Why do seals beat their bellies?


Dog seals are the acrobats of the sea, but they seem a bit clumsy on land. Although they can race through the water at 35 kilometers per hour with ease, they can only manage 2 kilometers per hour on dry land. With their beautiful, spherical eyes, dog seals can see well under water, but above water they would actually need glasses. Dog seals are completely adapted to life underwater. Nevertheless, you can meet them regularly over water. They like to lie in the sun and rest on a sand plate or on the beach.

Seal menu

Dog seals mainly eat fish. They use their whiskers to track down the fish. With it you can feel the slightest movement of the water. In this way, seals can 'see' where the fish is in the murky water, up to 100 meters away. They can also feel the shape and size of the fish in the distance. Dog seals have no preference for certain types of fish, but they mostly catch fish that live close to the bottom. Flatfish, sand eels, and codfish are among their favorites, but what they end up eating depends on what's on offer. Young seals have to teach themselves to catch and eat the fish when they can no longer get their mother's milk. Their mother doesn't teach them how to hunt. During this time, the young seals lose weight. If they do well, they learn to catch fish better and better.

Seals in the water

A seal swims just as easily on its stomach as it does on its back, and just as easily standing upright as it is upside down. When swimming, seals use their forefins as oars; with the body and the hind fins they generate the speed. Dog seals can dive hundreds of meters deep. They actively swim down for the first few minutes, after which they let themselves sink further down in a kind of 'gliding flight'. You can dive really long and deep. Seal blood can hold much more oxygen than human blood. The heartbeat drops from 40 to less than 1 beat per minute, which means that you have to breathe less often. When they come back up from a great depth, they can pump the air they breathe out of their alveoli. In this way, they prevent nitrogen bubbles from forming in the blood, which can be fatal. Once over water, the heartbeat of a seal picks up again, up to 120 beats per minute, in order to supply the organs with oxygen again.

Seals ashore

The seals' ancestors were mammals living on land. But in terms of locomotion on land, there is hardly anything left to be noticed today. Over the millions of years they have adapted to aquatic life in various ways. The many adaptations to marine life have made seals clumsy on land. You cannot walk properly because the hind fins are in line with the axis of the body. Sea lions, on the other hand, can still put their hind fins under their bodies. That's why they run much better than seals. Seals drag their bodies across the ground and push themselves off with their front fins. This movement is called “crawling”.

How does a seal sleep?

Seals can sleep both in the water and on land. They sleep in the water, drifting vertically like a swimmer or drifting horizontally on the surface of the water. Since you do not swim actively when you sleep, you can endure it longer without breathing. There are known cases of seals that have remained underwater for half an hour. Most of the time, however, they don't stay under water for more than a quarter of an hour.

Seals are well insulated

For the most part, seals are well insulated by a thick layer of fat. It can sometimes be more than two inches thick. Only the fins and the head have to do without a layer of fat. If it still gets too cold, you can 'cut off' the blood supply to the skin so as not to lose too much heat. The fins are also used to regulate body temperature. The blood vessels are located just below the skin in the webbed feet between the toes on the fins. Blood flowing into the fins is cooled by the returning blood and goes into the fins at a lower temperature. Before this blood flows back into the body, it is warmed up again by the blood flowing out. As a result, the fins always have a lower temperature, so that relatively little energy is released into the cold environment. With this type of temperature control, seals always have cold feet! That's why, once the sandbars dry out, they try to keep their fins afloat. They want to lose less heat and warm them up again in the sun. When it is too warm for them, seals leave their fins and head in the water to cool off.

Seal in a banana pose

On the sand plateaus in the Wadden Sea you can often see the seals lying in a typical banana pose. They probably do this to keep their head and fins dry and therefore warm. Seals are well insulated with a thick layer of fat, with the exception of the head and fins. If they are in a dry place but the water is coming up, hold them up to keep them dry. This almost automatically leads to a kind of banana farming.

The inside of a seal

How does a seal drink and pee?

Adult seals almost never drink. They get the fresh water they need from the fish they eat. If they don't eat for a while, the water comes from breaking down their layer of fat. Due to their life in the sea, they naturally also absorb salt water from time to time. Your kidneys are specially adapted and can excrete the salt in your urine. So you already pee, but not a lot. The urine is very concentrated and in some cases it can be salty than sea water. At the berths you can often see hollows in the sand where the seals were lying, where they peed and the sand was washed away. Often one can see from the position of this hollow in relation to the imprint of the seal whether it was a male or a female. The penis opening of the male is quite high, just below the navel. In the female, the bladder opening is closer to the tail.

Seals change fur

Seals are hairy to prevent dehydration and damage to the skin on land. The short hair also ensures that the cold sea water does not reach your skin as quickly. The fur is changed every year. Common seals get their new fur in summer, gray seals in March-April. During this time, they particularly like to lie on dry land in order to change their coat as quickly as possible. When it is very cold or the seals often have to flee into the cold water, the change of coat is delayed. The cold reduces the blood supply to the skin. The change process is delayed by the lack of blood in the skin. That is why it is very important for seals to be able to lie quietly on the sandbanks when they are changing fur.

The throwing and suckling of seals

Seals get their young on dry land. Common seals are born in the summer months, gray seals in winter. The cubs of the gray seals therefore have thick winter fur. They look touching with their thick white hair, but it's a little inconvenient. You can't swim with it. The young gray seals therefore stay on land for the first few weeks. The mother comes by regularly to suckle the young. Seal mothers suckle their young for three weeks. During this time the boys only get milk, so they do not learn to eat fish. After weaning from milk and leaving their mother, young seals have to learn for themselves that fish is edible and how to catch it.

Distribution and numbers of seals

The number of seals in the Wadden Sea has been well studied. Researchers from Wageningen Marine Research count the seals in the Dutch part of the Wadden Sea from an airplane. The seals are photographed when they are resting on the sand slabs. A third of the seals are in the water during the counts, so the researchers have to add another 30% to the counts from the plane in order to be able to come to a reliable overall estimate.

The common seal lives in the entire Wadden Sea (i.e. the Netherlands, Germany and Denmark); in 2018 there were around 40,000 animals (adults and young animals). But these seals are also found around the Scottish Islands, in the Wash, on the English, French and Belgian coasts, in the Skagerrak and on the south coast of Norway.

Gray seals live mainly in the Scottish Isles, on the British east coast and in Cornwall. In December 1999, a gray seal was even seen in the Thames beyond London.
Gray seals settled in the Dutch Wadden Sea in the 1990s. It has now grown into a large group, with 4565 gray seals counted in the Netherlands in 2018. The largest group lives west of Terschelling. More than a hundred animals live on the German islands of Helgoland and Amrum.

Satellite observations of seals with transmitters show that the seals from the Wadden Sea swim around the entire North Sea, for example they swim from the Wadden Sea to Scotland and back again.

Seals can survive well in fresh water. They used to swim up the rivers regularly. Since most rivers are now closed with sluices, it is no longer so easy to get into fresh water. Still, it happens often. Seals are sighted regularly in the IJsselmeer. You have probably wandered through the locks of the final dike. In 2011 a seal was run over while crawling over the dike.

Stray seals from more northerly areas

In addition to the common seal and the gray seal, seal species from more northerly areas come to the North Sea from time to time. Ringed seals from the northern Baltic Sea and harp seals from the far north can be found here from time to time. In 1987 there was a small harp seal invasion as a result of the lack of food in the Barentsz Sea. The stranding of folding hats is less common. In 1986, several hats were stranded on the coasts of France, Belgium and the Netherlands at the same time. This "mini-invasion" was possibly related to a hunt on the island of Jan Mayen in the Arctic Ocean. A female bearded seal was found once, on June 27, 1988 in the port of Yerseke. Furthermore, there were 6 strandings of walruses in the last century, the last one was in January 1998 on Ameland.

Seals threaten humans

Dog seals were heavily hunted in the North Sea area in the past. They were considered harmful animals. They damaged the nets and caught the fish from the noses of professional fishermen. Hunters received a bonus for every seal they shot. Although the seal hunt in the Wadden Sea and the North Sea ended 50 years ago and the number of seals has increased, they are still threatened. Polluting waste, whether through discharge or accidents, is a threat. In addition, disruptions from shipping and tourism are causing problems. And seals can drown in gill nets and fish traps.

Seal threat: marine pollution

Waste materials end up in the North Sea every day. Coming from agriculture, industry and cities, these substances find their way into the sea via the rivers that flow into the North Sea. Other waste materials get into the sea through direct discharges, ship accidents or through acid rain from the air. The plankton ingests these toxins, which is how they get into the food chain. The poison then finally reaches the marine mammals via soil animals and fish. That is where they accumulate. The concentrations of poison are highest in the species that live near the coast. A well-known example of a dangerous poison for seals are PCBs. At the end of the last century, many seals were no longer giving birth and they quickly became ill from this poison.

Seal threat: Disturbed calm

The hustle and bustle in the Wadden Sea and the North Sea is a problem for the seals. Tourists and the military, for example, disturb the peace of the seals on the mudflats. These disturbances are a particular threat in the summer when the common seal pups are born. The boys do not get enough milk or the mother and child are separated. That is why seal reserves were set up where the animals have to be left alone. The water police have to come into action an average of 4 times a day during the summer season because tourists disturb seals.

Seal threat: drowning in fish traps

A fish caught in a fish trap is tempting prey for a seal. She swims in the trap to catch the fish. Once the seal has swum in, it can no longer go out and it drowns. In the Netherlands it has been mandatory since 1994 to equip all fish traps in tidal waters with a sweeping net to prevent seals from drowning. This sweeping net is a coarse-meshed net at the entrance to the trap, through which eels and other fish can still pass, but no seals. However, seals that are not equipped with the prescribed sweeping net are still drowning in the traps. A dust net also reduces fishing.

Seal versus fisherman

Fishermen often see seals as a harmful competitor. In the past, seals have been used to kill seals because they are believed to have caused major damage to the fishery. In Norway and Canada this is still an argument in favor of reduction hunting for seals. Studies of the stomach contents and feces of seals show that all Dutch common seals together eat much less (1-2%) than what the fishery lands. Common seals eat a relatively large number of young flounder, a species that is hardly of interest for fishing.
Fishing with gillnets or traps can be troubled by hungry seals. If the fish is stolen, the animals damage both the catch and the nets. Scottish and Norwegian salmon farms are regularly visited by gray seals who love to eat salmon. Fish farmers claim the damage is huge, while nature lovers say the damage is negligible in terms of fish farm sales. Neutral sources indicate an average damage of 1 to 4% of the total proceeds.

Protection of the seals in the Wadden Sea

The common seal and gray seal are both on the Red List of Endangered Mammals. The government and the administrative organizations must therefore take the seals into account and ensure that the population does not decrease. In addition, the seal protection contract between the three Wadden Sea countries of the Netherlands, Germany and Denmark was concluded in Bonn in 1988. It regulates the protection of seals in the Wadden Sea. In addition to a seal hunting ban, it was also agreed to protect the seal's habitat in the Wadden Sea from disturbances. Reserves have been established to prevent disturbance to resting and nursing seals. These areas are restricted areas in the summer, exactly when the cubs of the common seals are born and nursed. In the other European waters, the common seal and the gray seal are protected on the basis of the EU Habitat Directive and the Bonn and Bern conventions.