Author: Marco Boshoven

Research in the Dollard has been ongoing since 2015. In all these years, several results have been achieved. We have learned more about the behaviour of seal mothers and their pups. We have also been able to use the results to adjust our rehabilitation policy: we now observe seal pups longer and catch fewer pups. Below we share the insights we've gained so far.

We used to think that a seal pup lying alone along the shore had been abandoned by its mother. They were also called 'howlers', because of the sound they made. But were they really abandoned? Our researchers have followed several mothers over the past few years. They looked at how long it took them to return to their pups.

Thus, we found that puppies were regularly left alone for a while (see the picture below). Some mothers even stayed away for eight hours. Then they would reappear and give their pup milk. This means that a pup alone is not necessarily abandoned. Seal mothers also have to look after themselves and hunt for food. It is quite normal that she cannot be with her pup all the time.

The researchers also looked at the behaviour of mothers and their pups as well as pups of other mothers. They found out that the pups not only drink from their own mother, but also join in drinking from other mothers. Not all mothers allow this; there are mothers who only give milk to their own pup (see image below). But there are also very social mothers, who let dozens of other pups suckle. One mother gave milk to 33 different pups!

Seal pups cannot survive on the milk of other mothers alone. For the first 7-11 days of their life, the pup drinks from its own mother, after which the mother sometimes lets other pups drink from her. If a pup loses their mother within that first 7-11 days time, they can get milk from other mothers. This does not mean that the abandoned pup will survive with only milk from other mothers. In fact, the pup gets most of its milk from its own mother. So other seal mothers cannot replace their own mothers, but collectively they can care for several pups. It is like a daycare centre for seals.

This research has allowed us to change our seal rehabilitation rules. Thanks to these discoveries, seal guards observe pups that are alone for up to 24 to 48 hours. This is a lot longer than before. This gives pups a greater chance of staying with their mothers. Only pups who have really lost their mothers are now taken in.

Did you ever wonder how a blind seal survives in the wild? Seals are used to hunting with low visibility. Most seal species look for their food dozens, sometimes even hundreds, of metres underwater. Light does not penetrate as deeply into the sea. As a result, seals often hunt in the dark. Although their eyes are large and see more underwater than we do, it is the whiskers that are indispensable when hunting.

Seals have very sensitive whiskers, which allow them to detect the smallest movements in water. As such, they find their way through the water current and feel the vibrations given off by fish. These are picked up by the hundreds of nerves in each whisker. Minutes after a fish has swum away, a seal can still detect it. This is because a seal's whiskers are not smooth, as a dog's or cat's whisker, but ribbed. Therefore, they glide smoothly through the water.

This allows Blind, the blind seal seen for years by our researchers in the Dollard, to find enough food without seeing anything.

One of the studies that our researchers conduct in the Dollard, is to analyse ten seal mothers. Over several years, they look at whether their mothering strategy changes. For example, they look at whether she feeds more or fewer pups from other mothers and whether she always feeds the same pups or whether this varies. In addition, they study whether the seal mothers give milk each other’s pups. These things combined is called maternity strategy. 

Allonursing means that a seal gives mothermilk to a pup that is not their own. But why would a seal mother give milk to another pup? The causes and function of allonursing in seals remain mysterious, as giving milk takes an enormous amount of energy for the mother seal. There is also an increased risk of transmission of pathogens between mothers and other pups. It is therefore a crucial research subject.

There are five hypotheses that could explain why seals nurse their other pups:

1. Allonursing is the result of misguided parental behaviour.
2. Seal mothers alternate to nurse each other's pups.
3. Females nurse foreign pups for additional health benefits.
4. Females feed foreign pups to discharge milk not drunk by their own.
5. Inexperienced seals spontaneously give milk without reproducing, or because they have lost their young. In order to improve their mothering.

Dit onderzoek doen we tijdens onze observatie in de Dollard: de periode waarin gewone zeehonden hun pups krijgen. Er worden filmpjes gemaakt van de moeders die melk geven. Met photo identification kunnen we de verschillende pups herkennen.

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.

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.

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.

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.

Met het veldonderzoek is ons team afhankelijk van eb en vloed. Onze onderzoekers zijn vier uur voordat het vloed is aanwezig bij de kijkwand. Met laagwater rusten zeehonden op drooggevallen zandbanken in de zee, maar als het vloed wordt verplaatsen ze zich naar de kust. Tijdens vloed zijn de zeehonden het beste van dichtbij te zien. Drie uur na vloed zijn ze klaar met observeren. Ze bestuderen in deze tijd het gedrag van de zeehonden.

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

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.

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.

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.

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.

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.