How do bats know it is night time?
Chris - Brilliant question. The answer is it is all down to the body clock. As mammals we humans are diurnal – we are active in the day time. But we have in our brains a circadian clock. It’s called the suprachiasmatic nucleus and that keeps time. It is a few thousand nerve cells, maybe 20,000 nerve cells that are all interconnected and they are running a genetic programme that works a bit like a molecular domino effect. So one gene turns on, it then turns on another gene, which turns on a third gene and that third gene feeds back and turns back off the first gene, and so on. And this molecular clockwork takes about 24 hours to go round a complete cycle, and as it goes round the complete cycle it is activating the cell in different ways.
So these nerve cells become more or less active and they secrete various hormones into the blood stream, and they go round the body and inform all of the tissues in the body that have their own body clocks, what time it is. So you have this master clock in your head and you have these other slave clocks – if you like – in every cell in your body. And that’s how your tissues know what time of day it is. And jetlag is all about a disparity between when you really should be getting up and when your body is thinking it should be getting up, and you reset that body clock – in your brain – the suprachiasmatic nucleus- because there is a feed from the retina. Your eye has a very special set of nerve cells called ganglion cells at the back of the eye that can detect blue light and they send signals into this body clock to reset it, so that your body clock is in sync with when it is getting light and when it is getting dark, and that is why you can get over jetlag.
But some animals, rather than us who get up when it’s light, some animals have reversed their response to their body clocks so when it gets light they go to bed and when it gets dark, they wake up. And they have all the same hormone signals that we do, they just respond to them in a different way. And mice are the same, mice are nocturnal, so they go to sleep when it gets light and they wake up when it gets dark and this is to avoid predation. And bats come out at night time as slaves to their body clocks and so they are actually using this very ancestral deep body clock inside the brain to work out what time it is.
Dave - And I guess they are active in the evening and the morning when there is a little bit of light around so that they can keep resetting that body clock?
Chris - When they wake up in the night time, the sun is setting and this helps them because they respond to that, because there is a bit of light, as you say. And the other point is that they also use their body clock to navigate - because they know where the moon is, and they know where east and west is, because they know where the Sun is setting and they know where it is rising and they set their body clocks with the Earth’s magnetic field as well. So they have a magnetic response element in their brains, they look at where the Sun is in the sky. They then know that the Sun rises and sets east and west. They know what time it is in their body clock and they lock their body clock onto the magnetic signal of the Earth, and they then know which direction to fly in and if you make them jetlagged, as one researcher I was talking to recently did, they can all fly off in the wrong direction. Isn’t that amazing?