Pinnipeds include the true seals (Phocidae), seal lions & furs seals (Otariidae – eared seals) and walruses (Odobenidae). Only true seals and walruses are found in Eastern Canadian waters. The two resident species of seal in Maritime coastal waters are the grey and harbour seal. Occasionally other (Arctic migrant) species (harp and hooded seals and walruses) are seen. Please refer to the “Marine mammal strandings and identification in the Maritime Provinces” book for information on pinniped species most likely to be encountered in Maritime waters.
True seals (phocids) probably evolved from otter-like ancestors around the shores of the North Atlantic 15-20 million years ago and are the latest of the sea mammals to have evolved, with the exception of the sea otter. There is some debate as to whether they evolved from the same stock as otariids (sea lions and fur seals) and the walrus.
They are very streamlined in shape (“torpedoes”), with no obvious neck, no external ears or genitalia, hidden teats and reduced limbs (“flippers”) and tail. This helps to reduce drag in the water (which is 800 times denser than air). The streamlining is enhanced by the layer of blubber rounding off all bony protuberances.
They are extremely agile in the water, propelling themselves by lateral flexing of the rear end of the spine and hind flippers, the spread digits and webbing acting like a fan. The foreflippers are used for stabilization and directional change. Seals achieve speeds of 5-25 knots.
Seals are the specialist divers amongst the pinnipeds. Dives usually last 5-10 minutes and reach depths of 30-70 metres. Only a very short recovery time is needed between dives. The nostrils are kept closed during dives (the relaxed position in seals) and voluntarily opened on surfacing. Unlike cetaceans, seals exhale on diving and the lungs and alveoli collapse at depths greater than 25m, thus pushing any residual air into air spaces where no gaseous exchange can occur. This prevents seals from suffering from buoyancy problems and reduces the risk of the ‘bends’. Oxygen needed to sustain dives is stored in large quantities attached to pigments in the muscle (myoglobin) and blood (haemoglobin), which are present in higher concentrations in seals than in terrestrial mammals. The pigments’ affinity for oxygen is also higher than in terrestrial mammals, so oxygen is given up sparingly to the tissues. Furthermore, seals have a relatively large blood volume (1.5 – 2 times that of terrestrial mammals). Shutting down the blood supply to non-essential organs and the slowing of the heart rate by one third (or even one tenth) also increases the efficiency of oxygen utilization. When the seal surfaces, it takes several rapid deep breaths until the oxygen supply is replenished. Longer dives can be taken – up to 30 minutes in grey seals – but these are sustained by anaerobic metabolism (i.e. the tissues function without oxygen), and consequently recovery times are a lot longer. Seals’ muscles are able to handle the large amounts of lactic acid produced during anaerobic metabolism.
Coping with Salt
Seals’ kidneys are very efficient at concentrating urine. They absorb water from, and eliminate excess salt into the urine, thus producing urine that has a higher concentration than seawater. Seals also restrict their seawater intake, to avoid taking on excess salt. They primarily derive fluids from the food they consume, e.g. 90% of fresh water taken in by common seals is from the fish they eat; most of the remaining 10% is metabolic water and inspired water vapour.
Seals’ eyes have a strengthened, heavily keratinized cornea and a protective third eyelid that wipes sand and other debris away. A continually produced tear film also helps to protect the eyes. The eyes are large and a spherical lens allows effective refraction of light underwater (the cornea’s refractive index is equal to that of water). The retina is adapted to dim light conditions, with a well developed tapetum and preponderance of rods, allowing good visibility underwater. On land, seals suffer from astigmatism. In brightly-lit conditions, constriction of the pupil corrects this, allowing reasonable vision. In dim light, when the pupil is dilated, vision is consequently blurred.
Seals have well-developed directional hearing underwater. This is aided by effective insulation of the inner ear (cochlea) from the skull, due to the delayed fusion of the bones of the middle ear with the rest of the skull. Therefore, background noise, transmitted through the skull, does not reach the inner ear. Seals have no external ears flaps and the muscles around the ear canal contract to prevent entry of water when diving.
If the visibility is good, sight is primarily used; otherwise sound and vibrations, picked up by the sensitive whiskers (vibrissae), are used. They catch their prey with a rapid burst of speed and forward extension of the neck, which is usually held ‘coiled’ in an S-shape. Small prey are swallowed whole. Larger prey are held in the fore flippers and eaten.
Blubber, the thick layer of fat between the muscle and skin, provides insulation in water, which has a thermal conductivity 25 times that of air. The low surface area-volume ratio of seals, with the reduction of external appendages and limbs, also aids heat conservation. There is also a sophisticated ‘counter current’ heat exchange mechanism in the blood supply to the hind flippers, where the outgoing arterioles are surrounded by an elaborate network of returning venules (arteriovenous plexuses). Returning cooled blood is heated by outgoing blood at core temperature. In hot weather on land, or after strenuous exercise, dilatation of the arterioles allows a rush of blood to the skin, particularly the hind flippers, promoting cooling.