BIOLOGICAL RHYTHMS (ANIMALS)
=Biological Clocks= [image:http://i.imgur.com/GiV5Buq.png] Many animals and plants seem to be able to respond to environmental events before they even occur. For instance, many animals that are active during the day come out just before the sun rises. These organisms must have some internal awareness of time. This is often referred to as a ‘Biological Clock’ A biological clock is an internal (endogenous) timing system that continues without any external cues and controls the activities of plants and animals '''Advantages of a Biological Clock:''' *Anticipate periodic environmental changes to maximise feeding time, avoid predators and avoid competition. *Anticipate and prepare for periodic environmental extremes (winter, drought etc). This can be a trigger for migration. *Time mating with physiological readiness (when most fertile) or synchronise mating times. ==Biological Rhythms== Because the function of a biological clocks is to anticipate environmental changes they tend to be set to environmental rhythms such as daily, tidal and seasonal changes. As a result the following are commonly observed periods for biological rhythms: [image:http://i.imgur.com/fbsRDCq.gif] '''Circadian –daily activity (~24hrs)''' The rotation of the earth about its axis causes a day / night cycle. Sleeping, feeding, changes in body temperature, etc usually exhibit daily cycles. There are three types of circadian rhythms: •'''Diurnal''' -active by day •'''Nocturnal''' -active by night •'''Crepuscular''' -active by dusk/dawn [image:http://i.imgur.com/P3kTUrA.gif] '''Circatidal –tidal activity period (approx 12hrs 25min)''' Cycle of high and low tides, roughly twice per day. Because the tides aren't exactly 12 hours apart, high and low tide are approximately 50mins later each day. Tides are caused by the gravitational field of the moon (and to a lesser extend the sun). The gravitational field created by the Moon weakens with distance from the Moon, it exerts a slightly stronger than average force on the side of the Earth facing the Moon, and a slightly weaker than average force on the opposite side. This causes the water to bulge out slightly on each side (high tides) [image:http://i.imgur.com/gvwC7DA.gif] '''Circalunar –monthly activity period (approx 29days)''' Some cycles are approximately monthly, resulting from the movement of the moon around the earth. More commonly, these cycles are circasemilunar = twice a month. These are usually the result of '''spring tides''' and ''neap tides'''. Spring tides are particularly high tides that occur twice each month when the moon and sun are in alignment (and the gravitational field of the sun contribute to the tides also). Neap tides are weaker tides where there is much less difference between high and low tide. They occur when the sun and moon are perpendicular to each-other and their gravitational fields have an opposing influence on the tides. [image:http://i.imgur.com/lFYnSn2.gif] '''Circannual –Yearly activity period (approx 365 days)''' The earth axis is on a slight tilt, causing a yearly cycle of seasonal changes. Countries experience summer when their hemisphere is tilted towards the sun. It's not warmer because they are any closer to the sun, but because the days are longer. In winter the days are shorter and the sun doesn't rise as high, staying closer to the horizon. The lower angle of Sun during the winter months means that incoming rays of solar radiation is spread over a larger area of the Earth's surface, so the light received is more indirect and of lower intensity. Lower intensity light is less able to heat the ground. Circannual rhythms are often associated with migratory behaviours, hibernation, dormancy, etc. ==Biological Rhythms in Animals== If the biological rhythm is controlled by a biological clock it will continue even in the absence of the exogenous (external) stimulus. The behaviour is not a direct response to an environmental factor, but is controlled '''endogenously''' -by an internal timing mechanism that is ultimately genetic in origin. It allows the organism to anticipate the environmental change and respond before hand. However, some responses are controlled purely by exogenous (external) factors. In this case the response always follows the environmental change.
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