Detection and direction discrimination of single vortex rings by harbour seals (Phoca vitulina)

Abstract

Harbour seals possess highly sensitive vibrissae that enable them to track hydrodynamic trails left behind by a swimming fish. Most of these trails contain vortex rings as a main hydrodynamic component. They may reveal information about their generator as the trails differ depending on the fish species, the fish's body shape, size and swimming style. In addition, fish generate single vortex rings in diverse natural situations. In this study, the ability of blindfolded stationary harbour seals to detect and analyse single vortex rings regarding directional information has been investigated. In three different behavioural experiments, the animals were trained to respond to single artificially generated vortex rings. The results show that harbour seals are able to respond to a variety of different vortex rings upon vibrissal stimulation. The investigation of the minimum hydrodynamically perceivable angle revealed that it is at least as small as 5.7 deg, which was the smallest adjustable angle. Moreover, harbour seals are capable of analysing the travel direction of a vortex ring perceived by the mystacial vibrissae irrespective of whether the vibrissae were stimulated ipsilaterally or contralaterally. In situations in which no complex hydrodynamic trail is available, it is advantageous for a hunting seal to be able to extract information from a single vortex ring.

Ecomorphology reveals Euler spiral of mammalian whiskers

“…Summarell et al (2015) found that smooth whiskers were stiffer than undulating whiskers. Since phocids, with undulating whiskers, tend to be better at hydrodynamic tasks (Gläser et al, 2011; Hanke et al, 2013; Krüger et al, 2018; Niesterok et al, 2017), the authors suggest that having some flexibility of the whiskers might be useful for hydrodynamic sensing in phocids, while stiffer whiskers might be better for touch sensing in otariids. While we suggest here that aquatic mammal whiskers are stiffer than those of terrestrial mammals, for otariids and phocids, a more complex three‐dimensional approach may be needed in order to fully compare whisker stiffness between these species, especially to better understand the functional significance of whisker stiffness.…”

Pinnipeds orient and control their whiskers: a study on Pacific walrus, California sea lion and Harbor seal

“…Pinniped whiskers, in particular, have been studied, due to their prominence, high sensitivity (Hyvärinen and Katajisto 1984;Hyvärinen 1989;Dehnhardt et al 1998;Mauck et al 2000;Marshall et al 2006;Hyvärinen et al 2009;Erdsack et al 2014;McGovern et al 2014) and their ability to be moved using a network of voluntary muscles (Berta et al 2005). Indeed, Pinniped whiskers are capable of the tactile discrimination of object textures, shapes and sizes to a similar sensitivity as human fingertips (Dykes 1975;Murphy et al 2015) and can also detect fine-scale water movements, termed hydrodynamic sensing (Dehnhardt et al 2001;Wieskotten et al 2010a, b;Krüger et al 2018).…”

Hydrodynamic reception in the Australian water rat, Hydromys chrysogaster

“…Harbour seals (Phoca vitulina) detect and analyse subsurface water motions not only of the sinusoidally oscillating dipole type (Dehnhardt et al 1998a), which bear some resemblance with the water movements generated by oscillating body parts, but also direct current water jets (Wieskotten et al 2011;Niesterok et al 2017a, b;Krüger et al 2018). Direct current water jets occur, for example, in the tail wake of fishes (Hanke and Bleckmann 2004;Niesterok and Hanke 2013), where they often form the central part of vortex rings, and in the breathing currents of fishes (Niesterok et al 2017b).…”

“…Direct current water jets occur, for example, in the tail wake of fishes (Hanke and Bleckmann 2004;Niesterok and Hanke 2013), where they often form the central part of vortex rings, and in the breathing currents of fishes (Niesterok et al 2017b). As vortex rings produced by fish tails constitute strong and stable flow structures, they appear to be promising subjects for future studies on the discrimination and directional sensitivity of Hydromys to hydrodynamic stimuli, corresponding to studies on stationary harbour seals (Krüger et al 2018).…”