Hydrodynamic trail following

Harbour seals are able to detect and follow hydrodynamic trails that are generated by objects moving through the water with their vibrissae (Dehnhardt, Mauck & Bleckmann 1998; Dehnhardt et al. 2001).

In the video seal Sam is following a hydrodynamic trail of a remote controlled submarine. During trail generation the seal stations with its vibrissae out of the water what prevents it from sensing hydrodynamic information already during trail generation. Additionally it is wearing headphones playing pink noise for acoustical masking and is blindfolded by a stocking mask to prevent visual cues to be perceived. When the submarine has come to rest and thus is not providing any acoustical cues anymore the seal is allowed to search and follow the trail still carrying the blindfold. This astonishing ability could be used to detect and follow the hydrodynamic trails of fishes during foraging.

Hydrodynamics with single vortices

Seals were shown to be able to extract information from complex hydrodynamic trails. Within these trails vortices can be found that seem to be important for a seal's decision on trail direction as well as on the size and shape of the trail generator. We then asked ourselves if harbour seals are also able to detect single vortices. In this experiment single vortices are generated with the help of a vortex generator. We can vary single parameters of the vortices in our attempt to understand the information a seal can read from it by means of its vibrissae.

The video shows seal Filou being blindfolded with a stocking mask in order to prevent a visual solution of the task. Then it dives down to its underwater station. A vortex ring is generated (for demonstration purposes the water within the vortex generator is dyed green). The vortex travels towards the seal that detects it with its vibrissae and tells the experimenter from which side the vortex has come from.

Hydradynamic predator-prey-interaction

Flounders are important prey fishes for harbour seals. Although cryptic in appearance and although they bury themselves in the sediment when a predator is approaching harbour seals are able to detect the flounders. We hypothesized that the breathing currents of the flounders form the basis for this detection. In this experiment it shall be investigated if harbour seals are indeed able to locate breathing currents, first artificially generated, then real breathing currents, on a 4 x 4 m area with the help of their vibrissae.

Determination of moving direction from a hydrodynamic trail

Besides being able to detect and follow hydrodynamic trails harbour seals are able to read the moving direction of e. g. a fish out of its trail. These results were obtained in behavioural experiments in which the seal had to indicate the moving direction of a fin-like paddle that had been pulled through the water (Wieskotten et al. 2011).

The camera is filming the top view of a water basin. Small neutrally buoyant and light reflecting particles were added to the water of the basin which are illuminated in one plane by a laser. The small fin-like paddle is pulled through the water from right to left, and the vortices left behind in the hydrodynamic trail of the paddle can be easily seen in the video. Shortly afterwards the seal wearing a stocking eye mask is allowed to enter the basin and analyses the trail with its vibrissae and extracts directional information within milliseconds. These pictures can be analysed in detail at the computer.