Chapter 1
Chapter 2
Chapter 3
Chapter 4
Chapter 5
Chapter 6
Chapter 7
Chapter 8
Chapter 9
Chapter 10
Chapter 11
Chapter 12
Chapter 13
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We’ve set up a camera, scanning both the trees and the open country that borders them. And there’s a couple of houses in shot as well. It’s a rather special camera, one of those that compresses an hour into a minute or two. Let’s turn it on, and leave it for a few days……… We watch the sun rush up from the East, pass over our heads, and sink into the West, like a great celestial tennis ball, but one that goes over an imaginary net in only one direction. Helped by the biologist friend who is watching with us, we begin to notice more. We notice that the leaves on the trees also seem to have a pattern: they move differently as the day passes – some open during the day, close at night. Just as we are beginning to feel quite pleased with our powers of observation, our biologist gently points out that a Frenchman named de Mairan , who was not even a biologist, but an accomplished astronomer, had also noticed the same thing (without using a high-tech camera) in 1729. And as we’ll see, his curiosity led him to do a simple, but striking experiment, which is what separates his observation from all those who had noticed similar things before - going back to the ancient Greeks…..
All animals, including humans, have a daily body programme that follows a remarkably consistent pattern. The day (or night) is full of different bodily events, all in the right sequence, and happening at the right time. In some cases - for example the digestive processes that follow a meal - the sequence of events depend upon each other, rather than upon an external synchronising signal. In others, there is a direct link between the synchronising signal (the sun). This means that different events must have a different lead time: for example, if your lunch is linked to sunrise, then your body has to have some means of assessing the passage of time, so that lunch occurs at lunch-time, not earlier. Many animals can do this: for example, you can train an animal to come for food at a certain time of day. The animal has no watch: but it’s internal clock tells it, in some way, when the time is right. If you move that animal across several time zones (for example, take it from the UK to the USA) then it will appear for food at the ‘right’ British time on its first day, gradually adjusting its internal clock so that, after a few days, it now arrives at the right local time. This implies, of course, that animals (and humans) can measure
the passage of time quite
accurately without looking at an external clock…Since the limbic system controls eating, drinking, hormones, the cardiovascular system and many other elements making up the daily programme, it seems likely that the clock, if there is just one, might also be somewhere in this part of the brain. Let’s focus a bit more: the hypothalamus is the part of the limbic system that monitors the internal environment, and thus the daily surges of physiological activity. So perhaps we should look in the hypothalamus. Despite what now may seem rather obvious, it took until 1972 for Friedrich Stephan and Irv Zucker, working in the USA, to do the essential experiment: they found the clock…..
In the overlying hypothalamus lie two little balls of nerve cells (one each side). So they’re called the ‘suprachiasmatic nuclei’ (the nuclei that lie above the chiasm). Quite a mouthful, so most scientists abbreviate them to ‘SCN’. ......They did the classical experiment: they destroyed the two little nuclei, and the clock stopped…..
….in countries appreciably distant from the Equator, the young of nearly all wild species are born in the spring. The reason is obvious: reproduction , a biologically and socially expensive and risky business (we talk more about this in Chapter 8) needs the very best conditions to ensure that the young survive. Most animals can’t afford to breed throughout the year: they have to ‘choose’ the best season. That’s spring and early summer, when food in most parts of the world is relatively plentiful, air temperature moderate, and rainfall adequate to provide drinking water. Recall that to arrange to have young in the spring requires some forward planning. Pregnancy lasts a predictable period (in most cases – see below), which differs by species. So to ensure that the young are born at the right time, animals have to mate at the ‘right’ time, which will differ depending on the duration of pregnancy (for that particular species). Which presents us with another adaptive problem: how on earth do animals know when spring will come? Or when its here? How do different species mate at the right (different) time? The annual baby-boom in the spring clearly shows us that they must do it somehow. Luckily, they have a means: if they could measure daylength (as opposed to the time of day) they might be able to compute the time of year. …
Knowing the time of day is a different piece of information from knowing the time of year, though both are essential for successful adaptation. So the two
are handled by separate bits of brain. The
SCN synchronises the daily programme , whereas the pineal coordinates
reproduction - and other seasonally important events, like
growing a thick coat for the winter, or shedding it in time for the hot
days of summer….Rhythms represent regularities in behaviour and physiology which form essential adaptive responses to a regularly changing environment. These adaptations must be flexible: though the light cycle stays the same each year, other important elements in the environment, such as food supply or temperature, are not nearly so reliable. ........... The limbic system makes sure that the internal and external environments stay in concert with one another. Your body dances to the music of time.
Scientific and literary quotes.