Hydrodynamic discrimination of wakes caused by objects of different size or shape in a harbour seal (Phoca vitulina)

Abstract

SUMMARYHarbour seals can use their mystacial vibrissae to detect and track hydrodynamic wakes. We investigated the ability of a harbour seal to discriminate objects of different size or shape by their hydrodynamic signature and used particle image velocimetry to identify the hydrodynamic parameters that a seal may be using to do so. Hydrodynamic trails were generated by different sized or shaped paddles that were moved in the calm water of an experimental box to produce a characteristic signal. In a twoalternative forced-choice procedure the blindfolded subject was able to discriminate size differences of down to 3.6cm (Weber fraction 0.6) when paddles were moved at the same speed. Furthermore the subject distinguished hydrodynamic signals generated by flat, cylindrical, triangular or undulated paddles of the same width. Particle image velocimetry measurements demonstrated that the seal could have used the highest velocities and the steepness of the gradients within the wake to discriminate object size, beside the size of counter-rotating vortices and the spatial extension of a wake. For shape discrimination the subject could have used the spatial extension of the whole wake, in addition to the arrangement of the vortices. We tested whether the seal used highest velocities, the steepness of the gradients and the spatial extension of the wake in a second set of experiments by varying moving speed and paddle size, respectively. The subject was still able to discriminate between the respective object sizes, but the minimum detectable size difference increased to 4.4cm (Weber fraction 3.6). For the shape discrimination task, the seal was only able to distinguish flat from triangular paddles. Our results indicate that the seal's discrimination abilities depend on more than one hydrodynamic parameter.

Vibrissal sensitivity in a harbor seal (Phoca vitulina)

“…Pinnipeds use their vibrissae for the tactile discrimination of surfaces (Dehnhardt, 1994;Dehnhardt and Kaminski, 1995;Grant et al, 2013) and the detection and following of underwater wakes (Dehnhardt et al, 2001;Gläser et al, 2011). Although behavioral and histological evidence suggests that the vibrissal system in pinnipeds is adapted to extract complex tactile information from the environment (Dehnhardt et al, 2014(Dehnhardt et al, , 1998(Dehnhardt et al, , 2001Dehnhardt and Kaminski, 1995;Hanke et al, 2012;Wieskotten et al, 2010Wieskotten et al, , 2011, the sensitivity of this sensory modality is not fully understood. A few studies have utilized different methods to directly measure the tactile sensitivity of seals to a range of stimulus frequencies.…”

“…In addition, the sensitivity thresholds overall were elevated relative to what would be expected from an animal that is highly reliant on this mechanoreceptive system for prey detection. Considering the behavioral abilities of these animals to track wakes and determine hydrodynamic features (Dehnhardt et al, 2001;Wieskotten et al, 2010Wieskotten et al, , 2011, the seal's vibrissal system would need to be highly sensitive. Although there are limited psychophysical data to allow for comparison with other whiskered mammals, it has been demonstrated that the rat (a known vibrissal specialist) can detect stimulus movements as slight as 11 µm at 80 Hz with their vibrissae (Adibi and Arabzadeh, 2011).…”

Passive wake detection using seal whisker-inspired sensing

“…A submarine was com- Figure 1-21: Different shapes that a seal was trained to distinguish based solely on the wake left while they were held fixed and towed. Figure from [140].…”

“…Object Size and Shape [140] showed the seals' ability to distinguish certain sizes and shapes of objects based on their hydrodynamic signature. Chemically inert paddles of different shapes ( Figure 1-21) and sizes were towed in front of a blindfolded seal.…”

Sensory biology of aquatic mammals

“…Harbour seals (Phoca vitulina) are able to detect and follow such hydrodynamic trails (Dehnhardt et al 2001). Subsequent studies showed that they can even further analyse the structure of water disturbances (Wieskotten et al 2010(Wieskotten et al , 2011. The detection and analysis of water movements by pinnipeds is described and reviewed by W. Hanke et al in this issue.…”