Wednesday 29 October 2014

The Rhythm of Life

Erebid Moth

What chronotype are you? In other words are you a lark or an owl? Personally I’m a lark; wide awake and raring to go at sunrise and dozy as a dormouse by sunset. About half of us, apparently, are neither. Today I thought we’d stroll down the hill and look at things from a slightly different perspective, that of rhythm.

As human beings we are generally diurnal creatures (active during the day) whereas other creatures such as bushbabies, tarsiers and my twenty year old niece (who, I am convinced, sleeps in a coffin during the hours of daylight) are nocturnal (active at night). Many beetles and moths (such as this Erebid which was on the terrace the other night), as well as your pet moggy, are crepuscular (active in the hours of twilight).  These circadian or daily rhythms which are linked to the rising and setting of the sun, and to a lesser extent the moon, are just one of the rhythms of life which governs the existence of every living thing on the planet.

Four O'clock Flower
It is not only animals that exhibit these circadian rhythms; plants do too. Certain baobab trees and at least one species of jasmine are night flowering in order to be pollinated by nocturnal creatures. Some flowers such as salsify open in the mornings and this one, the four o’clock flower (an introduced plant to Crete), opens in the late afternoon and produces a heady scent throughout the night. Pollination isn’t something that happens by chance, plants have evolved to attract certain pollinators not only by size, shape, colour and scent but also by the time of day when they flower. 

Back in 1751the Swedish botanist and father of modern taxonomy Carl Linnaeus even designed a flower clock by placing flowers at appropriate intervals around the face depending upon their opening times.

Willow Warbler
Listen. Do you hear that questioning “oo-whit?” call coming from the carob tree over there? I think that’s a willow warbler. Yes, there it is. It’s just alighted in the tamarisk trees.  Willow warblers are highly migratory birds and this one will be on his way from northern Europe to central Africa. Migration is another one of the rhythms of life, in this case an infradian rhythm which describes any rhythm that occurs in a time period longer than a day.

Beech Marten Trails and Scat
You won’t be surprised to learn that if there are infradian rhythms that last longer than a day there are also ultradian rhythms that last less than a day. Which brings us back to the naturalists’ favourite topic – pooh. Over there on the far bank of the gully you can see a beech marten’s den with tracks leading down to the water (we won’t see the marten as he is a crepuscular/nocturnal creature and won’t be out for a good few hours yet). And down here by my feet is a scat which, by the look of the seeds embedded within it, belongs to the owner of the den. Both eating and defecation are examples of ultradian rhythms. Eating is stimulated by an increase in appetite which involves the release of certain chemicals within the body and there are other chemicals which suppress bowel movements when an animal is asleep (typically between 10.30pm and 8.30am in us humans).

I see that the moon is just rising in the west. For us land animals this is of minor importance but for sea life it is more important than the sun. There are three lunar cycles: the lunar day which lasts for 24 hours and 50 minutes. (Incidentally some research into human circadian rhythms suggests that most people prefer a 25 hour day which makes you wonder if these rhythms of ours have been in-built since we were sea creatures); the semilunar cycle which lasts for 14.8 days; and the lunar month which is 29.5 days. If we trot down to Ferma harbour I may be able to introduce you to a little animal that can help to illustrate this.

Here we are, this little guy is a fireworm. He is a relative of the common earthworm (they both have a body cavity, moveable bristles and bodies divided into segments by transverse rings) and he’s called a fireworm because those white bristles running down his side glow white when he feels threatened and, in some species, can give a nasty sting. Research on a close cousin, the Bermuda fireworm has revealed that he breeds about 55 minutes after sunset, for about half an hour, only when there is no moon in the sky during the early part of the night. So he is being governed by the lunar day with the transition from sunlight to moonlight and also the lunar month as he chooses to mate when there is no moon visible which occurs from two or three days after the full moon until the new moon. Obviously a very shy creature that doesn’t like making love with the light on – of any sort.

So there you have it, a short introduction to chronobiology or the rhythms of life. Yet another fascinating aspect of the natural world around us. Meanwhile I’m still waiting for that special event that I mentioned a couple of weeks ago. That too is chronobiological in nature but as I did not witness the beginning of the cycle I cannot predict the next stage with any accuracy. I’ll keep monitoring the situation and let you know when it occurs either through the blog or in the Facebook Naturalists Group which you are welcome to join if you haven’t already done so. Until next week – happy hunting.

Naturalists (the facebook page that accompanies this blog)

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