Seals have a well-developed sense of hearing that is specialized for underwater acoustics. Hearing is greatly reduced when out of the water. Seals are able to respond to sounds from 1 to 180 kHz when underwater. In the air, hearing ability is only from 1 to 22.5 kHz.
Seals have large eyes to allow them better vision underwater. On land their vision is greatly reduced. Their lenses are enlarged and almost round, adapted for focusing on light that is refracted upon entering the water.
Their eyes are specially adapted for sight in dark and murky water. Eyes contain a high number of rod cells that specialize for black, white and grey pigments and are sensitive to low light levels.
Seals have a well-developed tapetum lucidum, a layer of reflecting plates behind the retina. These plates act as mirrors to reflect light back through the retina a second time, increasing the light’s gathering ability of the rod cells. (The tapetum lucidum is the same structure that makes a cat’s eye appear to ‘glow’ when reflecting light at night.)
Underwater the pupils dilate into a wide circle to let in as much light as possible. In bright light, the pupils constrict to a slit.
Mucous continually washes over the eyes to protect them. Unlike most land mammals, pinnipeds lack a duct for draining eye fluids into the nasal passages. When a seal is out of the water, mucous surrounding the eyes gives them a wet, ‘tear-rimmed’ look.
Seals use their sensitive vibrissae (whiskers) to find food, especially in the dark, deep waters or at night.
Each vibrissae can move independently. Underwater, a seal can thrust its vibrissae to and fro in a sweeping motion by pushing its mobile upper lip in and out.
Prey moving under water creates vibrations that the seal may detect with its vibrissae.
All marine mammals have special physiological adaptations for diving. These adaptations enable a seal to conserve oxygen while it is underwater.
Seals have a slower heart rate while diving. The heart rate slows from 75- 120 beats per minute to only four to six beats a minute. When a seal surfaces after a long dive, it experiences an accelerated heart rate for a short time.
When diving, blood is shunted away from the tissues that are tolerant of low oxygen levels and diverted to the heart, lungs, and brain where oxygen is needed. This is known as peripheral vasoconstriction.
A seal has a greater volume of blood than a land mammal of the same size to allow it to retain more oxygen.
The muscle of seals also has a high content of the oxygen-binding protein, myoglobin (10 times as mush as an average human). Myoglobin stores oxygen and helps prevent muscle oxygen deficiency. This gives seal meat its dark colour.
Seals sleep on land and in the water. In the water they sleep at the surface and often assume a posture known as bottling, their entire bodies remain submerged with just their heads exposed. This enables them to breathe when necessary.
A seals core temperature is about 37.8oC (100oF)
The skin remains about 1oC warmer than the surrounding water.
Seals have a metabolic rate somewhat higher then a land mammal of the same size. This helps keep them warm by generating body heat.
A thick layer of blubber insulates the body, reducing heat loss.
Blubber streamlines the body and functions as an energy reserve from which the seal can draw energy during periods of fasting.
In cold water, blood is shunted inward as blood vessels in the skin constrict (vasoconstriction), reducing heat loss to the environment.
When hauled out on land, blood vessels in the skin dialate (vasodialation). Blood is shunted to the rear flippers where it pools and allows heat to be released to the environment. This is made possible as there is no layer of blubber to insulate the flippers and prevent heat loss.
To prevent heat from escaping through extremities, seals hold them close to the body.
Harp seals obtain fresh water by consuming ice and snow, as well as from their food
If food intake is decreased, the metabolic breakdown of fat produces water. The metabolism of 0.45 kg (1 lb.) of fat produces 0.64 kg (1.4 lbs.) of water.
Biology of Pinnipeds : Seals, Sea lions and Walrus
The order Carnivora includes terrestrial (bears, dogs, raccoons, weasels, hyenas, cats and mongooses) and aquatic mammals. All marine Carnivores are currently placed in the Suborder Pinnipedia (seals, sea lions and walrus) or the Sub-order Fissipedia (polar bear and sea otters). The pinnipeds (‘fin foot’) have flippers. The fissipeds (‘toe-foot’) have feet with separated toes.
There are three families of living pinnipeds; the Phocidae (true seals), the Otaridae (fur seals and sea lions) and the Odobenidae (walrus). All of these marine mammals must come ashore to give birth and nurse their young. Some species are at sea for several months while others return to shore every day. All pinnipeds are large, as they have developed specific adaptations to minimize the amount of heat loss due to conduction. Water conducts heat twenty five times faster than air therefore a pinniped’s relatively large body size provides a low surface area to body mass ratio, thereby decreasing the portion of body exposed to water. A thick layer of fat called blubber also insulates their bodies.
In all species, the external ears (pinnae) are small or absent (as in the case of the phocids such as the Harp Seal), the external genitalia and nipples are hidden in slits or depressions in the body, and the tail is small. The forelimbs and hind limbs are formed into fins. Certain bones have adapted to become highly specialized for swimming, such as the spine. Most species have a short rostrum (snout), and the teeth are usually shaped like simple cones. These animals are capable of diving to extreme depths, up to 600 m in the case of the Weddell seal, and remaining under water for upwards of an hour, though most dives are kept short.
Phocid seals are known as ‘true seals’ because they lack pinna, unlike the otarids and odobenids. Their hind flippers cannot rotate forward thus they cannot be used as a means of support. There are 18 species of phocid seals including; elephant seals, several species of ice seals such as the harp seals and ringed seal, the harbour seal and the monk seal.
The otarids, (fur seals and sea lions), are found in temperate colder waters. They have external ear flaps; pinnae, and the arm pit or axilla encloses the forelimbs to about the level of the forearm. Sea lions and fur seals can rotate their hind feet forward therefore allowing them to propel themselves forward with considerable speed.
Walruses are one of the largest pinnipeds, with males reaching over 3,000 pounds (1,500 kg). Both males and females have tusks, vacuum-like mouths for sucking up shellfish from the ocean floor. They live in the North Atlantic and Pacific Oceans, in the arctic region.