Sleeping
Seals sleep in three ways — and all three look odd to a casual onlooker.
- Floating ("bottling")Vertical in the water, only the nose above
- On the seabedIn shallow water, breath held
- On the sandbarIn loose groups, often in large numbers
The first form — floating sleep, called "bottling" in English — has the animal hanging vertically with only the tip of its nose above water. The body bobs on the blubber like an upright bottle. For walkers along the coast it can be a puzzling sight: a seal that doesn't move, with what looks like a metre-long pole sticking out of the water.
The second form is sleeping on the bottom in shallow water, often in sheltered channels. The animal holds its breath and surfaces regularly to breathe — without fully waking up. Brain scans of seals in laboratories show a kind of half-awake pattern that resembles what dolphins do, though in Phocidae it is less pronounced.
The third form is the most familiar image: dozens to hundreds of seals on an exposed sandbar, in loose groups. That has nothing to do with social bonding as in elephants or orcas — it's functional. The bar is a safe place, exposed enough around high tide to be avoided by large predators, but dry long enough to rest and moult on.
Diving and the dive reflex
On every dive an entire cascade unfolds in the body within seconds. The heart rate drops from about 120 beats per minute at the surface to often less than ten per minute at depth. Blood vessels in flippers and skin constrict; blood is concentrated in heart, brain and core. Muscle tissue runs largely on its own myoglobin store, in part anaerobically.
A diving seal is almost a different animal: its heart beats slower than that of a sleeping human, while it is actively hunting.
The result: a typical hunting dive of a few minutes at 10 to 40 metres, with outliers reaching around 100 metres and twenty minutes. In between, the seal empties its lungs at the surface, refills — and dives again. The anatomical basis for all of this, from myoglobin to blubber, is on anatomy.
Communication
Seals are not silent — they produce a broad range of sounds, above and below water. Above water you'll hear on the sandbar growls, short barks and, in disturbed animals, a kind of hissing snort. Pups call their mother with a thin, high "mewing" sound (the "wail" that gives the Dutch word huiler its name).
Under water it gets richer. Male grey seals in particular produce long, melodic "songs" of cries and groans during the mating season, easily picked up on a hydrophone. It's the underwater equivalent of a territorial call. Male common seals have their own, shorter underwater calls, also aimed at attracting females and chasing rivals. Short clicks have also been described, possibly involved in spatial orientation, although echolocation as in dolphins has not been demonstrated.
Social behaviour on sandbars
At first glance the groups on a bar look tight — often dozens of animals lie just a few body lengths apart. In reality the groups are looser than those of eared seals (sea lions, fur seals), which form genuine colonies with dominant males and harems on land. In common seals in the Waddenzee, the composition of a haul-out changes from day to day; individuals are site-faithful — they prefer to return to the same spots — but not to a fixed group.
In grey seals, around the nursery (such as de Richel near Vlieland) you do see a pattern moving towards a colony: a dominant male defends an area of females and pups in winter. Outside the breeding season that structure dissolves again and the grey seal too is "loose".
| Aspect | On the bar | Under water |
|---|---|---|
| Social pattern | Loose group, site-faithful | Individual |
| Communication | Growling, barking, hissing | Song, clicks, groans |
| Activity | Resting, moulting, nursing | Hunting, mating, travelling |
| Alertness | High (quick to the water) | Variable |
Response to disturbance
A seal on the bar is permanently alert. The standard startle pattern is strikingly layered: first the animal lifts its head, then the entire upper body (the "banana pose"), and on further approach it bolts in a wave motion towards the water. To onlookers it can look endearing; for the seal it's serious. A flight costs hundreds to thousands of kilojoules — energy it could not afford to lose if it still needed to moult, nurse or carry a pregnancy.
Studies in the Wadden area have repeatedly shown that kayaks, SUP boards, drone shadows and walkers cause disturbance within a few hundred metres. With repeated disturbance, animals abandon whole sandbars — which in dense areas is a net loss for the species as a whole. On spotting you'll find the rule: 500 metres distance, binoculars, no movement. For what to do if you find an animal close up, see pup found.
Play behaviour
Pups and subadults show recognisable play behaviour, both on the bar and in the water. In the shallows around the bar you can regularly see young animals chasing each other, diving in circles, or slapping flippers against each other. That has a physical training function: muscle coordination, breath control, social calibration. Older animals also play sporadically, mainly in spring — but it is much less pronounced than in pups. A seal that survives its first full year learns a lot in this play that it will later use professionally.
The pattern of behaviour through the year
All the behaviours described above shift in intensity over the year. In June–July the action on the Waddenzee sandbars centres on pups and nursing (common seal); in August–October on moulting; in November–January on the grey seal nurseries; in between, on hunting and recovering condition. For seeing seals in a specific season, spotting helps. For the full annual programme and life course: life cycle. How the body makes all of this possible — blubber, vibrissae, dive reflex — is on anatomy.