How do bears know when to hibernate?

We put this to Eleanor Drinkwater, from the University of Cambridge...

Eleanor - Well first of all there hasn't been a huge amount of studies on bears, unfortunately, on hibernation because of the difficulty of bothering a bear when it's trying to sleep. But there has been this great study done on brown bears and they found that first of all, bears just tend to slow down in autumn until it hits a specific time before the denning period in which they go into almost beginning to shut down. So they tend to move around really slowly and they found that 50% of bears would start doing this while they were still quite a long way away from their den and the other 50% would wait until they were closer to the den to start doing this. It's difficult to tell because, obviously, you can't ask a bear "are you feeling sleepy?," but the wind down suggest that they're going into the sleep phase.

Also the other interesting thing too is that, apparently, during this period this is when brown bears are most dangerous. There's the greatest number of unfortunate encounters between brown bears and hikers because they reckon it could be that the bears are kind of sleepy, and they can't run away as well, so they tend to get a little bit more aggressive.

Chris - Giles?

Giles - An animal model which is being studied, is the siberian hamster. And what happened there is you get a fluffy white siberian hamster during the winter and then they become lean and mean and thin during the summertime.

Chris - A mean hamster - why are they mean?

Giles - To go after the acorn! The critical thing is scientists can induce changes in these animals just by changing the time of light. So if you go from 8 hours of light to 16 hours dark, you will then have a winter hamster and if you actually switch it around, you will end up with a summer hamster. So you are actually able to change this. I don't know if this is true.

Chris - The light day length is the trigger for initiating... pathway kicks in to say you need to be thinking about going to bed now?

Giles - Correct.

Eleanor - That's so cool. With bears they found that they correlated with the amount of food present and what the temperature was but I don't know whether if you put bears in a box and changed their patterns. I know with lizards.

Chris - I'm sure they'd be delighted! I'm sure they wouldn't mind at all!

Eleanor - Well, you never know.