Polar Bears (Ursus maritimus)

Circumpolar Arctic distribution

Canada, Alaska, Russia, Norway and Greenland

Were widely distribution in Pleistocene glacial stages - even Britian

Nuclear genomic DNA analysed on polar, brown and black bears

Sister species = brown-bears

Diverged in middle Peistocene (~600,000ya)

Allows time for polar bear evolution to arctic conditions = distinction

Conforms that polar bears carry introgressed brown bear mitrochondira DNA due to past hybridisation

These are polar and grizzly bears (brown bear subspecies)

2006 - male grizzly x female polar hybrid shot dead in Canada

• First confirmed wild hybrid

Another confirmed in 2010

Shows increasing contact between grizzlies and polar bears

• Grizzlies migrating into polar bear territory

= introgressive hybridisation - movements of genes from one species to another species gene pool by backcrossing

Most respects, typical bear

Proportionally long neck - help drag heavy prey from water & see across distance

Large skull -points for large jaw muscle attachment

• Carnivorous dentition but can eat other food if needed

Shoulder height of 1.7m - total length 2.8m (standing)

• Males 655-800kg (claims of >1000kg)

Strong sexual dimorphism exhibited - males larger

Large feet - help distribute their weight evenly on ice

Allen's rule - endotherms from colder climates have shorter limbs/appendages

• Polar bears = short, stocky legs, small ears and tail to reduce heat loss

Bergmann's rule - races of species that live in a cooler geographic range have larger body size to reduce their SA:V ratio

Hesse's rule (heart-weight rule) - species in colder climates have a larger heart in relation to body = move cooler, more viscous blood around body

Wilson's rule - Arctic animals have thicker layer of subcutaneous fat and longer hairs compared to warmer tropical equivalents

Large and insulated by thick layer of blubber (not as dense as cetaceans)

• Fur = white 
• Skin = black - absorbs heat more easily
• Hairs = hollow - hold air and enhance insulation
• Paw pads covered by thick fur - time walking on ice
  • Pads = traction for walking on slippery ice

Claws - long, curved and sharp

• Smaller than other bear species
• Useful for gripping ice and grabbing/killing prey
• Digits partially webbed = poweful paddle

Dive 25-40m without difficulty <3 minutes

Both sexes cover large distances when forging

• GPS tracked female - swam continually for 232 hours (9.6 days) - covered 687km

Strongly dependent on seals - ringed seal especially

Don't hunt in open water - poor swimming abilities for this

• Wait at seal breathing holes or stalk seals over distance = stealth

Hunt walruses at haul-out sites 

• Cause walrus stampedes by charging them

If they have chance - kill ice-trapped beluga and narwhals

Scavange extensively when desperate - eat berries, kelp, grass, seabirds, eggs, rodents, hares, reindeer

7 human deaths since 1968 - intrustion into human settlements is major problem

Mating on sea ice - April till June

Suspended animation of ova - not implanted till following Autumn

• Females can store fat during spring and summer - increase weight by 200kg
• Gestation occurs during winter
• Lie up in snow dens and stay for 8 months 
  • Freezing temp inside
  • Warmer than -20'C outside

Give birth late Nov-early Jan 

• Usually 2 cubs
• Each ~600g, 30cm long = TINY
• Feed on fat-rich milk (50%)
• Weigh 10-15kg by spring = leave the den
• Male cubs ~10% bigger

Mother takes them to sea ice - survial depends on successful hunting of seal prey