Mating
Mating takes place in the water, usually shortly after the female has left her pup from the previous season. In the common seal, males set up "underwater territories" around good haul-out sites: they sing and growl below the surface to attract females and chase off rivals. In the grey seal, things are rougher. The species is strongly polygynous: a dominant male defends a group of females on the nursery and tries to mate as many of them as possible. Fights between males regularly escalate to serious wounds — the scars on older bulls are usually easy to spot.
Males generally don't become effectively reproductive until they are big enough to beat their rivals. They are physically mature by their fourth or fifth year, but in practice they only succeed in mating from year six to eight onwards.
Delayed implantation
Seals have a trick they share with bears, badgers and sea lions: delayed implantation. After mating a fertilised egg does form, but for about three months it floats as a blastocyst in the womb without attaching. Only then does it implant and true gestation begin. Add it up: about three months of pause plus around 8.5 months of active gestation makes an apparent gestation of roughly 11.5 months. That way mating and pupping can fall in the same favourable season, year after year.
The seal mother keeps a future on pause for three months — only then does the clock really start ticking.
Birth: common seal
The common seal pups in the Dutch summer, usually between mid-June and late July. The birth takes place on an exposed sandbar in the Waddenzee or the Delta, right by the water's edge. Almost always a single pup is born — twins are extremely rare. The young weighs around ten kilos and already has the adult short coat; in common seals the white lanugo coat is shed before birth and stays in the womb.
Because of that shorter coat, a common-seal pup can follow its mother into the water within hours. At high tide, when the sandbar floods, it has to. Mother and pup find each other again under water by scent and sound; on every return to the bar, identity is re-verified.
Birth: grey seal
The grey seal is the exact opposite in timing and strategy. Pups appear in the middle of the Dutch winter, between November and January. They are born in a white woolly coat — the lanugo — which is not waterproof and which provides almost no insulation against cold once wet, only as long as the pup stays dry. That's why a grey-seal pup spends about three weeks on land, on sandbars or beaches that high tide (ideally) does not reach. De Richel near Vlieland is the best-known Dutch nursery; other uninhabited bars are also used for pupping.
A grey-seal pup weighs around fourteen kilos at birth and triples or quadruples that in just over three weeks — it's not unusual to reach about 45 kg in that period. By then the lanugo has been swapped for the first true coat and the animal can take to the water.
Nursing
The nursing period is short, intense and — thanks to the fat content of the milk — unique among mammals.
- Duration (common seal)~3–4 weeks
- Duration (grey seal)~3 weeks
- Fat content of milk~50%
- Weight loss mother~25% of her body weight
- Weight gain pupTripling
Roughly half of that milk is fat. For comparison: cow's milk contains about four percent fat. The mother fasts during nursing for the most part (grey) or eats very little (common). Right after weaning she leaves the pup to hunt again and, soon after, to mate. More on her annual cycle on life cycle; her energy demand is covered on diet.
Abandonment
What looks harsh to human eyes is in the seal world the normal schedule: the mother leaves her pup for good at the end of the nursing period. The pup then has to learn to hunt on its own — without lessons. First it lives off the layer of fat it has built up, then it experiments with small prey (sandeel, gobies, shrimp). Many young animals temporarily lose weight in this phase and become exhausted; some wash ashore as stranded pups. But not every pup on the beach is a problem case — a healthy young seal often just rests there. The rules of thumb are on that page.
Pup mortality
The first years of life are rough. Dutch and British-Scottish research consistently shows mortality on the order of 30 to 50% in the first year. The causes are cumulative: insufficient fat reserves at weaning, a storm that washes young pups off the bar, disturbance that separates mother and pup, bycatch in nets, and simply the inability to learn to hunt fast enough.
Sexual maturity
| Aspect | Common seal | Grey seal |
|---|---|---|
| Female sexual maturity | 3–5 years | 4–6 years |
| Male sexual maturity | 5–6 years (mate later in practice) | 6–8 years (mate later in practice) |
| Pup season | June–July | November–January |
| Mating system | Polygynous, "underwater song" | Polygynous, harem on land |
| Pups per litter | 1 | 1 |
From her first litter onwards a female has, in principle, one pup per year for decades. With good health, a female can raise more than twenty pups in her lifetime. How she gets through that schedule of moulting, gestation and hunting is set out on life cycle; more on the strategy of the big, polygynous male on grey seal.