he could explain any aspects of shark hunting behaviour. First he simulated a simple route, where a shark swam through the fish's field at 45° to its target. Then he looked at more complicated approach paths. He modelled the shark's electro-view first as it passed the fish and then as it swam along an arc, spiralling in towards the fish-dipole at the heart of the electric field. Most young sharks swim straight towards their fishy victim, but a few follow a spiral path, similar to the arc he modelled for the last approach. Brown thinks that his results might explain their strange behaviour.
His simulation suggests that the young sharks spiral to maintain the same orientation in the field as they approach their prey. Brown explains that this might be a good way to make sure that an inexperienced hunter always catches the fish. He thinks that the spiral approach could be a learning strategy for inexperienced sharks, which ensures that they don't go hungry when they're on the learning curve.