Hydrodynamic detection and localization of artificial flatfish breathing currents by harbour seals (Phoca vitulina)

Abstract

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.

Active touch in sea otters: in-air and underwater texture discrimination thresholds and behavioral strategies for paws and vibrissae

“…This study highlights how a behavioral approach can address questions about tactile cues relevant for prey capture in the wild. For example, this study focused on active touch, yet sea otters may use hydrodynamic information while foraging for burrowed invertebrates, similar to harbor seals' ability to detect simulated benthic flatfish breathing currents (Niesterok et al, 2017). Although sea otter vibrissae seem morphologically adapted to active touch rather than passive touch required for hydrodynamic detection, this may not preclude sea otters from detecting water currents emitted by prey as a byproduct of respiration.…”

“…Both passive and active hearing may assist in prey detection, but at close range, taction has emerged as a primary sense among aquatic and semiaquatic taxa, especially when hunting buried invertebrates or fishes (Dehnhardt and Mauck, 2008). For example, many shorebird species probe the tidally flooded substrate with touch structures at their beak tips (Piersma et al, 1998); star-nosed moles seek prey in subterranean streams using specialized appendages around their nostrils (Catania and Kaas, 1997;Catania and Remple, 2004); and seals, sea lions and walruses detect and pursue prey using their vibrissae while diving (Dehnhardt and Mauck, 2008;Dehnhardt et al, 2001;Kastelein and van Gaalen, 1988;Kastelein et al, 1990;Niesterok et al, 2017).…”

Hydrodynamic sensory threshold in harbour seals (Phoca vitulina) for artificial flatfish breathing currents

“…This study was conducted with two male harbour seals (Phoca vitulina Linnaeus 1758), Henry and Luca, which had also participated in our former experiment on the detection of artificial flatfish breathing currents (Niesterok et al, 2017). They were 19 years (Henry) and 13 years (Luca) old.…”

“…While the opening of the nozzles was just below the mesh wire grid in our previous study (Niesterok et al, 2017), for this experiment the openings of the nozzles were lowered (Fig. 1B) by 23 cm vertically, to imitate another potential situation in the wild: a seal swimming over the ground at some distance.…”

“…In our previous study (Niesterok et al, 2017), we demonstrated a harbour seal's ability to detect artificial breathing currents using its vibrissae under the influence of hydrodynamic background noise and self-motion. Here, we used artificial flatfish breathing currents to investigate the efficiency of the vibrissal system in terms of a threshold for this specific hydrodynamic stimulus under the influence of self-motion and background noise.…”