BIOLOGICAL CLOCKS

Biological timing responses All organisms respond to different cycles. They respond to: 1) annual cycles, with the changing of the seasons; 2) daily cycles (night and day) 3) lunar cycles (monthly cycles) 4) tidal cycles (related to the tides)

Synchronisation with the cycles It is advantageous for an organism to be in sync with these cycles in it’s environment. Important events such as mating, germination, migration are triggered by different stimuli which can be endogenous or exogenous.

Exogenous Rhythms When a rhythm is in direct response to an environmental cue it is said to be exogenous. Examples include day length, temperature, the height of tides. An example of an exogenous biological rhythm is the hopping of sparrows on a perch when a light is turned on, or hamsters running in their wheels when the lights are turned off.

Endogenous Rhythms Endogenous rhythms are set by an internal clock (biological clock). They will persist even in the absence of environmental cues. Examples include preparing for winter by storing food, animals coming on heat, solar navigation.

Resetting the biological clock To remain in synchrony with the environment, biological clocks need to be reset at regular intervals. This process is called entrainment. The environmental cue that resets the biological clock is called the zeitgeber (“time giver” in German)

Look at page 33 Biozone Read through the human biological clock and answer the pages on 33

Period The period of a rhythm is the length of one cycle of the rhythm. In this graph the period of the cycle is about 24 hours.

http://bcs.whfreeman.com/thelifewire/content/chp52/5202002.html

Types of rhythms/cycles

Circadian Cycles A circadian cycle has a period of about 24 hours. In other words it is about 24 hours long (one day). Examples of circadian cycles in humans include the wake/sleep cycle, body temperature changes, and heart rate (the last two drop at night).

Patterns of Activity within Circadian Cycles Diurnal - mostly active during the day, inactive at night. Eg. Humans, bees. Nocturnal – active at night, inactive during the day. Crepuscular – active at dawn and dusk. Eg rabbits and mosquitoes. Arrhythmic – no regular pattern

Circatidal cycles A circatidal cycle is about 12.5 hours long (one tidal cycle). Circatidal cycles affect marine organisms, particularly those which live on shorelines. Examples include feeding cycles for shellfish and rock pool animals.

Animals showing circatidal cycles

Circalunar (circamonthly) Cycles A circalunar cycle is about 29 days long (one lunar month). Because the position of the moon and sun generate tidal patterns, they can affect marine organisms.

Breeding in grunions Grunions are small fish that spawn on land. From April to June, when a spring tide occurs (3 or 4 times), the fish squirm onto the beach at high tide. Female releases her eggs in sand and the male wraps around her to release the sperm. They then catch the out-going tide. 15 days later, at the next spring tide, the young grunions have hatched and catch a wave back out to sea.

Circaannual Cycles A circaannual cycle is about 365 days long (one year) Examples of annual cycles: Reproduction of animals and plants Hibernation in animals Migrations Dormancy of seeds Leaf fall (abscission)

Hibernation – an example of a circaannual cycle Some animals survive over winter by slowing down their metabolism and hibernating. The American Woodchuck changes: it’s heart beat from 80/min to 4-5 /min it’s temperature from 37ºC to 3ºC.

Aestivation = summer hibernation When the weather gets too hot, some organisms hibernate in summer. Earthworms burrow deep into the soil,coil into a knot, and secrete a mucous over themselves to stay moist.

Photoperiodic responses = photoperiodism In most cases the cue which triggers these cycles is the change in day length. We call these photoperiodic responses.

Free running period The free running period of a cycle is the length of one cycle in the absence of environmental cues. For example: If people live in an environment of constant light, temperature etc, their free running period is more than 24 hours (24 hours, 42 mins).

Phase shift When the onset of the period of the rhythm is changed (either earlier or later) we have a phase shift. A phase shift occurs when you travel around the earth into a different time zone – this leads to Jet Lag. Phase shifts can be induced artificially in the laboratory by controlling the light and dark periods that an organism experiences.

Do pages 34-38 biozone. Look through first.

Actograms Often when plotting activity of animals on an actogram we place two days next to each other. This makes it easier to read.

Working out free running period 1. Work out when the first period of activity finishes (nearest ½ hour) 2. Count down 10 days 3. Work out when the 10th period of activity finishes (to nearest ½ hour) 4. Work out the difference between these two times 5. Convert into minutes to work out the avg time gained/lost over 24 hours 6. Add or subtract from 24 hours to work out free running period.