Under the Microscope: Why do animals hibernate?


Asked by: Rachel Arnold, London



Answered by: Professor John Altringham, Chair in Animal Ecology and Conservation, University of Leeds

What happens when an animal hibernates?

Hibernation is a state of physiological inactivity triggered by seasonal changes in an animal's habitat. Animals recognise shortening days and falling temperatures through evolutionary development, so are able to recognise the onset of winter. When certain external stimuli are detected, they produce a hormone called "hibernation-specific protein". This protein, identified in 2006 by scientists at the Life Science Institute of Tokyo, triggers the metabolic depression that leads to the "sleep state". Heart and breathing rate slow, blood supply is restricted, and non-essential organs may go into stasis.

As a result, their core temperature falls. This has two consequences. As the optimal internal temperature comes closer to the ambient temperature, less energy is needed for thermo-regulation. At the same time, energy demand is further reduced because of the drop in organ function. Both help the animal survive through the winter months.

Which animals hibernate?

A significant number of creatures in the temperate world hibernate. It is a notable behavioural feature in many mammals, such as the bat and bear; amphibians; butterflies and a number of other insects; and, of course, reptiles – the most widely known being the tortoise.

We used to think some birds hibernated – the ancient Roman naturalist Pliny the Elder wrote about hibernating swallows – but we now recognise that they actually use torpor, a truncated version of hibernation.

What triggers hibernation?

Hibernation is a response to the ecological imperatives of winter. Like humans, animals require a relatively stable internal temperature for their cells to function properly – even cold-blooded animals cannot brook too drastic a change in temperature.

When the ambient temperature falls, it becomes harder to maintain the internal temperature necessary for metabolic reactions to take place. Ever larger quantities of energy – either from fat stores or from new sources of starch – are required to maintain this thermal level. However, winter is a time of scarcity, so animals have to use coping mechanisms to survive. One such mechanism is hibernation. To put it simply, hibernating animals sleep their way through the difficult winter months.

Why don't all animals in the temperate world do it?

Not all animals need to. Most species have evolved in such a way as to render the process unnecessary. Humans, for example, have developed mental faculties which allow us to change our behaviour to take into account a fall in temperature. When we put on a woolly jumper and eat a bowl of soup we are making use of our evolutionary intelligence and effectively outsmarting the cold.

The only animals that need to hibernate are those whose physiological characteristics make them vulnerable to the cold. A large surface-area-to-volume ratio makes such animals highly vulnerable to reductions in external temperature, as they have a large area from which to radiate heat and a small volume of cells from which to generate it through the breakdown of starch.

An appropriate metaphor would be a small heater in a greenhouse in winter: with a large surface area of glass through which the heat will radiate, it is very difficult for the heater to produce enough warmth to keep the green house at a comfortable temperature.

Why don't wild animals hibernate for longer than domestic pets?

The first thing to note is that hibernation periods are not fixed. Variance occurs not only between species, but also within them. Individual British bats, for example, go into hibernation at differing points between November and March. And while the exact time hibernation begins is dependent on individual environment, they will all at some point go into stasis.

Domestication, then, will not eradicate the physiological reaction to winter: a warm shoebox and plentiful food won't prevent a hedgehog hibernating, for example. That said, domestication may have an effect on the patterns of hibernation. Domestic animals may wake more frequently and return to normal temperature more readily than a wild animal.

What happens if I wake a hibernating animal?

Contrary to popular belief, hibernation is not a long "sleep" during which an animal remains in suspended animation. In fact, hibernation is a transitory state and is punctuated by periods of arousal when the body comes out of "sleep" and metabolic processes resume at normal rates. So although you should avoid waking a hibernating animal, it will not necessarily cause any lasting trauma.

Send your science questions to microscope@independent.co.uk

Suggested Topics
Start your day with The Independent, sign up for daily news emails
New Articles
tvDownton Abbey Christmas special
Arts and Entertainment
Wolf (Nathan McMullen), Ian (Dan Starky), The Doctor (Peter Capaldi), Clara (Jenna Coleman), Santa Claus (Nick Frost) in the Doctor Who Christmas Special (BBC/Photographer: David Venni)
tvOur review of the Doctor Who Christmas Special
News
peopleIt seems you can't silence Katie Hopkins, even on Christmas Day...
News
news
PROMOTED VIDEO
News
i100
Arts and Entertainment
Left to right: Stanley Tucci, Sophie Grabol and Christopher Eccleston in ‘Fortitude’
tvSo Sky Atlantic arrived in Iceland to film their new and supposedly snow-bound series 'Fortitude'...
Arts and Entertainment
Jenna Coleman as Clara Oswald in the Doctor Who Christmas special
tvForget the rumours that Clara Oswald would be quitting the Tardis
Arts and Entertainment
Japanese artist Megumi Igarashi showing a small mascot shaped like a vagina
art
News
The Queen delivers her Christmas message
newsTwitter reacts to Her Majesty's Christmas Message
Arts and Entertainment
tv
Life and Style
fashion
ebooks
ebooksA year of political gossip, levity and intrigue from the sharpest pen in Westminster
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Ashdown Group: Marketing Manager/Marketing Controller (Financial Services)

£70000 - £75000 per annum + benefits: Ashdown Group: Marketing Manager/Marketi...

Recruitment Genius: Account Manager

£20000 - £35000 per annum: Recruitment Genius: This full service social media ...

Recruitment Genius: Data Analyst - Online Marketing

£24000 - £35000 per annum: Recruitment Genius: We are 'Changemakers in retail'...

Austen Lloyd: Senior Residential Conveyancer

Very Competitive: Austen Lloyd: Senior Conveyancer - South West We are see...

Day In a Page

A Christmas without hope: Fears grow in Gaza that the conflict with Israel will soon reignite

Christmas without hope

Gaza fears grow that conflict with Israel will soon reignite
After 150 years, you can finally visit the grisliest museum in the country

The 'Black Museum'

After 150 years, you can finally visit Britain's grisliest museum
No ho-ho-hos with Nick Frost's badass Santa

No ho-ho-hos with Nick Frost's badass Santa

Doctor Who Christmas Special TV review
Chilly Christmas: Swimmers take festive dip for charity

Chilly Christmas

Swimmers dive into freezing British waters for charity
Veterans' hostel 'overwhelmed by kindness' for festive dinner

Homeless Veterans appeal

In 2010, Sgt Gary Jamieson stepped on an IED in Afghanistan and lost his legs and an arm. He reveals what, and who, helped him to make a remarkable recovery
Isis in Iraq: Yazidi girls killing themselves to escape rape and imprisonment by militants

'Jilan killed herself in the bathroom. She cut her wrists and hanged herself'

Yazidi girls killing themselves to escape rape and imprisonment
Ed Balls interview: 'If I think about the deficit when I'm playing the piano, it all goes wrong'

Ed Balls interview

'If I think about the deficit when I'm playing the piano, it all goes wrong'
He's behind you, dude!

US stars in UK panto

From David Hasselhoff to Jerry Hall
Grace Dent's Christmas Quiz: What are you – a festive curmudgeon or top of the tree?

Grace Dent's Christmas Quiz

What are you – a festive curmudgeon or top of the tree?
Nasa planning to build cloud cities in airships above Venus

Nasa planning to build cloud cities in airships above Venus

Planet’s surface is inhospitable to humans but 30 miles above it is almost perfect
Surrounded by high-rise flats is a little house filled with Lebanon’s history - clocks, rifles, frogmen’s uniforms and colonial helmets

Clocks, rifles, swords, frogmen’s uniforms

Surrounded by high-rise flats is a little house filled with Lebanon’s history
Return to Gaza: Four months on, the wounds left by Israel's bombardment have not yet healed

Four months after the bombardment, Gaza’s wounds are yet to heal

Kim Sengupta is reunited with a man whose plight mirrors the suffering of the Palestinian people
Gastric surgery: Is it really the answer to the UK's obesity epidemic?

Is gastric surgery really the answer to the UK's obesity epidemic?

Critics argue that it’s crazy to operate on healthy people just to stop them eating
Homeless Veterans appeal: Christmas charity auction Part 2 - now LIVE

Homeless Veterans appeal: Christmas charity auction

Bid on original art, or trips of a lifetime to Africa or the 'Corrie' set, and help Homeless Veterans
Pantomime rings the changes to welcome autistic theatre-goers

Autism-friendly theatre

Pantomime leads the pack in quest to welcome all