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

On route with harbor seals - How their senses contribute to orientation, navigation and foraging

Seals and sea lions are well-oriented in their habitat, the coastal regions and oceans, and are, moreover, successful hunters. During their movements between haul-out places and foraging grounds as well as during foraging, the sensory systems of seals and sea lions provide useful information, although the animals, and thus their sensory systems, face considerable challenges in their habitat and due to their amphibious lifestyle. In this review, in the first chapter, we compiled and later (chapter 4) discuss the information on the senses of seals and sea lions in general and their specific adaptations to habitat and lifestyle in particular. We hereby focus on the senses of harbor seals. Harbor seals turned into a model organism regarding the sensory systems due to intensive sensory research of the last decades. In the second and third chapter, the sensory basics are put into the context of orientation, navigation, and foraging. This allows formulating new research questions, such as where and how the information from different senses is integrated

Coping with heat: function of the natal coat of cape fur seal (Arctocephalus Pusillus Pusillus) pups in maintaining core body temperature.

Cape fur seal (Arctocephalus pusillus) pups spend the first weeks of life exclusively or mainly ashore. They are exposed to intense solar radiation and high temperatures for long time periods, which results in temperatures up to at least 80°C on their black natal coat. To test the hypothesis that the natal coat has a crucial function in coping with these extreme conditions, we investigated the insulating properties of the natal coat in six captive newborn Cape fur seals during the first 50 days after birth. The natal fur differs from the adult fur not only in colour, but also in density, structure, and water repellence. We measured temperature on the fur surface and within the fur, as well as skin and rectal temperature under varying environmental conditions, comparable to the species' habitat. Experiments were designed to not influence the spontaneous behaviour of the pups. Rectal temperature was constant as long as the pups stayed dry, even during long-lasting intense solar radiation for up to 3 h. Skin temperature remained close to rectal temperature as long as the fur was dry, while with wet fur, skin temperature was significantly reduced as well. Our results show that the natal coat provides an effective insulation against overheating. The severely reduced insulation of wet natal fur against cold supports the assumption that the natal fur is an adaptation to the pups' terrestrial phase of life

Brightness discrimination in the harbor seal (Phoca vitulina)

The brightness discrimination ability of a mate harbor seal was investigated at an ambient illumination of 0.9 lx. The visual stimuli consisted of circular gray discs that were presented on a black background on a TFT monitor. Eight standard intensities were tested against sets of lower comparison intensities. In accordance with Weber's law we observed a constant gain of the just noticeable intensity difference with increasing intensity of the standard stimulus. The calculated Weber fraction is 0.14. This result indicates that the brightness discrimination ability of the harbor seal is comparable to that of humans

Visual fields and eye movements in a harbor seal (Phoca vitulina)

AbstractThe boundaries of the visual fields of a harbor seal were measured using static perimetry. In the seal lying on a plane surface (fixation point “0°” straight ahead at eye-level), the visual field with fixed eyes extended over 208° horizontally and reached from −12° to +69° vertically. The binocular visual field amounted to 67°. Eye movements of 12° (±2)° to both sides and 64° upwards could be induced. In the seal performing eye movements, a visual field of 210° in the horizontal plane and 121° to the dorsal side was determined. From the measured eye movements, a visual field of 232° in the horizontal plane appears possible

Why do seals have cones? Behavioural evidence for colour-blindness in harbour seals.

All seals and cetaceans have lost at least one of two ancestral cone classes and should therefore be colour-blind. Nevertheless, earlier studies showed that these marine mammals can discriminate colours and a colour vision mechanism has been proposed which contrasts signals from cones and rods. However, these earlier studies underestimated the brightness discrimination abilities of these animals, so that they could have discriminated colours using brightness only. Using a psychophysical discrimination experiment, we showed that a harbour seal can solve a colour discrimination task by means of brightness discrimination alone. Performing a series of experiments in which two harbour seals had to discriminate the brightness of colours, we also found strong evidence for purely scotopic (rod-based) vision at light levels that lead to mesopic (rod-cone-based) vision in other mammals. This finding speaks against rod-cone-based colour vision in harbour seals. To test for colour-blindness, we used a cognitive approach involving a harbour seal trained to use a concept of same and different. We tested this seal with pairs of isoluminant stimuli that were either same or different in colour. If the seal had perceived colour, it would have responded to colour differences between stimuli. However, the seal responded with "same", providing strong evidence for colour-blindness

A harbor seal can transfer the same/different concept to new stimulus dimensions

We investigated the formation of an abstract concept of same/different in a harbor seal by means of a two-item same/different task. Stimuli were presented on a TFT monitor. The subject was trained to respond according to whether two horizontally aligned white shapes presented on a black background were the same, or different from each other, by giving a no-go or go response. Training comprised of four stages. First, the same/different task was trained with two shapes forming two same problems (A-A and B-B) and two different problems (A-B and B-A). After the learning criterion was reached, training proceeded with new pairs of shapes. In the second experimental stage, every problem was presented just five times before new problems were introduced. We showed that training to criterion with just two shapes resulted in item-specific learning, whereas reducing the number of presentations to five per problem led to the formation of a same/different learning set as well as some restricted relational learning. Training with trial-unique problems in the third stage of this study resulted in the formation of an abstract concept of same/different which was indicated by a highly significant performance in transfer tests with 120 novel problems. Finally, extra-dimensional transfer of the concept was tested. The harbor seal showed a significantly correct performance on transfer tests with 30 unfamiliar pattern and 60 unfamiliar brightness same/different problems, thus demonstrating that the concept is not restricted to the shape dimension originally learned, but can be generalized across stimulus dimensions

Multifocal lenses in a monochromat: the harbour seal

Previous photorefractive results from harbour seals indicated the presence of a multifocal lens. This was surprising because the evolution of multifocal lenses has served to compensate for chromatic aberration in animals with colour vision, which harbour seals as monochromats should not be capable of. To gain insight into the lens optics of these animals, we extended our photorefractive measurements in live seals under water and in air and, additionally, analyzed eight extracted juvenile harbour seal lenses with schlieren photography and a laser scanning technique. The results from all methods applied support the presence of multifocal lenses in harbour seals. However, the functional significance of multiple focal lengths in harbour seal lenses remains speculative. Interestingly, the slit pupils of harbour seals cannot be considered to be an adaptation to the multifocal optical system of the eye

Cape fur seal pups hauling out under intense solar radiation.

<p>The pups, aged 18 and 28 days, hauled out in the sun for 1.5 h, T_air  = 31.3°C. T_surface increased up to 78.7°C on their backs.</p

Reduction of T_surface on hauled out pups by wind forced convection.

<p>Pups hauling out under solar radiation with sudden appearance of a light breeze of wind force 1–2 (Beaufort), corresponding to 0.2–3.3 m/s. (A) Pup with T_max = 61.2°C on the surface, T_air = 17.3°C. (B) Decreased T_surface after light breeze for 5 s to T_max = 58.8°C (3.9%). (C) Pup with T_max = 72.3°C on the surface, T_air = 23.8°C. (D) After 42 s of light breeze T_surface decreased to T_max = 47.3°C (34.6%).</p