Switching off: hibernation in Australia
As the winter chill sets in and snow falls over our mountain ranges, many of us are at home by the fireplace, rugged up in blankets – but how do our animals tackle the cold?
WE MIGHT THINK of America’s grizzly bears when we think of hibernation – but it is actually surprisingly common in our own backyard.
In Australia, four species of pygmy possum, a handful of bat species and the short-beaked echidna are all known to hibernate for extended periods of the year.
And the eastern pygmy possum – found along the east coast from Queensland to South Australia and throughout Tasmania – holds the title for the longest hibernation of any mammal in the world, capable of spending a whole year in the less-active state.
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But what exactly is hibernation, and why do animals use it?
Hibernation is an adaptation found in endothermic animals – those which can maintain an internal body temperature by producing metabolic heat – often labelled as ‘warm-blooded.’
These animals constantly lose heat to the environment, so to conserve energy – often in the cooler months – they lower their body temperature and metabolic rate to just a fraction of their normal levels, and remain in this state for extended periods.
A short-beaked echidna emerges from its burrow after a period of torpor. (Image courtesy of Taronga Zoo)
Gemma Morrow, a keeper in the Australian Fauna Precinct at Taronga Zoo says the short-beaked echidna – found throughout mainland Australia and Tasmania – is one of the best examples.
“They go into deep hibernation,” Gemma explains.
“Their body temperature is normally about 31-32 degrees, but during hibernation they can drop to within 0.5 degrees of the soil temperature – in Tasmania that means they can drop to about four degrees.”
But it is not always by the book – Gemma says the Tasmanian subspecies enters hibernation in summer rather than winter, males hibernating from February to May and females from February/March to June, before rewarming to breed.
Entering this inactive state in hotter temperatures is often called aestivation or ‘summer sleep,’ seen in some species of frogs, fish, insects and the critically endangered western swamp tortoise.
Torpor versus hibernation
While many Australian animals do not technically ‘hibernate,’ they employ very similar energy-saving measures on a short-term basis, called daily torpor.
“Hibernation is prolonged, multi-day torpor,” Gemma says.
“They are essentially the same thing – a drop in temperature and metabolic rate.”
An estimated 43 per cent of Australia’s terrestrial native mammals use torpor – some on a daily basis and others on a multi-day basis.
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Dr Fritz Geiser, professor of zoology at the University of New England, has spent decades studying hibernation in Australian animals, and says that deep torpor is only seen in mammals weighing less than 10 kilograms.
“Heat loss is a function of surface area, and surface area is a function of size – the smaller you are, the bigger your surface area,” Fritz explains.
“So if you have an animal with a big surface area and it’s cold, heat just flies out of its body, and it’s expensive, so they switch off.”
It is used by most carnivorous and insectivorous mammals, including many dasyurids – quolls, dunnarts and planigales – as well as bats, gliders, numbats, and even tawny frogmouths and kookaburras.
Fritz says many of these animals enter daily torpor, where instead of sleeping, they switch off thermoregulation – lowering their metabolic rate and temperature – for five to ten hour periods.
Reptiles also enter a state of torpor during winter – however, the process is to a large extent caused by the fall of ambient temperature.
As ectothermic or ‘cold-blooded’ animals, their internal heat is determined by external sources such as warm rocks, so unlike endotherms, they have limited control over their temperature and metabolic rate.
Therefore in the colder months they are less active.
“The difference is that in mammals torpor is under precise control,” Gemma explains.