 Weddell
seals are animals of the ice. They live further south than any other
mammal. Between the end of August and early November in the southern hemisphere
spring, the mother seals to be haul themselves out of holes in the ice and
give birth to their pups.
When born, Weddell seal pups look like
unstuffed pyjama 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.
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 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.
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 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.
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 Weddell
seals live on the edge of the ice all year round and dive down beneath it
to feed. When underwater there is frequently little light, particularly
if there is a layer of snow over the ice which can make it very dark indeed.
The seals therefore need good vision to catch their food. |
 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.
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 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.
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  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 probably 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 metres (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 and lasts for 5-25 minutes.
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8/ Weddell Seal (Leptonychotes wedelli)
at breathing holes |
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 top 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 lower
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. |
White Island is an Island in the Ross sea that
has the most southerly population of Weddell seals. These seals are only 1 300 kilometres
from the South Pole, but this is not the only remarkable thing about them.
They are isolated from the rest of the world as
the nearest open sea for them to is too far under the very thick ice of the Ross
ice shelf for them to get out.
These seals are thought to have travelled to this
area between 40 and 90 years ago when a large chunk of permanent ice shelf broke
off. They were then trapped when it reformed behind them and have remained here
ever since. They use cracks in the ice immediately beside White Island to reach
the sea, they must dive about 70 metres here through cracks in the ice before they
get down to the open sea below. In the summer when the sea ice has broken up, it
is still at least 22 kilometres to the next breath at the edge of the ice shelf,
too far for the seals to manage.
So here they remain unable to leave the area and
with a deep dive past walls of ice before they can even begin fishing.
Antarctica Fact File Index
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