Polar Bears: How do the coolest animals in the Arctic stay warm?
Most of the life of the iconic polar bear is spent wandering vast sheets of drifting sea ice. They are largely considered to be semi-aquatic, and this status is well supported by their remarkable swimming abilities. One female was even observed to swim continuously for over 9 days, covering a distance of 687km. The arctic environment is extremely harsh, air temperatures can be as low as -50°C. Polar bears are warm-blooded animals and have an internal body temperature of around 37°C. There can be a temperature difference of 100°C between the external and internal environments. So how does the polar bear maintain this core body temperature?
The warm coat of the Polar Bear
Famously, polar bears are often reported as being invisible to infrared cameras, a phenomenon achieved by their exceptional ability to retain heat. Although it isn’t clear whether this is entirely true as most evidence is anecdotal, it gives some context into the wonderfully insulated world of the polar bear.
While the title of ‘warmest coat’ is held by the Arctic Fox (Vulpes lagopus); the ability of the fur of the polar bear to retain heat is undeniable as it must maintain body temperatures of around 37°C in temperatures of -50°C. How does the fur of a polar bear do its job so well?
Polar bear hairs have a membrane pore structure, a structure that is different to the hair of all other mammals. The membrane pore structure essentially means that the individual hairs are made up of hollow fibres. Strangely enough, the hairs of polar bears are not actually white, rather they are colourless and the skin of polar bears is actually black as illustrated in the image to the right. Though it seems odd at first, these adaptions make perfect sense.
The structure and lack of colour of the hairs make them transparent to shortwave radiation which can then be absorbed by the skin; the efficiency of which is increased by the black skin. The hairs, on the other hand, are opaque to infrared radiation, meaning that heat cannot easily pass through. This could explain why polar bears might be as good as invisible to infrared cameras.
Occasionally things can get too cold even for a polar bear, to combat the cold in particularly harsh conditions they have developed a number of behavioural strategies. During blizzards, both male and female polar bears have been observed to dig simple burrows that protect themselves and prevent heat loss. These are very different from the dens that females make when they go into hibernation in order to reproduce as they are a direct response to the environment. Individual bears can stay in them for days at a time, sometimes even covering themselves in snow to form a sort of ‘snow blanket’.
The snout is a major source of heat loss for a polar bear, more heat is lost from it than is lost from any other part of its body. To minimise the amount of heat lost, Polar bears tuck their snout into their fur when resting. Allowing for the recycling of heat, as the inhaled air has been partially heated before it reaches the thinner tissues in the nostrils.
Moreover, if they curl up into a ball, they reduce the surface area over which they will lose heat whilst maintaining the same heat producing mass. These strategies can be used alongside the den strategy in order to further conserve heat when temperatures fall.
How does a polar bear cool off?
It seems counterintuitive to think that polar bears might get too hot considering they live in the arctic but they can be prone to hyperthermia. Polar bears are extremely well adapted to thrive in environments of ‘extreme’ cold. However, for a polar bear, a warmer climate is in many ways a more extreme environment. Core body temperatures of polar bears begin to rise if they experience heat of over 5°C, even if the polar bear is inactive.
Humans have a number of ways to cool off if things get too hot, sweating is probably the most obvious example. Many of these responses are orchestrated by the brain. If we get too hot our brain tells our sweat glands to produce sweat, which then evaporates and takes some of that heat away from our body in the process. Polar bears have no such brain cooling mechanisms, so if things get too hot they struggle to cool themselves down.
They do have a few ‘manual’ strategies to cool themselves down and thus lower their body temperature. They are often observed to take ice baths when they get too hot; luckily they often have the largest ice bath on the planet in the available much of the time in the form of the Arctic Ocean. They have also been observed to ingest snow to cool themselves down, not too different from us humans eating an ice-cream on a hot summers day.
Polar Bears in a warming world
Polar bears are quite clearly masters of heat retention, this becomes clear in their struggle to lose heat when things get too hot. The question is – will polar bears be able to adapt to the warming world? A worst case scenario of a 4°C rise in temperatures would pose a serious issue for polar bears. A warmer world would inevitably lead to the melting of sea ice in the arctic, which represents a substantial loss of habitat for these sea ice mammals. Polar bears might be excellent swimmers, but there is strong evidence that long distance swimming in polar bears can lead to a major decline in body condition.
The abilities of polar bears to retain heat serve as a nice parallel for the state of our own planet. Insulation is a good thing, greenhouse gases keep us warm and make our earth habitable. Too much and we’ll overheat; making things harder for our polar bears too.