Philopatry in seals

Knowledge repository

Philopatry means that an animal stays with a certain area or returns to it. In the study of Sealcentre Pieterburen about this topic, we examine birth philopatry. This means that the seal would return to their birthplace to breed.

See also

Research on birth philopatry

Our research team studies whether pups from the previous season return to the Dollard to breed and whether this also happens every year. Each seal pup is identified at birth. We do this with the photos we take every year of every seal that comes on the beach, so we can see if they have returned. This method is called photo identification.

Disturbance impacts the results

We also look at several factors that may influence this result. If a seal is severely disturbed several times by noise or dogs running loose, it may become stressed and remember that the place is not safe and therefore won’t return. This is also the reason why our research team does its research from a distance, behind a wall with holes in it. The seals do not see us.

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Field research into behaviour of seals

Knowledge repository

With the research we conduct in the field, we study and analyse the behaviour of seal mothers and pups in the wild. An example of this is our ongoing research in the Dollard area. On this page you learn everything about studying seals in the wild.

See also

  • Field research

  • Wetenschappers bij kijkmuur Dollard

  • Camera door observatiemuur - veldonderzoek

  • Veldonderzoek - zeehonden door kijkmuur observeren

Working with the tides

The work our research team does is dependent on the tides. The observation hours are determined, so to speak, by high tide and low tide. Our researchers are present at the observation wall four hours before high tide. At low tide, seals rest on dry sandbanks in the sea, but when the tide comes in, they move to the shore. During high tide, the seals are best seen up close. Three hours after high tide, they finish observing. They study the seals' behaviour during this time.

Research equipment

Do you know what all the research team needs? The team carries various materials to make the investigation run smoothly. For example::

  • Cameras for the photo identification of seals
  • Lenses
  • Batteries and SD cards
  • Tripods on which the cameras can be placed
  • Field notepad and pens
  • Tides time table and weather forecast
  • A lot of patience

    They also take materials that can be useful when a seal needs to be taken care of after observation. The seal is then taken in a large basket. Below are the materials that are needed for a seal rescue:

    • Towels
    • Scrubs and overals
    • Jar, funnel, tubes, water, ORS
    • A scale
    • Blue spray to mark the pups (in the rest of the province Groningen the colour orange is used)
    • Plastic bags for rubbish
    • Gloves, masks and blue shoes

    Wrap-up observation day

    At the end of the day, the team returns to the Sealcentre to officially close the observation day. The collected data are transferred to a hard disk. All important events such as births, marked animals or pickups are written down in the notebook. Lastly, an update is given to the rest of the team.

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    How do you identify a seal?

    Knowledge repository

    There are two ways to identify a seal. The first way is the 'capture-mark-recapture' method. This involves capturing a seal and marking it with a spray. We prefer to avoid this method. Because capturing a seal is very stressful. Stress changes the seal's behaviour and thus the results of the study.

    See also

    Foto identification of seals in the Dollard

    Our researchers therefore use the method photo identification. They take a photo of all seals coming out of the water. They do this immediately because the seal's fur is still wet. On the wet fur, the seal's pattern can be seen more clearly. For each photo, our researchers circle what strikes them most about the seal. Think patterns or scars. If the pattern matches photos taken before, we know it is the same seal.

    To make this process faster and easier, they work with programme Photo-ID. A photo of a seal is put into the programme. The programme examines this photo and recognises the seal's patterns. Then the photo is automatically matched to one of the seals in the list.

    Pattern is unique for every seal

    Below is an example of how we can recognise seals. Near the arrows are prominent spots. As you can see in the photo, the fur has changed colour, but the spots are still visible. The arrow below his flipper points to three dots that are the same in both photos. The arrow to his head indicates a kind of white spot with four dots. Besides the programme automatically recognising the photos, it is important for us to double-check for accuracy ourselves. After all, we already have 400 seals in the database.

    Hoe herken je een zeehond?

    Identifying a seal

    In the summer of 2021, there was a seal in De Onlanden – an area in the province of Drenthe. A special event, because you don't often see a seal in fresh water. After many hours of taking photos from a great distance, we managed to get a good picture of its tag number (this is on a small label on its rear flipper) and spot pattern. It turned out to be an old friend of us. The seal has a tag number 20-115. That's how we knew it was seal Thor. We also put the photo in the photo-ID programme. Through Bea and Marga's research, every seal that has been at the centre has been photographed.

    Zeehond Thor in de opvang
    Zeehond Thor in De Onlanden

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    Knowledge repository

    You have probably heard of the word 'albino'. Most likely, you think of white rats or white rabbits with red eyes. There are also people with albinism. But did you know that there are also albino seals? In this article you can read about what albinism means and what it looks like in seals.

    See also

    What is albinism?

    Albinism, the opposite of melanis , is a rare disorder in which little or no pigment melanin is made at all [1]. Without pigment, the skin color, hair and eyes look different. Usually, albino animals have red eyes and white fur, but that is not always the case. There are multiple forms of albinism with differences in hair, skin and eye color.

    As far as we know, we have treated at least seven albino seals in our Sealcentre. Seal Sealas the most recent case of this (2017). 

    Partially albino?

    We have had different forms of albino seals in the sanctuary. One of the variations is called ocular albinism. Seal Sealas is a good example of this. In ocular albinism, it is mainly the eyes that lack the pigment melanin. That is why these albino seals have red eyes, but no white fur, for example. The fur is lighter in color than that of the congeners.

    Albino zeehond in Pieterburen

    Albino seals are sensitive to light

    Albino seals are more sensitive to light and therefore behave differently. We saw the seal Sealas close her eyes above water to protect her eyes from the sun (see photo 4). The sun is dangerous for the sensitive skin and eyes of albino animals. It doesn't matter much for a seal to hunt, because seals hunt underwater and their whiskers are more important than their eyes. But above water, they are more at risk of sun damage.


    There is also leucism. In leucism, the pigment melanin is produced, but it is not expressed in the coat or skin. This means that only the coat is lighter in color and the eyes are normally colored. The eyes of these seals are in any case less sensitive to the sun than those of albino seals. In 2009, we captured a leucistic seal, appropriately named Golden Queen.

    Leucisme bij zeehonden

    Watch this video

    Would you like to see what an albino seal looks like? Then watch the video below of seal Sealas at our centre:

    1. Fertl, D., & Rosel, P. E. (2009). Albinism. Encyclopedia of Marine Mammals, 24–26. doi:10.1016/b978-0-12-373553-9.00006-7 

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    The values of the Wadden Sea

    Knowledge repository

    The Wadden Sea is a heritage. That means we think the area is very important. It has special values for us. We want to pass that on to our children (and their children after them). So we want to protect the area. Would you, too? And why?

    See also

    • Zeehondenmoeder en pup

    • Wadden Sea

    • Zeehondenpup

    • Scholeksters

    What does the Wadden Sea mean to you?

    The Wadden Sea is extremely important to us. Not just because it is the habitat of the seals. But many other animals and plants also depend on the Wadden Sea. Furthermore many people depend on the Wadden Sea; they live on the coast or on the islands and eat the fish that are caught there. We also think the Wadden Sea is simply very beautiful. These are the values of the Wadden Sea for us.

    Many people find the Wadden Sea important. This differs per person: different people have different values. Some people find the Wadden Sea especially important because many birds make a stop in the Wadden Sea area on their migration from the Arctic to Africa. Other people especially like the wide landscape and walking in the mud. And other people find it very special that the landscape is constantly changing; after all, the Wadden Sea is different every hour.

    UNESCO World Heritage

    As so many people think that the Wadden Sea has special values, UNESCO recognizes the Wadden Sea as a World Heritage Site. This means that UNESCO believes that the Wadden Sea is important for everyone in the world: it has an Exceptional Universal Value. UNESCO has determined that by a criteria of ten points. The Wadden Sea meets the last three of those ten criteria.

    Criterion 8: Geological processes

    The Wadden Sea is constantly changing. Ebb and flow alternate: twice a day the Wadden Sea changes from sea to land and back again. Together with the wind, the tide ensures that sand and silt disappear in some places, while remain in other places. The tide makes the area very dynamic. Islands, sandbanks, arbours, channels, salt marshes and dunes are formed, shifted and broken down again. That process goes on constantly, in most places without human intervention. It happens almost nowhere in the world that an area can form itself in this way. It is very special that this is possible in the Wadden Sea.

    Criterion 9: Ecological and biological processes

    Many different plants and animals live in the Wadden Sea. Many of these plants and animals occur in large numbers. In other words, there is a large biodiversity and a large biomass. Many animal and plant species like places where two areas merge. In the Wadden Sea, land turns into sea and fresh water turns into salt. That is why many animals and plants feel like home there.

    The Wadden Sea is especially important for migrating birds who stop there during their journey from, for example, Greenland to West Africa. They find a lot of food here. The Wadden Sea is therefore also extremely important for life in other places in the world where other animal and plant species depend on these migrating birds.

    Criterion 10: Biodiversity

    Up to 10,000 species of plants and animals live in the Wadden Sea. Because there is so much food for birds, up to 6.1 million birds can live in the Wadden Sea at the same time. Up to 12 million birds pass through the area every year. In addition to having plenty of food, there is little disturbance to birds, allowing them to rest in peace during their long journeys from the north to the south of the planet and back again. Some species fly all the way from northern Greenland, Canada and Russia via the Wadden Sea to western and southern Africa, such as the Bissagos Islands in Guinea-Bissau and Banc d'Arguin in Mauritania.

    Did you know...

    The Wadden Sea is not only important for biodiversity in the Netherlands, Germany and Denmark, but The Wadden Sea is important for a huge part of the world? 

    Conservation of the Wadden Sea

    Because of these geological, ecological and biological processes and the large biodiversity and mass, UNESCO believes that the Wadden Sea should be protected. We wholeheartedly agree with that. The Wadden Sea is extremely important. Not only in itself, but also because many species and areas in other places on Earth depend on this area. In addition, attention to geological, biological and ecological processes and biodiversity is important for everyone The existence of everyone in the world depends on it. Nevertheless, the Wadden Sea is still threatenedin various ways. That is why we are committed to a healthy and sustainable future for the Wadden Sea.

    Rijksdienst voor het Cultureel Erfgoed. Waddenzee: Uitzonderlijke Universele Waarden (OUVs). 2019.
    Wadden Sea World Heritage. One Wadden Sea: One Global Heritage.

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    Viruses in seals

    Science institute

    Just like humans, seals can get sick from viruses. Some viruses only make seals temporarily ill, but some can be fatal. In the centre, we conduct research into the viruses that seals can carry with them, and — wherever possible — try to fight them.

    See also

    • Zeehondenvirusuitbraak

    • Vele dode zeehonden

    • Uitbraak zeehondenvirus

    • Monster afnemen bij zeehond

    The seal virus

    One of the deadliest viruses is the Phoca Distemper Virus (PDV)This virus, also called the seal virus, is very contagious. It is transmitted when animals come into contact with each other or by virus droplets floating in the air.  

    Symptoms of the seal virus

    When seals are infected, they develop symptoms such as heavy breathing, coughing, fever, and problems with their nervous system. The virus also makes them more sensitive to other infections. Ultimately, PDV can result in death.

    Virus outbreak

    The seal virus caused a massive number of deaths among seals in 1988, and again in 2002. In 1988, more than half of the Dutch seal population died. Back then, the cause of the deaths was still unknown, so Lenie 't Hart asked virologist Ab Osterhaus to investigate it. That was when he discovered the phocine distemper virus (currently known as the phocine morbillivirus).

    In 2002, the virus struck again. And again, almost half of the seals in the Netherlands died. The seals that washed ashore were taken from the beach to a research laboratory in Groningen. More than 1200 dead seals were investigated for the cause of death on behalf of the Seal Centre. After fifteen years, the phocine distemper virus had returned. This means that there is a chance that another outbreak will occur in the future.

    Did you know...

    It is unknown how common seals (Phoca vitulina) first got the virus. One theory is that the virus comes from another seal species: the harp seal (Pagophilus groenlandicus). The harp seal is a species of seal that lives in the Arctic. There is a chance that these seals already carried the virus and passed it on to common seals.

    Finger on the pulse

    We think it is important to keep an eye on the seals in the wild. One way we do this, is by taking blood and saliva from every seal we rehabilitate. These samples are analysed by our veterinarians to check for virus particles. This way, we keep an eye on the health of the seal population in the Wadden Sea. It also aids us preparing for a possible new virus outbreak. 

    Watch a mini documentary below about how we monitor a virus outbreak in the Wadden Sea:

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    Antibiotic resistance in the Wadden Sea

    Science institute

    When you get sick from a bacterial infection, you can treat it with antibiotics. These are drugs that can kill bacteria. If antibiotics are used too often, bacteria can become resistant to them, making them less effective against infections. Our veterinarian Ana Rubio Garcia tests rescued seals for bacteria that have become resistant to antibiotics. This research can tell us more about antibiotic resistance in the Wadden Sea.

    See also

    • Zeehondenmoeder en pup

    • Medicatie voor zeehonden

    • Onderzoek naar antibioticaresistentie bij zeehonden

    What is antibiotic resistance?

    If people use antibiotics too often, the bacteria they are supposed to treat can become resistant. This means that the bacteria are no longer sensitive to the drug—they won’t be killed by it anymore. This is very problematic. If it keeps happening, the chance of healing a bacterial infection becomes smaller and smaller. New medicines would have to be developed to fight these bacteria. That is why it is important to only use antibiotics when necessary.

    How does this end up in the Wadden Sea?

    There is a chance that antibiotic-resistant bacteria end up in the Wadden Sea. When these bacteria leave our body, they get into the sewers through the water we use. From there, they can find their way into rivers and eventually the sea. For example, seals might come into contact with this type of bacteria and ingest them. By examining these seals, we can demonstrate that there are antibiotic-resistant bacteria in the Wadden Sea.

    Onderzoek naar bacteriën bij zeehonden

    Bacteria in seals

    Ana wants to know if seals have bacteria in their bodies that are not sensitive to antibiotics. Her doctoral research (PhD) concerns all microscopic organisms that can occur in a seal, such as bacteria, viruses, and fungi. As a part of her research, Ana investigates whether antibiotic-resistant bacteria are common in seals.

    We carry out this research on the seals that we have in rehabilitation. With each intake, we take a sample from the seal’s anus with a cotton swab. The cells in this sample are then grown in our laboratory. By multiplying cell samples in the lab, we can see which types of organisms there are.

    Did you know...

    We work together with several parties to monitor antibiotic resistance in the Wadden Sea. The research is supervised by the Faculty of Veterinary Medicine of Utrecht University (UU) and the Department of Medical Microbiology and Infection Prevention of the University Medical Centre Groningen (UMCG). Determining the type of bacteria and the presence of antibiotic resistance is done by the IZORE laboratory in Leeuwarden.

    Results of the research

    Ana's research shows various results. For example, seals have many of the same types of bacteria in their bodies as humans. She also found very few bacteria that were no longer sensitive to antibiotics. In other words: there are not a lot of antibiotic-resistant bacteria in seals. That is good news! For now, things seem to be going well with antibiotic resistance in the Wadden Sea.

    Antibiotic treatment in seals

    We also looked at what happens to a seal in rehabilitation if it needs antibiotics to treat an infection. It appears that there is no difference between the bacteria of seals that received the treatment and seals that didn’t. This means that the seals’ gut bacteria returned to normal after treatment. This is very striking, since the recovery of natural gut bacteria in humans can sometimes take months or years. This implies that the use of antibiotics in rehabilitated seals will have no effect on the Wadden Sea when they are released.

    The seal as bio-indicator for antibiotic resistance

    Seals are an important bio-indicator for antibiotic resistance in the Wadden Sea—they show us how things are going with their environment. By continuing this research, we can keep an eye on whether or not the situation changes.

    Watch a mini-documentary below about how the centre monitors antibiotic resistance in seals:  

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    Entangled seals

    Knowledge repository

    Every year, we receive dozens of reports of entangled seals. They get trapped in litter floating around in the sea. This could be a fishing net, balloon ribbon or a frisbee. We unfortunately come across the craziest things. The waste can cause deep wounds, or worse: it can kill the seal.  

    See also

    • Verstrikte zeehond

    • Zeehond verstrikt in visnet

    • Verstrikte zeehond in zeehondenopvang

    Plastic soup and seals

    Trash floating around in the sea is often plastic floating around on the water or underwater. Seals (and other animals) swim through it due to their curious nature. Or they mistake it for prey and get stuck that way. 

    Veterinarian Anna conducted research

    Since 2010, we have been tracking the number of reports of entangled seals. Veterinarian and researcher Anna Salazar Casals conducted research into the numbers and types of entanglements in the Netherlands. The data comes from a joint study by seal sanctuaries Zeehondencentrum Pieterburen, Aseal and Ecomare in collaboration with the Max Planck Institute. 

    Did you know...

    Anna is lead author of the scientific publication, which you can read back here:  

    Zeehond in duinen

    Amount of entanglements

    In the graph below, you can see that this number of reports has increased on average: in 2010, there were 'only' 8 reports, while in 2019, it totalled 38. They came to the startling conclusion that the number of reports has increased four times over the past 11 years.  

    Meldingen verstrikte zeehonden 2010-2020

    Type of waste

    Interestingly, 88 per cent of entanglements among seals are caused by material from fishing, such as fishing nets, fishing lines and fish hooks. The remaining 12 per cent comes from other material. We have caught entangled seals at the centre that were even caught in a t-shirt or a frisbee. 

    88% van verstrikkingsmateriaal komt uit visserij

    Type of entanglement in seals

    Seals can become entangled in different ways, as the picture shows. Most of these seals are trapped with their necks. These are mainly grey seals (Halichoerus grypus). They can also swallow litter such as fish hooks - with disastrous consequences for the animal. The danger of these entanglements is the damage it causes. A fishing line can cut through the seal's skin and damage its muscles, blood vessels and organs. As long as the depth of the wound is limited to their fat layer, seals can survive an entanglement. 

    Verschillende soorten verstrikkingen in plastic bij zeehonden

    Young seals fish behind the net

    The majority of entangled seals are young seals (juvenile). Due to their curiosity or playfulness, they probably put their head through plastic or rope or they want to play with it. As a result, the animal becomes so entangled that they are unable to free themselves. Because these are often young animals, they are also still growing. As a result, the entanglement gets tighter and tighter. And the wounds become much worse. 

    Verschillende verstrikkingen gewone en grijze zeehond

    How do we help entangled seals?

    If we find an entangled seal, we can usually help. In 3 out of 10 cases, we can help the seal on site and release it immediately. We prefer this, of course, because then the animal does not have to go to the shelter.

    For 4 out of 10 entangled seals, shelter and further care are needed. We cannot help the remaining 30 per cent of entangled seals: in 20 per cent of cases, we cannot catch the animal and in 10 per cent of cases, the animal has already died.   

    Seal guards look at an entangled seal on the beach to see what help is needed. For example, if the fishing net has not yet caused injuries, then the person can immediately cut the fishing net loose and release the seal. If the seal needs medical attention for the injuries, the seal will be taken to a shelter.  

    Zeehond Tefity

    At the centre, our veterinarians check and take care of wounds on entry. Seals usually have a deep fat layer of a few centimetres; this means that the wound can sometimes be quite deep without the seal being injured inside the body. Once the wound is clean and grown closed, the seal can sometimes be released after a few weeks. The only thing the seal is left with is a scar, as you can see in the photo of seal Tefiti.

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    Common seal

    Knowledge repository

    Scientific name: Phoca vitulina
    Family: Phocidae
    Size:: male: 1.60 meter; female: 1.50 meter
    Weight: male: 87 kilo; female: 65 kilo
    Habitat: entire Northern hemisphere
    Endangered status: not endangered

    See also

    • Gewone zeehond moeder en pups

    • Gewone zeehonden in de Waddenzee

    • Zeehonden rusten op zandbank

    "The other name of this species reveals where it is common: harbour seal"

    External characteristics of the common seal

    The common seal has a fairly round head and a blunt snout. The nostrils are a v-shape. Their face looks a bit like a cat. Their fur is very short, smooth and has a light brown or grey colour with dark spots on it. The spot pattern is unique for each seal. You can think of it as a human fingerprint.

    The common seal is relatively small compared to other seal species. In the Netherlands, the males can reach an average length of 160 cm and weigh 87 kg. The females are on average 148 cm long and weigh 65 kg. The average size of the common seal varies by region.

    In Japan, for example, there is a common seal population that is getting a lot bigger. Namely 186 cm long and between 87 and 170 kg for the males and 169 cm long and between 65 and 142 kg for the females.

    In the wild, common seals live about 20-30 years. In captivity, they can grow even older. For example, common seal “Piet” spent his entire life in Ecomare on Texel, until he died in 2009 at the age of 41.

    Gender differences

    There are few differences between males and females. This is because their coats are the same colour and they are almost the same size.

    Did you know...

    Harbour seals regularly swim up in rivers and can stay there for a long time before returning to the sea. In 2022, for example, a common seal may have lived for a month in a spur of the Lek in Gelderland.

    Distribution and status

    The English name of the common seal is "harbour seal". That says a lot about where this species occurs. The common seal lives quite close to the coast and can therefore sometimes be seen in harbours.

    The habitat of the common seal is spread throughout the Northern hemisphere, but they always stay close to the coast.

    The common seal is not considered to be endangered. According to IUCN, the international conservation organisation, the population worldwide consists of about 600,000 common seals. That number is large enough and ensures that the world population is stable. This means that no intervention is necessary for the species to continue to exist.

    The common seal in the Netherlands

    Every year, about 8,000 common seals are counted in the Netherlands and scientists estimate that about 10,000 common seals use Dutch waters.

    Did you know...

    That there are two types of seals living in the Netherlands? The common seal is one of them. Do you know the other? You can find the answer here.

    The largest population of seals live in the Wadden area. There they use the sand banks to rest, reproduce and give birth to pups. They hunt and eat in the North Sea. Common seals also live in Zeeland, but a lot less than in the Wadden area. There, they mainly live on sandbanks in the Oosterschelde and along the North Sea coast.

    In recent years, the number of seals that are counted annually in the Netherlands and in the entire Wadden Sea has been more or less the same. In the Netherlands, it was quite recently that the population of common seals was in danger. Until 1962 there was still a lot of hunting of the common seal. There have also been outbreaks of the seal virus (Phocine Distemper Virus) in 1988 and 2002 in which half the population died. In both cases, the population quickly recovered. Also with the help of shelters along the entire Wadden Sea coast.

    Diet and foraging

    The common seal is an opportunistic hunter. It means that this species is flexible when it comes to where and what kind of prey it hunts. This allows them to look for food close to their resting place. But sometimes seals swim miles away from their resting spot, either along the coastline or further out to sea to hunt. They can dive to depths of more than 500 meters for food.

    In addition, seals do not hunt one particular type of fish, squid or shrimp. Their diet varies greatly depending on the region and season. In the Netherlands they mainly hunt small to medium-sized fish species, such as cod, hake, mackerel, herring, sardines, allis shad, capelin, sculpin, various sandeels, flatfish and salmon species. In addition to those fish species, the common seal also often hunts squid and crustaceans such as shrimps and crabs.

    Did you know...

    That the common can hunt effectively in poor visual conditions, for example in dark murky waters?

    The common seal, like many other seal species, uses its whiskers to find prey. Their specially shaped whiskers are super sensitive and can detect the smallest vibrations. Due to their irregularity and wavy shape (unlike the smooth whiskers you've probably seen on a cat or dog), the whiskers are so streamlined that they glide smoothly through the water.

    Thanks to research we know that the common seal can still feel the vibrations in the water of a fish that swam away a while before. So they use their whiskers in a similar way that whales and dolphins use echolocation: by picking up vibrations of sound waves in the water. With seals it is all about movements in the water.

    Common seal behavior

    Common seals are solitary animals. That means that they prefer to be alone. On land, seals may lie in groups, because then it is more likely that one of the seals will see predators approaching. They also want to remain close to the water's edge, so that they can quickly flee when disturbed. They often do this at the slightest threat.

    Have you ever seen a seal 'wave'? They are not doing this to say hello. On the contrary: it is a threat. With this waving, the seal means that if the threat approaches, it risks being scratched with the long nails on the front flipper.

    Towards the end of summer/beginning of autumn, common seals are more often found on land. They lie together in larger groups. Seals moult two to three months after the mating season: then they shed their old fur for a new coat.

    Reproduction in common seals

    Mating behaviour

    Common seals mate in the water, around the time the pups are weaned (late summer). Males will compete with each other for females. They do this by making noise, diving and fighting with each other. Males often mate with multiple females.

    Diapause and pregnancy

    As with all seal species, the fertilization is followed with an embryonic diapause. A diapause means that there is a time between fertilization and the actual pregnancy. In the common seal, the diapause lasts up to 2.5 months. After this, the fertilized egg implants in the uterus. The total pregnancy (including diapause) lasts 10.5 months.

    In most seal species, more than 85% of females become pregnant each year. Once the common seals reach sexual maturity, they remain reproductive throughout their entire lives.

    Birth and nursing period

    Common seal pups are born on land in the summer. These pups can go into the water almost immediately after birth, which is often necessary when the tide comes in. In the water, pups often hang onto their mother’s back to hitch a ride. Common seal birthing season in the Netherlands usually lasts from June to July.

    Did you know...

    That pups from the common seals grow 17 kilos in the first 4 weeks of their life?

    The pup are nursed by their mother for 4 weeks with milk containing approximately 40% fat. The puppies then gain weight quickly. They grow from 8 kg at birth to 25 kg in 4 weeks. After this they are completely abandoned and they have to learn to hunt on their own. Fortunately the instinct of young seals to learn this on their own is very strong.

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    Pinnipeds (pinnipedia)

    Knowledge repository

    The pinnipeds (Pinnipedia) are a group of marine mammals comprising of three families: the seals (Phocidae), the sea lions (Otariidae) and the walrus (Odobenidae). They are all members of the order Carnivora, the carnivorous mammals. 

    This order can be divided into two sub-orders: the felines (Feliformia) and the canids (Caniformia). Pinnipeds belong to the latter group: the canids. They share an ancestor with the marten families (Mustelidae). This ancestor will have looked a bit like an otter.

    See also

    • Zeehond in zee

    • Zeeleeuw op het strand

    • Grijze zeehonden op het strand

    Did you know...

    The paws of seals are called flippers?

    What do pinnipeds look like?

    Pinnipeds all have a long body with large chest and narrow flared abdomen. Their main characteristic is the shape of the legs. This is also where they get their name from. The legs of pinnipeds are short with very long fingers and toes. The space between the fingers and toes are webbed, making their legs look more like fins. We also call their legs flippers.

    Because they spend much of their lives in cold seas. Their skin is also covered with a short, dense fur. This keeps pinnipeds warm. They can put that to good use in cold water or in the icy areas where they sometimes live. 

    How do pinnipeds move?

    Some marine mammals, such as whales, only live in the water. Pinnipeds are different. They live (largely) partly in the water and partly on land. They all have their own way of moving around, both on land and in the water.

    Mostly pinnipeds use the land to rest, shed hair, mate, and raise pups. But their life mainly takes place in the water. Most pinnipeds can spend days in the water to hunt and eat. They can even sleep in the water. Some species also mate in the water.

    Because they live this way, we call pinnipeds semi-aquatic. Literally it means half in the water.

    Under water

    All pinnipeds are most agile underwater. Their build allows them to move much faster there. This does come in handy, as their food is all found in the water!

    Seals (Phocidae) keep their back flippers against each other while swimming . They then move their abdomen back and forth to push through the water. When you see a seal swimming, they make a bit of the same movement as a fish. Their front flippers are mainly for steering, not for speed.

    Walruses under water
    Walruses (Odobenidae) swim in the same way as seals, so by moving their back flippers back and forth. They use their front flippers to steer and paddle.

    Sea lions under water
    Sea lions (Otariidae) have a different way to swim. They have much longer and stronger front flippers than seals, and use them to gain speed. They move the front flippers up and down, pushing themselves through the water. It resembles the way a bird flies through the sky. Sea lions are a lot more agile than seals and walruses, but can swim for less time.

    On land

    Because their bodies are well adapted for life in the water, pinnipeds on land are much less agile. But there is a difference between the families in how easily they move on land.

    Seals on land
    Seals cannot fold their rear flippers forward. Due to the construction of their bodies, they always point backwards. Instead of walking, they move more like a caterpillar. They lift their backs up, making their rears stick out a bit. Then they push themselves off with the back of their body. By doing this often and in quick succession, it looks like the seal is bouncing a bit. 

    Walruses on land
    The walrus (Odobenidae) can fold the back flippers forward. But their bodies are too big and heavy to lift off the ground. Instead of actually walking, they slide across the land on their stomachs by leaning on their flippers.

    Sea lions on land
    Sea lions (Otariidae) are the most mobile of the three families on land. Their front and rear flippers lift their entire body off the ground (folding the rear flippers forward under their bodies). For example, sea lions can walk on all fours, and even gallop!

    How do pinnipeds live?


    The sea is where pinnipeds get their food from. They hunt all kinds of animals there. Most pinnipeds are opportunistic hunters.. This means they will eat pretty much anything they can get their hands on. Generally, they go for fish, squid, shellfish that they can swallow whole. But sometimes there will be a few that will also go after larger animals such as seabirds and small marine mammals.

    In groups or alone?

    Most pinnipeds are very social. Outside the mating season, walruses and sea lions often lie together in huge groups on land. Large groups of the same species are called colonies. Sea lions also sometimes hunt in smaller groups. They then work together to herd schools of fish together.

    Walruses mainly eat shellfish that live in the seabed. They will stick together in groups while feeding, but do not need each other to hunt.

    Seals vary by species whether they live in groups or alone, but most seal species hunt alone.

    Mating season

    Pinnipeds gather in large numbers on land or ice during the mating season to mate. They usually do this with several partners. Pinnipeds are therefore polygamous. In sea lions, walruses and some seal species, a male will have a group of several females around him. Such a group is called a harem. The male defends his harem against other males and then has the right to mate with all those females.


    Pinnipeds have one pup at a time. Twins are extremely rare. If they did, twins would in most cases not survive. How pinnipeds give birth and care for their pups varies by family and by species, but it is always the females who raise the pups. Males have nothing to do with this.

    Sea lions and seals get a pup almost every year. Walruses care for their pups much longer. They get one pup every 4-5 years.

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