Bits on Bats 8: Goodnight - see you next Spring.

Trapped in by domestic requirements, the icicles dangling off the conservatory gutters seemed like prison bars.  The wintry scene set me thinking - why is it that so few animals hibernate and why can't we?  Apart from bats, only the hedgehog and the dormouse hibernate among British mammals. Yet surely it has wider merit than that as a way of riding out rough patches.  I mean, we're all mammals with similar physiology, so how come all mammals can't do it?  I guess that there are a number of competing factors that determine whether it is a sensible strategy or not.  For our bats its a no-brainer. They can't eat anything except flying insects and you'll probably have noticed that there's not a lot of those around right now.  So they have three options - migrate, hibernate, die.

British species of insectivorous bats are pushing at the northern limit of their ranges and all originated in tropical areas.  Although some do migrate, the commonest strategy is to hiberate.  But the fact is, hibernation is not all it's cracked up to be. If you don't get stocked up on reserves you won't make it through. Also if you get disturbed and woken up this can use up significant energy reserves that can make the difference between surviving and not.  It's a risky strategy.

 

I read an interesting take on why humans don't hibernate which I find persuasive. Humans evolved in tropical regions where seasonal food shortage wasn't really a problem. No advantage to be had from developing hibernation.  We only spread out to the colder parts of the globe within the last 100,000 years or so and this isn't long enough to evolve the physiological changes to enable hibernation. Plus, there's no point (however appealing it may seem at this time of year!) as we are versatile eaters and the discovery of fire, clothing, shelter etc enable us to survive cold periods much more successfully than if we hibernated. By contrast, bats were catching insects in prehistoric skies while our ancestors were no more than small lemur-like primates. So they have had millions of years to evolve hibernation as a response to being totally dependent on a food source that disappears for months on end.

Hibernation is a fascinating business.  Bats find cold places with steady, low temperatures (caves, tunnels, thick walls etc) and let their body temperature drop to a degree above ambient - ideally between 6 and 9 degrees C - and just close down.  The heart rate drops to 10 beats per minute in a bat whose heart will be belting away 1000 times a minute in summer flight.  Energy needs reduce to almost zero (less than 1% of that needed when fully revved up at 39C). The bat goes cold and stiff and a tiny pipistrelle can last like this for 100 days with ease.  The bat in the photo below - which is courtesy of the Leeds university - is covered in condensation. 

 

 (photo c/o Bat Research Group at the University of Leeds) 

Coming back to the question of whether humans could develop hibernation if need be there are a couple of tantalising stories of people who were rescuscitated hours after they were thought dead. One case involved a women trapped head first under water beneath a frozen river for over an  hour. Another involved a japanese man found on a snowy mountain side 24 days after vanishing.  When searchers discovered him, he appeared to be in a frozen coma. His pulse was almost undetectable. His body temperature had dropped to 22c and his organs had mostly shut down. He recovered fully with no lasting ill effects. One of his doctors said: "He was frozen alive and survived. If we can understand why, it opens up all sorts of possibilities for the future."  Indeed.

Meanwhile, back to bats. They have one other neat trick. Apart from hibernation, at any time of day, at any time of the year they can let their body temperature drop to ambient if there is a cold snap or poor feeding. The technical term is torpor but one book I have refers to it as 'daily lethargy'.   I must have bat genes tucked away somewhere then....