Gills Hog blog No 3. Hibernation.

A Post from the Palace.

Hibernation.  One of nature's miracles.

Gill Dixon, PGCE,MA,BHScHons,RGN.

(Guy Troughton)

Hedgehogs are one of the few mammals that practice hibernation and it is thought that they do so in an effort to conserve energy when food availability is poor.

Interestingly in recent years rescuers are noting that some hedgehogs do not hibernate at all and if you are one of the wonderful people who support feed hedgehogs, please continue to do so. hedgehogs arouse periodically during hibernation and a handy feast may make all the difference to their success.

Hibernation is a perilous business because it isn't just a matter of snoozing away the winter months. It is a process whereby a warm blooded mammal  almost becomes a  cold blooded one. They undergo massive physiological changes to enable this fascinating transformation. They don't always get it right and there are no guarantees that they will be successful and survive.
Sleep is not an apt description as during sleep all bodily functions remain more or less the same as they are when awake. Bodily functions remain normal, keeping the brain and other organs unconsciously active.

A sleeping hedgehog. The feet and nose are visible. 

When hedgehogs are sleeping they move and twitch and can be seen dreaming but when they are in hibernation they come to a complete standstill. To the untrained eye they appear dead. The hedgehog is cold to touch and inactive although its spines may undulate or stand up if touched or subjected to noise. However it's brain is closed down whilst remaining functional. 

A hibernating hedgehog is an impenetrable ball.

 A normal heartbeat for a hedgehog can be up to 280 beats per minute but this may drop to a barely detectable 20. Its body temperature which is normally 35c drops to 5 degrees and it may only take a breath every few minutes. Some are reported to be apnoeic for up to an hour.  A fall that would kill most other mammals.
This amazing transformation doesn't stop there, around 90% of the white blood cells congregate around the stomach and intestines, ready to fight invading bacteria.  The blood vessels around the kidneys close to avoid excretion of urine (wasting vital fluids).  The pancreas is the one organ that remains fully functional in order to secrete insulin and keep the blood sugar levels low.


This phenomenal change in bodily functions enables the hedgehog to use its fat reserves to sustain it.
Ideally two types of fat are laid down prior to hibernation. White fat which is like a slow burn fuel, while brown fat is a supercharged fuel. If the hedgehogs temperature falls too low (risking it freezing) the brown fat is ignited and quickly produces heat which warms the hedgehog over a few hours enabling it to wake and move somewhere better insulated. A hibernating hedgehog usually moves nest at least once.
It is not until November, December or January that the larger hedgehogs settle down to hibernate. It is a last resort, as an inactive mammal is a vulnerable one.
Hibernation is thought to be triggered by a prolonged fall in temperature, reduced daylight hours and lack of food, but  while It is commonly thought that hedgehogs hibernate from autumn through the winter until the spring, this is not so. Hedgehogs do not have the luxury of calendars; they react to the environment around them
Hedgehogs build themselves a hibernacula, a secure nest in which they spin the bedding materials to make a weather resistant and well insulated shelter.They are masterpieces of natural engineering.
The hedgehog may rouse periodically (and use a great deal of energy doing so) and some may not hibernate at all, so don't worry if you see a good sized hedgehog out and about on a winters' night. Just offer it some meaty cat food or cat biscuits and a drink of water and wish it well. 

Hedgehogs  need to have some fat reserves to survive hibernation and that is why Pricklington Palace and other rescues fill up with autumn juveniles that are simply too small to survive the process. They are kept warm until they are big enough to survive hibernation (over 650g) and then the heat is removed. 

At Pricklington Palace I do not overwinter hedgehogs endlessly just because it is winter. Once hogs reach a respectable weight to size ratio,  I continue to release in winter. Current research suggests that this is perfectly acceptable and our winters are on the whole very mild compared to decades ago. A study conducted by  R Yarnell et al in 2019   outlines definitively that hedgehogs can be successfully released back into the wild over winter.

More enlightened rescuers will release IF the weather is calm, there is a food source and adequate nesting sites and materials. The decision to release will also reflect where in the country the rescue is as temperatures can vary enormously.

Despite numerous studies on hibernation in hedgehogs, a specific trigger (or triggers) has yet to be identified. Reeve (1994) suggests it is triggered by a complex interplay of environmental and hormonal  factors. Anyone who is interested to know more would be well advised to read Reeves' excellent chapter Hibernation and Energetics in his book as referenced below.

The commonly agreed suggestion is that it is a response to a prolonged period of cold (below 5 degrees centigrade) reduction in daylight hours and a lack of available food.

There’s no evidence that failing to hibernate is detrimental to a hedgehog’s health, provided there’s sufficient food around and it is possible that our increasingly mild winters and an increase in the numbers of people support feeding hedgehogs may result in less hedgehogs undertaking the process.

Not all hedgehogs will hibernate, so those that are too small  may continue to put on weight if there is a food source and they do not need to be scooped up and 'rescued' unless of course they show them selves to be in need by appearing in daylight or being obviously injured or unwell.  Hibernation is a dangerous enterprise and there is no reason a hedgehog should undertake it if they really don't need to. People should definitely not be hunting for and weighing animals at night. This is contrary to various pieces of wildlife  legislation and not in the hogs best interest at all. A hedgehog out after dark should be allowed to go about its business. To imprison a fit nomadic animal in a small box or hutch for months on end is surely tantamount to cruelty and will cause untold stress leading to illness in the hedgehog. 

Far better to offer shelter and a reliable food source.

Here on my patch in rural  East Yorkshire, hedgehogs continue to visit my feeding stations throughout the year.

People often ask me what is a safe weight for hedgehogs to survive hibernation and the answer is that there are no guarantees that it will survive at all. However hedgehogs 450g and under are considered to be at more risk of dying if they attempt the process. The general consensus amongst rescuers seems to be that 650g is the minimum safe weight during the worst weeks of winter but some wildlife experts consider that 500g and above is perfectly reasonable.

Note:
If you dig up/disturb a hibernating hedgehog, try to replace it without further disturbance. Do not warm it rapidly or it will die.

 

Gill Dixon, until recently  ran Pricklington Palace Hedgehog rescue in Howden. Purely voluntarily and single handedly. 

She now administrates an information hub which can be found at https://www.facebook.com/pricklingtonpalace

Ref: 

Reeves, N (1994) Hedgehogs Pub: Poyser Natural History. London.

Yarnell,R et al (2019) Should rehabilitated hedgehogs be released in winter? A comparison of survival, nest use and weight change in wild and rescued animals. European Journal of Wildlife Research.