One of the most astounding things about spending a long time on a seal colony is witnessing the remarkable transformation that female grey seals go through. When they first arrive at the colony and give birth to their pup, they are in prime condition; having spent the preceding months feeding at sea, they build up a thick layer of blubber. This blubber will be their energy reserve for all their activities while on the breeding colony, including producing fat rich milk for their pup. The mums remain with their pups, and do not go back to sea to feed, but rely entirely on these energy stores they have amassed prior to the breeding season.
When they give birth, a typical female grey seal could weigh as much as 180 kg, though some manage over 200 kg (our heaviest this year was 230kg). Their pup, by contrast, will be a mere 17 to 20 kg when first born, and looks rather like a skinny bag of bones!
But that all changes – and very quickly. Mothers nurse their pups on high fat milk (approx. 50% fat content!) for about 18 days. During that time the mother can lose about 4kg per day, and the pup increases by about 2 kg per day. By the end of the 18 days, the tables have turned, the mothers look like a bag of bones, having lost nearly half their original mass, while their pup will have ballooned, with many pups reaching 40-50 kg (some even heavier!) by the time their mums wean them.
Weaning occurs abruptly, after the approx. 18 days, the mum will leave the island to go off to sea and try to fatten up again and regain condition after her hard work on the island. The weaned pups are left to fend for themselves, but that’s a story for another day….