How hibernation happens
We've all heard of the bear who sleeps through winter or the bird that migrates to warmer climates, but how do they know when the time is right? Francis Ebling is Professor of Neuroendocrinology at the University of Nottingham and lets Kat in on the happenings of hibernation.
Francis - Well the key is that most organisms measure day length and night length, which provides a reliable geophysical cue, and they can use that information to regulate an internal calendar and prepare for the changing of the seasons. Animals have to prepare for winter when climatic conditions are harsh, when food supply might be limited or very unpredictable, and the most dramatic example for preparation for winter are those animals that go into hibernation which, in effect, is suspended animation. They drop their metabolic rate, the drop their body temperature.
Kat - So how are animals actually measuring this change in day length? I mean assuming they don't have a clock and go - ohh you know, nights are drawing in. How do animals who don't have clocks and calendars work out what time of year it is? You know what's going on at that kind of level?
Francis - Our understanding is that they can measure the actual hours of light and hours of darkness. So, we and other mammals perceive light through our eyes, but it's not the rods and cones that people might have heard of as our normal cells that measure light and help us see colour in black and white. It's a set of special cells at the front of the ganglion that measure light intensity and the duration of the day and they use that information to signal to our pineal gland, and the pineal gland secretes a hormone, melatonin, but just at night. So, as the length of the night gets longer in autumn as winter approaches, the duration of melatonin secretion increases and that, in turn, signals to the pituitary gland at the base of the brain, and to the hypothalamus just above the pituitary gland, and it's that part of the brain that controls body temperature, controls reproduction, controls food intake and fat intake and fattening.
Kat - We know about animals like bears and things like that, that go into hibernation in the winter, and I certainly feel right now like crawling back under my duvet, but humans don't seem this hibernation - you know kind of wanting to bed down for the winter - do we?
Francis - Well, humans probably evolved as tropical apes in equatorial regions but we are very much aware of the changing seasons. For some people, that impacts upon their moods such that they develop 'seasonal affective disorder,' a form of depression in winter, but it's a very interesting form of depression in that they tend to sleep more and eat more, particularly carbohydrates and fats, which is atypical for other forms of depression. So, we are subliminally aware of the seasons and it affects some of us more than others.
Kat - How are you studying these processes of seasonal change and hibernation?
Francis - My laboratory at the University of Nottingham is funded by the Biology and Biotechnology Research Council, specifically to understand how it is that some animals prepare for winter by reducing their appetite and increasing the way that they burn the fat reserves they've stored. So we study these in Siberian hamsters: these hamsters show a form of hibernation called "torpor" where they drop their metabolic rate, and their heart rate, and their breathing rate for just part of each day, but in addition to that, they spontaneously reduce the amount that they eat. And they've evolved that strategy because, of course, in winter there are limited food supplies and, instead, they survive winter, keep the body going, by burning huge amounts of abdominal fat that they've stored over the summer in anticipation of winter.
Kat - And very briefly, what can we learn from your work about maybe how other animals, and maybe even humans, respond to winter or just regulate our metabolism generally?
Francis - I think the important thing from our work is that is shows that the mammalian brain is capable of spontaneously reducing our motivation to eat and our appetite, and capable of increasing the way we burn fat. So, if we could understand those mechanisms, then potentially we have some new targets that the pharmaceutical industry and the health service can look as potential targets to help us lose weight and be healthier.