Weddell Seals
Leptonychotes wedelli

1/ What are Weddell seals like?

When born, Weddell seal pups look like unstuffed pajama cases, all skin and flippers and not much content. Over the next few weeks the change in mother and pup is like one balloon deflating and filling up another.  

Weddell seals are animals of the ice.

Newly born Weddell seal pups have to be some of the worlds cutest creatures as they flop about the ice in the early days after their birth, not able to co-ordinate their over-sized flippers before they grow into them.  

The mother arrives pregnant and with enough resources of blubber and protein to double the 25kg (55lb) birth weight of a pup in 10 days. She doesn't feed for about the first month and goes from an extremely plump barrel shape just before she gives birth - to a skinny shadow of her former self with ribs visible while the pup reverses the process.

Weddell seal milk is one of the richest produced by any mammal. It contains about 60% fat (go and compare that to the label on the milk carton in the fridge) and it is this that is responsible for the rapid weight gain made by pups shortly after birth.

The pups are weaned (stop drinking milk and begin eating normal seal food, i.e. fish) at around 7 weeks when they should have reached about 110kg (242lb). When adult, they will weigh up to 400kg (880lb) and be up to 3m (10ft) long. Unusually, the males are slightly smaller than the females.

Pups are encouraged into the water very early on by their mothers, perhaps only a week or so after birth. The water is their natural habitat and with their thick protection of blubber is a more comfortable place to be most of the time for these seals than out on the ice where the temperature can be -40° C or less with winds frequently of gale force or greater.

Weddell seals usually have their pups on sea-ice, getting in and out of the sea through a breathing hole. These breathing holes are guarded and kept open by the males during the time when the females give birth. 

The male guarding the hole will defend a territory beneath the ice against other males for access to mates. The females are ready to breed again shortly after the birth of the pup so the males that successfully defend a breathing hole will mate with the mother seals that use this hole, typically this will be a ratio of about 10 to one.

Weddell seals prefer to live on ice that is broken up somewhat, in this way there are often natural cracks and holes through the ice that they can use to get in and out of the sea. There are also holes and cracks around ice bergs that are trapped in sea-ice and often "tide-cracks" appear near when near land, all of these help.

These holes are fine to begin with, but when temperatures are well below freezing, they begin to freeze up - quickly. The seals keep the holes open by rasping them with their teeth. They open their mouths wide and move their heads back and forward in a wide arc attacking the ice that is building up around the sides of the hole. This is a very fast and vigorous process that takes a lot of energy and a toll on the seals teeth.

Keeping breathing holes open like this wears away the teeth of Weddell seals and it is this that means that the Weddells only live to about 18 years old, about half the life-span of a crabeater seal for instance.

Weddells can swim great distances across apparently continuous sea-ice by detecting the natural cracks and holes along the way. When covering distance rather than fishing, they only dive to a shallow depth and find the next breathing hole in the gloom under the ice by sonar - they emit a series of high pitched sounds and pick up the difference in sound when the sounds reach a hole.

The first picture is of a juvenile Weddell seal  weaned from its mother about 2 or 3 months previously and already completely in control in its aquatic environment.

The scene a few months ago was rather different though. Weddell seal pups don't automatically realise that they can or should dive and the early attempts are amusing to watch.

"Attempts" is the wrong word. What actually happens is that the mother pushes the pup into the water against its will. She then pushes its head under the water - again against its will. There is much coughing, spluttering and panic before the pup realises that it can hold its breath under the water and that this in fact does help!

The pups soon get the hang of it though and as adults will dive to up to 600 meters (2 000ft) or more staying under for up to an hour and going as much as 12 kilometres from the breathing hole.

A typical feeding dive takes the seal to 200-400m depth and lasts for 5-25 minutes.

Weddell seals are very hardy, resourceful and quite remarkably behaviourally adapted for life in the Antarctic pack and fast-ice as these two pictures show. In the right picture, the seal has found a breathing hole through pieces of only partially consolidated pack-ice where there is a non-frozen portion that is nonetheless filled with slush. From below such a region will let considerably more light through than the thick pack-ice pieces and stand out like a beacon to a seal swimming by, even if it is solid, it will be thin and probably thin enough for the seal to break through. Weddell seals have no land-based predators and so there is no danger to them of coming up to such breathing holes, just the odd surprise if there's a wandering scientist nearby to capture the moment on camera.

The left picture is of a Weddell seal that has made a hole in apparently unbroken, though quite thin fast-ice and hauled out for a rest. We came across this seal while out several miles from the shore on recently formed and very hard and strong, but disconcertingly thin ice. In fact we didn't realise how thin the ice was until we came across this seal and the hole it had made. It was entirely unperturbed by a group of 5 people manhauling a heavily laden sledge with camping gear as we went off on our holidays and treated us as if we weren't really there at all. Seals probably live a fairly surreal life anyhow.