Seal fun facts

We get a lot of questions about seals that can be about anything. From why gray puppies are white to whether a seal drinks salt water. We are always happy to answer these questions and therefore there are many nice seal facts that we have collected for you and that you can find here.

seal pup maximo is given fish porridgeSeals at the Sealcentre are fed fish porridge or herring, depending on their age and condition. They also get water in order to provide the necessary hydration until they are eating enough fish to satisfy their water requirements.

The fish porridge and the water are given by tube feeding because that is the easiest and least stressful way of doing it. Although this does not look pleasant, it requires less time to handle the animal compared to other type of feeding techniques. In this way, we make sure the animal suffers the least stress. We feed defrosted fish to the seals. By law we are not permitted to feed live prey.

We feed in 3 different ways: force feeding, hand feeding and self-eating. The handling of the animal decreases with each type of feeding, respectively. Medication is given when necessary and always under veterinary management. It is given with the food, either the liquid or the solid diet. Handling a seal and feeding them by hand is challenging and requires experience.

Seals are the largest predators in the Netherlands. They are wild animals that can be unpredictable and the whole process needs to be as fast as possible to ensure the shortest handling time.

seals in phase 2 In Phase 1 the seals come in. These are small, quiet and heated spaces. There is a small pool with a gate that can be closed. The vet determines whether an animal is allowed to swim and for how long. The seals are usually alone in this phase. The care is intensive: the seals are fed fish porridge through a funnel 4 times a day. This porridge is made from salmon, because it is a fatty fish. This allows the seal to gain weight. When the seal’s condition improves and an animal needs less intensive care, it can go to phase 2.
In Phase 2, the pools are slightly larger and seals themselves decide how long they are in the water. There are often several seals in here together, so that they get used to their peers again. The seals are now eating herring. Sometimes they get help by being force fed. When the animal has gained enough weight, is eating independently and no longer needs medication, it can proceed to the final phase.
In Phase 3 they go to one of the big pools. Here they can swim well. Usually, there are a few more seals in this phase. The seals now eat independently and are weighed every week. When the seal remains at a good weight, the release of the animal is planned. And then they can go to a really big pool: the sea! 

seal mother with pup on the beachDid you know seal pups can remain alone for up to 8 hours without it having negative consequences? This is much longer than was anticipated before. We made this discovery through a two-year-long scientific research, named the Dollard project. Marga Mendez Arostegui and Beatriz Rapado Tamarit conduct this research as part of their PhD at the University of Groningen. In their research they study the behaviour of common seals in the Eems-Dollard (a nature reserve in the North of the Netherlands).

The seals are viewed from a distance through a wall which is covered in holes. Marga and Beatriz study the individual behaviour from more than 400 seal mothers and pups during the birth season. Other results from the research are regarding the feeding of the pups. The pups do not only drink from their own mother, but also from other seal mothers. This is called communal suckling.

When you see a pup alone at the beach, he or she is not always starved or an orphan. Leave a crying pup alone because the largest danger is being disturbed. The mother seal can get scared and flee. She won’t return to the beach when it is too busy or crowded.

grey seal shedding white furThere are 2 different seal species in The Netherlands: grey seals and common seals. Both species have a different type of fur.

Grey seals are born with a thick white “fur” coat. Do you remember how pup Snelle looked like when he was in the Sealcentre during his first weeks at the end of last year? Thanks to this thick coat, the pup can resist the cold winter weather. The white fur is also called “lanugo”, or sometimes baby fur. Many mammals, humans as well, have fur like this. In seals it has an extended function.

Eventually, the seal pup loses the white hairs. This type of fur is less suitable for swimming, because it retains a lot of water. The coat that remains is grey, shorter and isolating. Therefore, it is better for swimming and hunting purposes. In the picture, you see grey seal Snelle has partly shedded his white fur.

However, the common seal pup does not have such baby fur. They are able to swim almost immediately after being born. A unique fur structure has been found in common seals. Not only do they have underhairs and guard hairs, but there’s a third type of hairs in their coat: intermediate hairs. This fur composition has not been seen in mammals before and seems to serve for thermal insulation as well as drag reduction during swimming.

zeehond op strandMuch has changed within the organisation in the past 50 years. The vision and mission have been adapted and expanded. We are not only looking at the seal population and its health, but the entire health of the Wadden Sea. Our current logo has been adapted to fit this purpose. There is now a large S in our logo. But what does this S stand for?

The S stands for our organisation’s three main goals - Sea, Seal and Science. We work with an international team and annually have around 100 volunteers who are mainly international. This is part of the reason we use English words in our goals.

The Sea stands for our mission; keeping the entire Wadden Sea healthy. The Seal stands for the seals. Each day, a team of dedicated veterinarians and seal carers from all over the world care for sick, injured or orphaned seals at the Seal Centre. The last S also fits in with this, which is Science. To keep the sea and the seals healthy, research needs to be done. We research the behaviour of seal mothers and pups in the wild, to avoid unnecessary admissions into our Sealcentre. In the centre itself, we test each seal’s blood for the presence of diseases with the new blood analysis machine.

It is with good reason that we are the largest seal hospital in Europe.

zeehond op strandSeals have a very small tail at the end. This small tail and the tail bone are a vestigial organ (like humans have an appendix). A remote ancestor of the seal, an otter-like species, once had a tail that was used for swimming. This tail became unnecessary when the hind legs turned into flippers, which seals now swim with. The tailbone only functions as an attachment point for various skeletal muscles such as the rear flippers.

These attachments enable the movement of the skeleton. The rear flippers are often mistaken for a tail, as it is often called in dolphins. This is not the case with seals however, as these flippers are originally their hind legs. The seal has two rear flippers with 5 fingers each, which are located at the end of the seal. These rear flippers are mainly used for swimming and enables them to gain speed. They move forward by powerful strokes of the rear flippers and the sideways swinging movement of the rear body. When the flippers are fully extended, it looks like a wide fan and there is large webbing between the toes.

seal whiskersMost seals species search food up to hundreds of metres under water. Light doesn’t penetrate this deep in the sea, which means seals often hunt in the dark. Despite their eyes being large and seeing more under water than the human eye, the whiskers are what makes seals able to hunt in this environment.

The whiskers can detect the tiniest movement in water. This is how seals find their way through water flows and notice moving fish through the vibrations they emit. This is due to the unique shape of the whiskers. Unlike with cats and dogs, they aren’t smooth, but wavy. As a result, the whiskers are streamlined in a way they glide smoothly through the water. When the water starts to move, so does the whisker and this sign is picked up by the hundreds of nerve endings at the base of each hair. This makes it possible for a seal to pinpoint a fish even minutes after it has stopped swimming!

seal earsThey may not be easy to spot, but seals do have ears. They only lack the auricles, the external part of the hearing organ. This reduces their ability to swim and hunt. Despite the missing auricles, a seal can hear clearly both under and above water. Underwater, they react to sounds between 1 and 180 kHz (Humans hear from 0,02 to 20 kHz).

The inner part of the hearing organ is very similar to those of land mammals such as bears. When going underwater they close it with a flap, and despite this flap they can still hear well underwater. Low tones can be heard from several kilometres away, high tones carry less far.

Seals use their ears mainly to hear danger in advance. Think for example of people walking on the sea bank or a polar bear. They also use their ears for tracking down prey, in combination with their whiskers. With the whiskers, they can detect movement in the water. How they do this with their whiskers will be explained in next week's post. Do you have a question for us? Leave it in the comments and we might answer your question next time! .

seal evolutionDid you know that seals (Phocidae), sea lions (Otaridae) and walruses (Odobenidae) are closely related to bears (Ursidae) and mustelids (Mustalidae). The most recent common ancestor between this families was a species called Enaliarctos, which lived about 23 million years ago. A transitional fossil of a walking seal, named Puijila, is the missing link between these families. The animal was similar in body shape to an otter, but its teeth and skull indicate it is an ancestor of seals. It probably came from freshwater habitats and slowly moved to the sea.

seals herringAll seals that are being rehabilitated and recover, are released back into their natural habitat. Because they are wild animals, we try to keep the rehabilitation period as short as possible. The shorter they are with us, the better it is for them. The seals we take care of are often underweight and weakened due to sickness or injuries. It is important that the nutrition we give them ensures that they regain their strength quickly. This requires a diet with a lot of fat. Herring is a fish that contains it in large quantities. The herring that we feed is defrosted fish.

In nature a seal hunts life fish and is not used tof rozen and immobile fishes. Therefore it may take a while before a seal eats the fish by itself. We always give the animal the opportunity to eat by itself, if it doesn’t, we feed it by hand. Herring is a smooth and firm fish which makes it easy to feed. A seal in the last phase (phase 3) of rehabilitation eats an average of about 5 kilos of herring per day.

seals in winterSome marine mammals can keep half of their brains awake while the other half is asleep, but seals have retained the bihemispheric sleep of their terrestrial ancestors. Difficult words, but it means that species like grey and harbour seals have to "turn off" their brains completely in order to sleep (just like dogs or humans), and it happens regardless of whether they sleep on land or in water.

However, very little is known about the sleep duration in seals. For example, a seal that sleeps underwater will need to wake up regularly to surface so that it can breathe. That makes it difficult to study their sleeping habits. A study observing 3 to 5 month old harp seals showed that they slept on land for an average of 6 hours a day, while adult caspian seals slept for about 4 hours a day.

seals in winterEspecially in winter we are often asked whether the seals swim in heated water. The answer is actually quite logical: no. Because when they swim in the sea, the water is not heated either. Seals can withstand very low temperatures due to a thick layer of fat under their fur.

This layer protects them from the cold and ensures that they keep their body at a good temperature even in very freezing weather. Seals build up this fat layer by eating well, it is a fat reserve. A very young puppy or a weakened animal has little or no reserves. Then we do use floorheating that is available in the intensive care units. There are also a few phase 2 outdoor pools with heat lamps for the seals to lie under. In both cases, we only use this heating when animals are very vulnerable and it is important that they rebuild reserves without using energy to stay warm.

witte pup The beautiful white fur of the grey seal pups is also called lanugo. It is a baby coat, intended to keep the body warm during the period when the puppy has yet to develop a thick layer of fat that will keep it warm as an adult seal. Until then, the lanugo protects the puppy from the cold on the land. So it is really just a very thick winter coat.

Grey seals are found in the northern hemisphere, also in areas much colder than the Netherlands and there the white color of the lanugo has another important function: it camouflages the pup in the snow. This allows them to remain safely on land for the first weeks of their lives without being noticed by predators.

drinkenzeehondenSeals never actually drink. Their kidneys can handle salt water, but they get the moisture they need from their food. Fish and crustaceans largely consist of water, which gives a seal sufficient moisture. They can store excess fluid in their fat layer as a reserve or urinate it out.

If a seal is found weakened on the beach because the animal has not eaten anything for a while, it may also be dehydrated. That is dangerous. The seal will then receive ORS from us as soon as possible. ORS stands for Oral Rehyration Salts, it is a powder that dissolves in water and is administered through a tube in the throat. This allows the moisture balance to regain quickly. ORS is often given immediately at the place where the seal is found so that the animal feels a little better before it is transported to Pieterburen.