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Evolution It is believed that seals probably evolved from otter-like ancestors about 15 - 20 million years ago. Streamlining |
Keeping Warm
Seals are warm blooded. The loss of appendages allows them to
conserve more heat due to their low surface area : volume ratio. Heat is lost
from all parts of the body that are in contact with the water. Blubber provides
insulation in water, which has a thermal conductivity 25 times that of air.
In hot weather, on land, seals can often be seen flipping damp sand onto their
backs in an attempt to keep cool. Also dilation of the arterioles, particularly
in the flippers, allows a rush of blood to the skin promoting cooling.
Locomotion
Seals are extremely agile in the water. The hind limbs provide
mechanical motion from the rear end with powerful sideward thrusts. They can
achieve speeds of up to 25 knots.
On land, seals find it difficult to move about quickly. This loss of agility
on land is the cost of powerful movements in water.
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Diving Dives usually last 5 - 15 minutes and from 30 - 70 metres. The nostrils are kept closed during dives and opened on surfacing. Only a very short recovery period is needed between dives. Seals exhale on diving and the lungs and alveoli collapse at a depth between 25 - 50 metres, thus pushing any residual air into non-absorptive air spaces. This prevents seals from suffering from buoyancy problems and any risk of the bends. |
The oxygen needed to sustain
dives is stored in large quantities in the muscle (myoglobin) and in the blood
(haemoglobin), which are present in higher concentrations in seals than in terrestrial
mammals.
These pigment\'s affinity for oxygen is also higher than in terrestrial mammals.
It is the ability to store large amounts of oxygen and rapid waste gas exchange
that makes the seal so well adapted to the marine environment.
On deeper dives, seals have the ability of shutting down the blood supply to
non-essential organs and the heart-beat drops considerably to about 4 - 15 beats
per minute. Seals have a relatively large blood volume (up to twice that of
terrestrial mammals).
Seal\'s muscles are able to handle the large amounts of lactic acid produced
during the anaerobic metabolism of deep dives.
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Feeding All food is taken underwater. If the visibility is good, sight is primarily used, otherwise sound and vibrations, picked up by the sensitive whiskers are used. Their eyes are very well adapted for low light levels underwater. However, completely blind seals are able to live quiet happily for many years. We do not know whether seals use sonar for navigation. They do produce clicking noises which might be used for echo-location of food. Seals have well developed directional hearing underwater. |
The teeth bear a strong resemblence to those of a typical carnivore,
e.g. dog or otter, in that they include incisors and well developed canines.
An adaptation of the premolar and molar teeth is the presence of backward pointing
cusps, used to hold onto such slippery prey as squid or octopus.
Vision
The eyes are large with a strengthened cornea adapted to the
refractive index of water, and a retina adapted to dim light conditions, allowing
good visibility underwater. They also have a protective third eyelid that wipes
sand away and a continually produced tear film also helps to protect the eyes.
On land, in brightly lit conditions, constriction of the pupil reduces the light
intensity, which in turn reduces the refraction of light through the lens, allowing
the eye to effectively focus.
Coping with Salt
Seals 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 and most of the remaining
10% is metabolic water and inspired water vapour.
Seals kidneys are very efficient at concentrating urine. They produce a urine
that has a higher concentration than seawater.
Predators
Our seals have few natural enemies. Killer whales may take the
odd seal in our waters. Man is the main predator and seals have been hunted
for food for many centuries. The commercial hunting for skins in the 18th Century
left the population of Grey Seals in the British Isles at approx. 500 animals
in 1914.
This led to the 1914 Grey Seal Protection Act being introduced. A second Act
in 1932 extended the close season. This deems the killing of Grey Seals illegal
during the breeding season. In 1970, the current Conservation of Seals Act was
passed. This also covers the Common Seal.
These Acts allowed the Grey Seal numbers to increase, but this increase has
brought them into direct conflict with fishermen and seal culls were carried
out in the 1960's and 1970's to reduce the population. These culls were ceased
in 1978. Fishermen may still legally shoot seals caught in or near their nets.
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