BIOLOGICAL RHYTHMS -PHASE SHIFTS

branch ANIMALS

=Phase Shifts= Below is an actogram that shows the activity of a given organism over a period of 80 days. However on day 22 (marked in red on the y axis), all external cues (Zeitgebers) were removed. This may have involved placing the organism in complete darkness and keeping it at a constant temperature. We can see from the first 22 days that the organism exhibits a biological rhythm, with a period of approx 24 hrs (circadian). We can also see that once the external cues (Zeitgebers) are removed the free running period is slightly over 24hrs as the period of activity is getting a little later each day. The amount by which the period shifts each day is known as the '''Phase Shift'''. [image:http://i.imgur.com/YNoCBHZ.png?1] For the first 22 days the period of the rhythm (in presence of zeitgebers) was 24 hrs. The free running period (Zeitgebers removed) is a little over 24 hrs (we know this because the activity gets later each day / shifts right). The phase shift is about 1hr 16mins 80 hrs. To calculate phase shift: Draw a line down from the point at which the Zeitgebers were removed (where the phase shift started). Draw a second line diagonally following the activity periods (as above.) Draw a line connecting the two to show by how much the period of activity has shifted. Divide this by the number of days to give the Phase Shift; 80hrs divided by 63 days = 1.27hrs = 1hr 16 mins / day The biological rhythm is getting later by approx. '''1hr 16 mins''' each day. Thus '''our free running period''' is effectively '''25hrs 16mins'''. ==Example Question== [image:http://i.imgur.com/xjfBXaJ.png?1] a) The crabs are active twice each day, starting just before high tide. This is a circatidal rhythm and the onset of activity is slightly later each day. There is a high tide approximately twice each day. Note that because a high tide actually occurs every 12.4 hrs; the two high tides occur about 50mins later each day. This is often mistaken for a phase shift. b) An endogenous rhythm is an internal rhythm, which involves an internal biological clock. The rhythm persists in the absence of external cues. However, the absence of any Zeitgebers results in a Free Running Period that does not correspond perfectly to the period of the environmental rhythm. This is known as a phase shift (indicated in Red on the above graph). c) Normally, when the crab is active, its environment is covered by the high tide. Hence, the water may provide protection from potential predators (e.g. birds), prevent desiccation (drying out) and increase the likelihood of finding food. During low tide the crabs remain inactive. This will make them less visible to potential predators. Therefore, this response is advantageous as it allows the crab to conserve energy during low tide when they would be more exposed to predation and less food is available, and to be most active when the risk of predation is less and the chance of obtaining food is higher.