*Extract from a longer chapter on comparative locomotion. This post contains no horses.
The first polar bear I encountered
was in the Anatomy Department of the Bristol Veterinary School, and he was
already dead. Bears mark a point close to our hearts in the evolutionary tree –
being capable of both two-legged (bipedal) and four-legged (quadrupedal)
locomotion, but most impressively being able to casually choose between the
two. Polar bears don’t just use an upright stance to reach up a tree or get a
better angle on an anthill; they can stand and walk upright any time they want
to look a little taller or see a little further. They manage this with a
specialised spine and pelvis, and that is what makes them exciting to anatomy
departments.
Bear skeletons on the whole are so
similar to human skeletons that the first forensics lecturer I met at Bristol
claimed to have moved there from North America partly because he could not face
being called out to another “crime site body” that was actually a bear skeleton
with not a single bone of human remains. There are enough subtle differences
between human and bear skeletons though, that bears can achieve high speeds
using four legs, on land or in water, and we can only watch. The polar bear is so
good in water it is almost amphibious. There are some disadvantages to
this adaptability, as they had learnt in Bristol.
The Bristol polar bear skeleton
(pictured) had a name, Nina, and a friend, Misha, and both bears had come from
Bristol Zoo. Nina was imported into Bristol Zoo to partner Sebastian, a polar
bear who was born there. When Sebastian passed away in the 80s yet another
bear, Misha, was imported to keep Nina company. People don’t like to see
lonely animals.
Misha had spent her life as a
circus bear and arrived with a habit of pacing forward and back and nodding.
Whilst the zoo enclosure represented some seriously improved conditions for this
previously-caged bear, polar bears were already controversial zoo exhibits. The
general public had and still have a natural concern for animal welfare and so
it was considered necessary to keep Misha and her weird movements away from the
public gaze. By 1992 both bears had reached a remarkable age but Misha’s pacing
had grown more pronounced, accompanied by an even more exaggerated head waving.
The attempt to rehabilitate her was sadly exhausted at this point and she was
put down. Within six months, Nina the remaining bear had also began to pace,
and the decision was made that it would be kindest if she too was quietly put
to sleep. Both skeletons were sent to Bristol University Veterinary School for
post-mortem, and there it was discovered that these marvellous spines that
allowed these bears to do so many things also suffered from arthritis.
The very balance of
stability and motion that gave the bears options also caused their spines to
degenerate, and the bears had been stiffening up and becoming less able to
enjoy a full range of motion. In order to stop their spines from seizing up
altogether the bears seemed to have developed a series of exercises and
stretches – pacing up and down, waving their heads, putting their spines
through their paces and keeping the motion alive. It was this, the imperative
to stay mobile, and not the mental distress suspected, that eventually saw the
bears destroyed.
For all their popularity and
charisma when not caged, polar bears are rather dangerous to work with. They
belong to that special selection of creatures which, even when captive, are too
fluffy to put in front of a blue screen in film making. If you use a blue or
green screen with a polar bear you end up with brilliant action footage of what
looks like a completely shaved bear, when the fine hairs fail to be captured.
As a result polar bears have a skeleton and motion that is frequently recreated
using CGI for visual effects film work. It was wearing this hat that I
encountered my next bear, this time very much alive and called Mercedes.
I met Mercedes in Edinburgh zoo
but unlike Nina and Misha she had been born in the wild. Mercedes came from
the Hudson Bay region of Manitoba, Canada, where she could have lived out the
rest of her life if she had just stayed away from the humans. The Hudson Bay
contains Churchill, which is one of the most bear-prone towns in the world sitting
neatly between polar bear breeding and feeding grounds. Here polar bears get a
three strike rule. If a polar bear wanders into town and disturbs the peace or
generally endangers people it is tranquilized, captured, and labelled with
spray paint before being released away from inhabited areas. These labels mean
that a bear that is a repeated offender is easily recognised and our bear was
labelled dangerous bear number 39. On the third offence the usual course of
action is for the animal to be shot, but in our bear’s case once she had raided
town a third time a local car firm took up the case and offered to pay to have
the polar bear flown to a zoo or place of safety. The bear took on the name of
his benefactor and saviour, but finding a zoo prepared to take a polar bear was
not that simple, even once the search was extended internationally.
Bears may be
unwanted in suburbia, but another place that polar bears really aren’t popular is the zoo. For all the
reasons that Bristol struggled, most zoos will just not rehome a bear. They are
massively expensive to keep, the sizable enclosures are a substantial
investment, and the public just do not like to see them in captivity. Zoo after
zoo declined to help, until Edinburgh offered to take on the bear. Almost immediately
Edinburgh started to struggle with public perceptions – the bear was dirty, was
the bear meant to be dirty (yes), could the bear cope without company (yes),
did the bear have enough space? No captive polar bear could ever have as much
space as a natural territory, so psychologically, no, but Mercedes was getting
enough stimulation to keep her musculo-skeletal system in working order, yes,
and as alternatives go she also wasn’t dead. The public worried that the bear
looked sad and lonely and, as with Bristol, a mate was found. Polar bears are
naturally quite solitary animals and Mercedes hated Barney. The pair successfully
produced two cubs but keepers reported a great improvement to Mercedes’ health
and mental well-being once Barney finally passed away, having accidentally
choked to death on a child’s toy dropped into the enclosure.
The bear’s captivity continued to attract
criticism from the public and in particular their plight attracted violent
threats from the Animal Liberation Front, who started to attack and threaten
the keepers and all those associated with Edinburgh Zoo. Eventually a beautiful
territory was established for Mercedes in the purpose-built Highland Wildlife
Park, and despite concerns about how well she could re-adapt to the larger space
in her old age, she moved in in 2009. By this stage she had reached her
thirties, an age not usually seen in the wild, and like the Bristol bears
struggled as osteoarthritis slowly claimed her mobility. Another bear was taken
on, a two year-old needing to relocate from Holland to keep him a safe distance
from impending cub arrival. Luckily this time lessons had been learnt and
procedures were in place to allow the two bears to stay apart. In 2011, shortly
after her move, Mercedes’ arthritis reached the point where her movements
become strange, repetitive and “indicative of mental deterioration” and the
bear had to be put down.
Arthritis is also
seen in humans, including spinal arthritis. By making the commitment to
bipedalism, or two legs, we’ve sacrificed mobility for stability. Our pelvis is
fully rotated for our upright stance and so supports our more curved spine
better in that position, but the system still fails. Other bears, despite their
long hibernation patterns, do not deteriorate with age and disuse as polar
bears and humans do.
Happily the
similarities don’t end with arthritis. Like humans, ancient mammals, crocodiles
and hedgehogs, polar bears are plantigrade, meaning that they walk on the soles
of their flat feet. This is the primitive unevolved foot position for walking –
with animals that are more specialised for fast locomotion going on to develop
toe-running (digitigrade) and even toenail/hoof running (unguligrade). Landing
on your heels like a polar bear produces a lumbering gait – great for
stability, giving you more support on ice and if you need to balance on just
the two legs, but not so good for speed or shock absorption. Those huge, heavy feet
are never going to swing as rapidly as the hooves of the animals that evolved
to bounce along on the tips of their toes. The polar bear’s feet have some of
the largest, heaviest claws seen on bears and even have fur on, both handy for
ice-grip. Their paws are also webbed, and in water the shuffling gait becomes
an effective but rather cute doggie paddle. At 6mph it’s also speedy and manoeuvrable
enough to be not so cute when chasing you.
In return for all
this extra size and weight, the polar bear gains not speed but power. Humans
have a pair of bones in each forearm, and the same in their legs. Most species
have lost these extra bones and just use the one bone to connect elbow to
wrist, or ankle to knee, making them lighter, faster and more efficient. Polar
bears, like humans, have kept this paired arrangement intact, and keep all
their limb bones thick, dense and strong, and a result are impossibly
inefficient in their movement. Over short distances a polar bear can reach
35mph but it costs them, and this is the main reason that now that the ice is
retreating you hear about polar bears starving to death – they use up a ridiculous
amount of energy just to move, and reduced sea ice requires more movement. In
the summer when the ice retreats altogether polar bears out hunting actually
have an embarrassing and potentially fatal problem with heat stroke. Their only
means of defence is to stay as still as possible, even at the expense of a
meal. The advantage of the double bones, particularly in the forearm, is the
extra anchoring point for muscles which gives polar bear’s limbs the dexterity
to rotate. This makes the polar bear an impressive all-terrain traveller, and
when combined with their short, stocky, limb-bone structure you have a powerful
animal perfectly adapted to swim, dig, and hunt. Just not to run. These ice
bears do not just need a bigger enclosure, they need ice.