Sensitivity of the mystacial vibrissal system of harbour seals (<i>Phoca vitulina</i>) to size differences of single vortex rings

Seals can detect extremely weak water disturbances with their highly sensitive vibrissae. This allows them to detect and track hydrodynamic trails that swimming prey fish inevitably leave behind. As these trails differ depending on the characteristics of the fish generating them, they may reveal information about not only the swimming direction, but also the fish species, its body shape and size, its swimming style and speed, and the time that elapsed since the fish swam by. Most fish trails provide this kind of information by consisting of a continuous three-dimensional chain of vortex rings. However, when a fish performs an escape response, it generates a very complex flow pattern consisting of three individual jets, each traveling in a different direction, with only two forming vortex rings. This particular flow pattern might camouflage the fish's actual flight direction hydrodynamically, possibly thus offering the fish a means of deceiving the predator. Identification of the three distinct structures, by comparing and analysing their size, is a feasible way to overcome the fish's deception. The aim of this study was to investigate whether harbour seals are able to disclose such a camouflage and decipher the fish's escape direction. A harbour seal was trained to differentiate two artificially created vortex rings, perceived successively by the mystacial vibrissae, based on their size. The seal was indeed able to successfully differentiate vortex rings down to a size difference of about 18 mm. In natural situations in which a fish performs an escape response, this capability should enable harbour seals to pursue the fish's actual swimming direction.