Common seal (Phoca vitulina) - Made for living in the water

The Common seal is probably the best-known seal species of the northern hemisphere. This might be due to its wide distribution and the fact, that Common seals live near the coasts and can be watched, resting on sandbanks, from the dry land. Common seals prefer regions, where rivers flow into the sea, because these waters are permanently ice-free even in the northern part of their distribution range. The Common seal's second name - Harbour seal - is due to its affinity to coastal regions. Common seals inhabit the European coasts of the Atlantic, the North sea and the Baltic sea as well as the Atlantic coast of the northern part of North America and North America's Pacific coast. Due to the Common seal's wide distribution there are several subspecies.

Seals (Phocoidea) are even better adapted to living in water than the second group of Pinnipeds, the Sea lions (Otarioidea), represented by the California sea lion (Zalophus californianus), for example. After the Whales (Cetacea) and the Sirenians (Sirenia) Seals are those mammals, which are best adapted to living in water. Their physique is optimized for locomotion under water. It is torpedo-shaped and the limbs are short. The shoulder and the upper arm are extremely shortened and included in the shape of the trunk, whereas the hands and feet are stronger and widened. Stretched among the Seals' fingers and toes are webs. In contrast to the Sea lions Seals don't have an external ear; their auditory canal can be closed by muscle contraction. Everything that might make gliding through the water more difficult is moved inwards. The males don't show any external sexual organs; the testicles are inside the body and the penis is hidden in a skin fold. The Seals' spinal column is very strong and movable. Together with the hind limbs the lateral moving spinal column is the motor of locomotion under water. The front limbs serve as rudders. Seals appear very elegant moving under water but quite clumsy moving on solid ground. In contrast to the Sea lions Seals are not able to stand on their hind limbs and thus use them for locomotion on land. The front limbs are not very suited for that purpose either. So Seals have to crawl more or less on their trunks.

Beside these external alterations in the Seals' physique according to living under water, there are a lot of changes concerning inner organs too. The lungs and the blood circulation system are extremely powerful. A Seal has almost twice the amount of blood as a human of the same weight. In addition Seal blood is extraordinarily rich in oxygen binding haemoglobin. So Seals are able to stay under water for a long time without having to come up and breathe in. The record-holder is the Weddell seal (Leptonychotes weddelli), which is able to reach depths of about 600 metres and stay under water for 70 minutes. All these adaptations to living under water are of secondary origin; the Seals' ancestors have been ground-living marten-like animals.

The Common seal is a rather small Seal species. Adult males are almost two metres long and weigh about 100 kg. Females are a little smaller. A considerable part of the weight is due to fat tissue beneath the skin, which prevents Seals from chilling through. The Common seals' fur is grey with darker spots, which form larger areas on the back. According to the local distribution there are differences in fur colour. Although the Common seals' fur isn't very thick, they have been hunted for their fur for a long time. Even nowadays it's still allowed to hunt for Common seals in many regions of their distribution range. Otherwise Common seals are mostly endangered by marine pollution. Especially near the river mouths, where Common seals prefer to live, the pollution is even more severe than on the open sea, due to chemicals transported by the rivers. This often results in skin diseases, which are troubling many Common seals. Further threats are disturbances by tourists, which might put female Seals with young under stress, often resulting in abandoning the young. The orphaned pups become "howlers". They are called this way, since their calls for their mothers are similar to the bawling of a child. At many coasts within their distribution range there are rescue stations for abandoned Seal pups.

Normally females give birth to a single pup, which is weaned after four to six weeks. During the lactation period the mother is following her young and guarding it. Seal milk is very nutritious and the pups grow very fast. Now and then twins are born. Since the mother is only able to follow one young, the second one orphans and becomes a "howler". The mother-young-relationship ends simultaneously with the weaning. Common seals are mating at that time. Pregnancy takes eleven months, so that a female raises one pup each year under optimal circumstances. Although Common seals are social animals, no special social structure can be determined. They congregate forming groups, but animals easily leave or new ones join the group.

[Choose another article about animals!]