At a glance
On a Dutch sandbar common and grey seals often lie mixed together. Once you've looked at both carefully, the difference jumps out. Below are the five points you can use in the field, with the comparison table. At the bottom: one misconception you should let go.
| Feature | Common seal | Grey seal |
|---|---|---|
| Scientific name | Phoca vitulina | Halichoerus grypus |
| Nickname | — | Kegelrob (cone seal) |
| Head | Round, "cat-like" | Cone-shaped, "horse-like" |
| Snout | Short, blunt | Long, straight |
| Nostrils | V-shaped, meeting at the bottom | Nearly parallel, separated |
| Length | 1.3–1.9 m | 1.8–2.5 m |
| Weight | 65–130 kg | 150–300 kg |
| Pup season | June–July (summer) | November–January (winter) |
| Pup at birth | Short coat, swims immediately | White lanugo coat, ~3 wk on land |
1. The head
The single most important point — and the only one that works even at several hundred metres — is the head profile. The common seal has a strikingly round head with a high, domed forehead; many people say it looks like a dog or a cat. The grey seal, by contrast, has a cone-shaped head: the forehead runs into a long, straight snout without an obvious break, a silhouette that suggests a horse, an older dog, or a greyhound. Once you've seen both profiles side by side through binoculars, you rarely get them confused again.
2. The snout
The snout is essentially a follow-on from the head shape, but it's worth looking at separately. The common seal has a short, blunt snout that barely projects from the head — as if someone had pressed it in with a thumb. The grey seal has a long, straight snout that clearly juts forwards; in older males a pronounced "Roman nose" develops on top of that. You'll see the difference most clearly when the animal lifts its head in response to a sound or scent.
3. The nostrils
A fairly decisive detail when the seal lies quietly on the bar and you have good binoculars is the arrangement of the nostrils. On the common seal they form a neat V: the two openings meet at the bottom and splay apart towards the top. On the grey seal they sit almost parallel; a clear vertical band of skin remains visible between the two openings. This feature is commonly shown on comparison plates and is highly reliable — provided you can zoom in far enough.
4. Size
The grey seal is simply much larger. Adult male grey seals weigh 230 to 300 kg and reach up to 2.5 m; females 150 to 200 kg and 1.8 to 2.1 m. Adult common seals top out at 65 to 130 kg and 1.3 to 1.9 m. On a shared haul-out, grey seals almost always tower visibly over the common seals around them, with the heavy hindquarters and thick neck particularly striking. Caveat: young grey seals are of course smaller than older common seals — size only works in combination with head shape.
5. The pups
The difference between the pups is perhaps the most dramatic. Common seal pups appear in June and July, are born already in their adult, mottled coat, and can follow their mother into the water within hours. They have to: their birth site is usually submerged at high tide. Grey seal pups are born between November and January, almost always on a higher, dry sandbar. They wear a dense, almost white lanugo coat that keeps them warm but is not waterproof, and so stay on dry ground for three weeks while nursing. Anyone who sees a white youngster on a Wadden sandbar in the middle of winter need not hesitate: that's a grey seal.
And the coat?
Coat colour sounds like a logical clue but doesn't work well. Both species are extremely variable. A common seal can be dark silver-grey with fine rings, or almost uniformly sandy. A grey seal can be pale beige-grey with a few dark spots (females), or dark anthracite with light patches (males). On top of that, a wet coat always looks darker than a dry one. Coat alone is therefore not a reliable criterion; always use head and snout as the main cue and let the coat play a secondary role.
Practical field tip
Work in a fixed order. Pick up your binoculars, choose one animal on the bar and ask yourself, in this order, four questions: (1) Is the head round or cone-shaped? (2) Is the snout short or long? (3) Do the nostrils splay apart at the top (V) or run parallel? (4) How does this animal compare in size with its neighbours? If all four answers point the same way, you've identified the species with high confidence. Still unsure? Go by the season: summer pups = common; white winter pups = grey.
For full biology, read the profiles of the common seal and the grey seal, or check where to see both species in the Netherlands on the spotting map.