Spatial arrangement of the whiskers of harbor seals ( Phoca vitulina ) compared to whisker arrangements of mice ( Mus musculus ) and rats ( Rattus norvegicus ).

Most mammals have specialized facial hairs known as vibrissae (whiskers), sensitive tactile structures that subserve both touch and flow sensing. Different animals have different numbers and geometric arrangements of whiskers, and it seems nearly self-evident that these differences would correlate with functional and behavioral use. To date, however, cross-species comparisons of three-dimensional (3D) whisker array geometry have been limited because standard morphometric techniques cannot be applied. Our laboratory recently developed a novel approach to enable quantitative, cross-species vibrissal array comparisons. Here we quantify the 3D morphology of the vibrissal array of the harbor seal ( Phoca vitulina ), construct a CAD model of the array, and compare array morphologies of harbor seals, mice ( Mus musculus ) and rats ( Rattus norvegicus ). In all three species whisker arclength decreases from caudal to rostral, whisker curvature increases from caudal to rostral, and whiskers emerge from the face in smooth orientation gradients. Two aspects of whisker orientation are strikingly consistent across species: the elevation angle is constant within a row, and the twist of the whisker about its own axis varies smoothly in a diagonal gradient across the array. We suggest that invariant whisker elevation within a row may aid localization behaviors, while variable twist-orientation may help the animal sense stimulus direction. We anticipate this work will serve as a starting point for quantitative comparisons of vibrissal arrays across species, help clarify the mechanical basis by which seal vibrissae enable efficient wake detection and following, and enable the creation of whole-body biomechanical models for neuroscience and robotics.

Underwater sound localization abilities of harbor seals (Phoca vitulina) for high-frequency noise band stimuli in the median plane.

Pinnipeds use a variety of acoustic information underwater for social interactions, hunting, and predator avoidance. Thus, the ability to accurately localize a sound source in the environment can have a clear survival value. Nonetheless, the sound localization mechanisms for seals underwater still have to be clarified, especially for sounds received in the median plane. In this study, the sound localization abilities of five harbor seals for high-frequency noise band stimuli were measured underwater in the median plane. The seals' minimum audible angles (MAAs) were determined for two different high-frequency noise band stimuli using a two-alternative forced-choice procedure. Noise 1 had a frequency range between 8 and 16 kHz. Noise 2 contained frequencies between 14 and 16 kHz. Psychoacoustic results for the tested harbor seals show that the seals were able to localize these stimuli in the median plane underwater with MAAs between 5.1° and 17.5°. The results suggest that spectral cues improve the seals' ability to localize high-frequency sound signals in the median plane.

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

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.

Underwater sound localization of pure tones in the median plane by harbor seals (Phoca vitulina).

In an underwater environment the physical characteristics of sound propagation differ considerably from those in air. For this reason, sound localization underwater is associated with difficulties, especially in the median plane. It was the approach of the present study to investigate whether harbor seals (Phoca vitulina) are able to determine the direction of a tonal signal form above or below in the underwater environment. Minimum audible angles (MAAs) or the angular range in which the animals could localize a pure tone stimulus in the vertical plane were obtained for frequencies from 0.35 up to 16 kHz. Testing was conducted with four male harbor seals in a semi-circle area of 6 m in diameter in about 2.5 m depth, by using a two alternative forced choice method. The results show that harbor seals are able to localize a pure tone in the median plane under water with a high performance for low frequency stimuli between 350 Hz and 2 kHz with MAAs ranging from below 2.5° up to about 25°. For higher frequencies the animals show strong individual differences.

Feeding kinematics, suction, and hydraulic jetting performance of harbor seals (Phoca vitulina).

The feeding kinematics, suction and hydraulic jetting capabilities of captive harbor seals (Phoca vitulina) were characterized during controlled feeding trials. Feeding trials were conducted using a feeding apparatus that allowed a choice between biting and suction, but also presented food that could be ingested only by suction. Subambient pressure exerted during suction feeding behaviors was directly measured using pressure transducers. The mean feeding cycle duration for suction-feeding events was significantly shorter (0.15±0.09 s; P<0.01) than biting feeding events (0.18±0.08 s). Subjects feeding in-water used both a suction and a biting feeding mode. Suction was the favored feeding mode (84% of all feeding events) compared to biting, but biting comprised 16% of feeding events. In addition, seals occasionally alternated suction with hydraulic jetting, or used hydraulic jetting independently, to remove fish from the apparatus. Suction and biting feeding modes were kinematically distinct regardless of feeding location (in-water vs. on-land). Suction was characterized by a significantly smaller gape (1.3±0.23 cm; P<0.001) and gape angle (12.9±2.02°), pursing of the rostral lips to form a circular aperture, and pursing of the lateral lips to occlude lateral gape. Biting was characterized by a large gape (3.63±0.21 cm) and gape angle (28.8±1.80°; P<0.001) and lip curling to expose teeth. The maximum subambient pressure recorded was 48.8 kPa. In addition, harbor seals were able to jet water at food items using suprambient pressure, also known as hydraulic jetting. The maximum hydraulic jetting force recorded was 53.9 kPa. Suction and hydraulic jetting where employed 90.5% and 9.5%, respectively, during underwater feeding events. Harbor seals displayed a wide repertoire of behaviorally flexible feeding strategies to ingest fish from the feeding apparatus. Such flexibility of feeding strategies and biomechanics likely forms the basis of their opportunistic, generalized feeding ecology and concomitant breadth of diet.

Vibrissal touch sensing in the harbor seal (Phoca vitulina): how do seals judge size?

"Whisker specialists" such as rats, shrews, and seals actively employ their whiskers to explore their environments and extract object properties such as size, shape, and texture. It has been suggested that whiskers could be used to discriminate between different sized objects in one of two ways: (i) to use whisker positions, such as angular position, spread or amplitude to approximate size; or (ii) to calculate the number of whiskers that contact an object. This study describes in detail how two adult harbor seals use their whiskers to differentiate between three sizes of disk. The seals judged size very fast, taking <400 ms. In addition, they oriented their smaller, most rostral, ventral whiskers to the disks, so that more whiskers contacted the surface, complying to a maximal contact sensing strategy. Data from this study supports the suggestion that it is the number of whisker contacts that predict disk size, rather than how the whiskers are positioned (angular position), the degree to which they are moved (amplitude) or how spread out they are (angular spread).

Hydrodynamic perception in true seals (Phocidae) and eared seals (Otariidae).

Pinnipeds, that is true seals (Phocidae), eared seals (Otariidae), and walruses (Odobenidae), possess highly developed vibrissal systems for mechanoreception. They can use their vibrissae to detect and discriminate objects by direct touch. At least in Phocidae and Otariidae, the vibrissae can also be used to detect and analyse water movements. Here, we review what is known about this ability, known as hydrodynamic perception, in pinnipeds. Hydrodynamic perception in pinnipeds developed convergently to the hydrodynamic perception with the lateral line system in fish and the sensory hairs in crustaceans. So far two species of pinnipeds, the harbour seal (Phoca vitulina) representing the Phocidae and the California sea lion (Zalophus californianus) representing the Otariidae, have been studied for their ability to detect local water movements (dipole stimuli) and to follow hydrodynamic trails, that is the water movements left behind by objects that have passed by at an earlier point in time. Both species are highly sensitive to dipole stimuli and can follow hydrodynamic trails accurately. In the individuals tested, California sea lions were clearly more sensitive to dipole stimuli than harbour seals, and harbour seals showed a superior trail following ability as compared to California sea lions. Harbour seals have also been shown to derive additional information from hydrodynamic trails, such as motion direction, size and shape of the object that caused the trail (California sea lions have not yet been tested). The peculiar undulated shape of the harbour seals' vibrissae appears to play a crucial role in trail following, as it suppresses self-generated noise while the animal is swimming.

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

Harbour 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 two-alternative 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.

Hydrodynamic trail following in a California sea lion (Zalophus californianus).

The mystacial vibrissae of pinnipeds constitute a sensory system for active touch and detection of hydrodynamic events. Harbour seals (Phoca vitulina) and California sea lions (Zalophus californianus) can both detect hydrodynamic stimuli caused by a small sphere vibrating in the water (hydrodynamic dipole stimuli). Hydrodynamic trail following has only been shown in harbour seals. Hydrodynamical and biomechanical studies of single vibrissae of the two species showed that the specialized undulated structure of harbour seal vibrissae, as opposed to the smooth structure of sea lion vibrissae, suppresses self-generated noise in the actively moving animal. Here we tested whether also sea lions were able to perform hydrodynamic trail following in spite of their non-specialized hair structure. Hydrodynamic trails were generated by a remote-controlled miniature submarine. Linear trails could be followed with high accuracy, comparable to the performance of harbour seals, but in contrast, increasing delay resulted in a reduced performance as compared to harbour seals. The results of this study are consistent with the hypothesis that structural differences in the vibrissal hair types of otariid compared to phocid pinnipeds lead to different sensitivity of the vibrissae during forward swimming, but still reveal a good performance even in the species with non-specialized hair type.