Harbour seals are known to be opportunistic feeders, whose diet consists mainly of pelagic and benthic fish, such as flatfish. As flatfish are often cryptic and do not produce noise, we hypothesized that harbour seals are able to detect and localize flatfish using their hydrodynamic sensory system (vibrissae), as fish emit water currents through their gill openings (breathing currents). To test this hypothesis, we created an experimental platform where an artificial breathing current was emitted through one of eight different openings. Three seals were trained to search for the active opening and station there for 5 s. Half of the trials were conducted with the seal blindfolded with an eye mask. In blindfolded and non-blindfolded trials, all seals performed significantly better than chance. The seals crossed the artificial breathing current (being emitted into the water column at an angle of 45 deg to the ground) from different directions. There was no difference in performance when the seals approached from in front, from behind or from the side. All seals responded to the artificial breathing currents by directly moving their snout towards the opening from which the hydrodynamic stimulus was emitted. Thus, they were also able to extract directional information from the hydrodynamic stimulus. Hydrodynamic background noise and the swimming speed of the seals were also considered in this study as these are aggravating factors that seals in the wild have to face during foraging. By creating near-natural conditions, we show that harbour seals have the ability to detect a so-far overlooked type of stimulus.
The authors declare no competing or financial interests.
W.H. and B.N. conceived the project, built the setup and quantified stimuli. B.N. performed the experiments with the aid of Y.K. and S.W., who trained and guided the experimental animals. B.N. and W.H. analysed the data and wrote the paper. G.D. provided constructive scientific input.
B.N. was supported by a University of Rostock Research Fellowship (Landesgraduiertenförderung Mecklenburg-Vorpommern). The study was supported by grants of the Volkswagen Foundation (VolkswagenStiftung) to G.D. and the German Research Foundation (Deutsche Forschungsgemeinschaft) to W.H. (DFG HA 4411/8-2).
Supplementary information available online at http://jeb.biologists.org/lookup/doi/10.1242/jeb.148676.supplemental
- Received August 23, 2016.
- Accepted October 18, 2016.
- © 2017. Published by The Company of Biologists Ltd