The pup
The first life stage lasts three to four weeks in the common seal and about three weeks in the grey seal. During that period the pup nurses and roughly triples in weight. Right after weaning, the independent phase begins. The pup first lives off its accumulated fat layer, switches to small prey (sandeel, gobies, shrimp) and practises in progressively deeper water. No mother teaches her pup how to hunt — it has to work that out itself, with the right anatomy as its only starting capital. Many young animals temporarily lose weight in this phase. For the birth itself and the nursing period: see reproduction.
Subadult (year 1 to 4)
Biologists call the years between weaning and sexual maturity subadult. It is the stage with the highest mortality: roughly 30 to 50% of all pups do not make it through their first year. Subadults roam widely — more than older animals. Tracked common seals from the Waddenzee have been followed deep into the German Bight and the southern North Sea; young grey seals even cross to British-Scottish colonies. In these years they learn which hunting grounds pay off, which sandbars are safe to rest on, and how to position themselves relative to older animals in the group.
Being a subadult is no fun — it's the stage where every mistake can be your last.
Adult (from 4 to 5 years)
Females usually reach sexual maturity between three and five years (common seal) or four and six years (grey seal). Males are physically ready at the same age, but often only participate effectively in mating from year six or eight — earlier they are too small to compete. An adult seal has a fixed annual cycle: hunting, mating, gestation, moulting, pupping, mating again. He or she is faithful to a limited set of sandbars and hunting grounds within tens of kilometres of the birthplace.
The annual cycle
The cycle of an adult female common seal looks more or less like this, year in, year out:
- June–JulyPupping + nursing
- July–AugustMating (shortly after leaving pup)
- August–OctoberMoulting on the sandbar
- September–DecemberDelayed implantation
- December–JuneActive gestation (~8.5 months)
In the grey seal the whole schedule shifts by six months: pupping in November–January, moulting mainly in spring. The net result is that the two species move through the year on a different tempo on the same sandbar — which actually makes it easier for researchers to count them separately.
Moulting
Once a year every seal renews its coat. The common seal does this mainly in late summer (August–October); the grey seal in spring (often March–May). The animals lie on the bar a lot during moulting — about a month in total, in loose peaks. The reason: the skin receives extra blood flow to nourish the new hairs, and cold water then drains away a lot of heat. Disturbance during this period therefore literally costs extra energy. Research in the Waddenzee shows that moulting is one of the times when the counts are highest — almost everyone is on dry ground. It is therefore also the reference moment for the official counts; see counts.
Older seals
With the years, hunting efficiency slowly declines. The teeth wear down — seals do not get a second set — and older animals lose teeth or develop cracks in them. Cataracts and scars from earlier wounds accumulate. Females often keep producing pups well into their twenties, but the probability of becoming pregnant drops. Males lose their position in the mating system from a certain age onwards — a young, strong male beats them in a fight on the nursery.
Many old animals do not die of old age itself, but from a build-up of poor condition: a winter with little fish, a disease (such as the PDV epidemic that caused skin disease in 1988 and 2002) or an injury that fails to heal. On threats these factors are spelled out.
Lifespan
| Aspect | Common seal | Grey seal |
|---|---|---|
| Average lifespan in the wild | 20–25 years | 25–30 years |
| Maximum documented | ~35 years | 46 years (female, Scotland) |
| Females vs males | Females live longer | Females live longer |
| Sexual maturity | 3–5 years | 4–6 years |
| First pup of female | 4–5 years | 5–7 years |
The oldest reliably dated grey seal is a female from Scotland aged 46, determined by counting growth rings in a canine. In the common seal the upper limit lies lower, around 35 years. The averages are considerably lower because the high mortality in the first years of life drags them down; anyone who makes it past their second year has a good chance of getting much older.
What a life costs
An average seal with a 25-year life puts away around 25 tonnes of fish. It changes coats 25 times, raises — as a female — more than twenty pups, mates more than twenty times, dives an estimated hundreds of thousands of times and probably swims tens of thousands of kilometres through the North Sea. All of that in an increasingly crowded sea, with shipping, fisheries and recreation as new neighbours. How it organises all of that behaviourally is on behaviour. And the current numbers — who makes it and who doesn't — are on counts.