BIOL 331 Lecture Notes - Circadian Rhythm, Circadian Clock, Freerunning
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Responses to the environment must be timed appropriately
CIRCADIAN RHYTHMS CONTROL THE DAILY CYCLE OF BEHAVIOUR:
o The persistence of daily cycles in the absence of environmental time cues suggests that
animals have an endogenous clock (seldom exactly 24 hours) circadian rhythms.
o When two rhythms completely match, they are in phase, and if a rhythm is shifted, it is
phase-advanced or phase-delayed.
o Since the period of circadian rhythm is not exactly 24 hours, it must be phase-advanced
or phase-delayed to remain in phase with the daily cycle of the environment; rhythm
has to be entrained to the cycle of light and dark in the animal’s environment.
o Animal kept in constant conditions will not be entrained to the cycle of the
environment, and its circadian clock will run on its natural period free-running.
o The free-running circadian rhythm is under genetic control.
o Animals that are active at night are nocturnal and have different sensory capabilities
than those that are diurnal. Diurnal animals tend to be highly visual, whereas nocturnal
animals depend more on their abilities to hear and small and use tactile information.
o Diurnals have retinas made up entirely of cone cells and nocturnal retinas are composed
entirely of rods (highly sensitive to light stimuli, but do not provide info about colour).
PHOTOPERIOD AND CIRCANNUAL RHYTHMS CONTROL SEASONAL BEHAVIOURS
o Most animals reproduce most successfully if they time their reproductive behaviour to
coincide with the most favourable time of year for the survival of their offspring.
o A change in day length—photoperiod—is a reliable indicator of seasonal changes to
o Hibernators and equatorial migrants have circannual rhythms.
Animals must find their way around their environment
PILOTING: ORIENTATION BY LANDMARKS—most animals find their way by knowing and
remembering the structure of their environment piloting.
HOMING: RETURN TO A SPECIFIC LOCATION—the ability to return over long distances to a nest
site, burrow, or other specific location homing.
MIGRATION: TRAVELLING GREAT DISTANCES
o Distance-and-direction navigation: requires knowing in what direction and how far
away the destination is. With a compass to determine direction and a means of
measuring distance, humans can navigate.
o Bicoordinate navigation: true navigation, requires knowing the latitude and longitude
of both the current position and the destination.
o How do animals determine distance and direction Two obvious means of determining
direction are the sun and the stars.
o The stars offer two sources of info about direction: moving constellations and a fixed
point. One point that does not change during the night is Polaris (North Star) and always
How Do Animals Communicate with One Another?
Visual signals are rapid and versatile
Visual signals are easy to produce, come in an endless variety, can be changed rapidly, and
clearly indicate the position of the signaller.
Most animals are sensitive to light and can receive visual signals.
The extreme directionality of visuals signals means that the received must be looking directly at