Biological Clocks

Free-running endogenous clock.Can be entrained by external cues

Selective advantage

Hypothesis 1: biological clocks control behaviours where some synchronisation is necessary

eg1 intraspecific (for breeding or migration)

eg2 interspecific (to exploit a seasonal food source)

Hypothesis 2: physiological preparation for an anticipated environmental change

eg storing food reserves prior to hibernation, which is more energy efficient.

3

Timing

Need to know time for navigationDay-length - internal clockNeed to “date” for migration

Wilson’s Plover

Free-running Rhythms

Generally have a slightly longer period than that of the environmental rhythm they are associated with.

Rhythm is entrained or synchronized to the environmental cue (zeitgeber).

May involve a phase-shift if the environmental pattern is changed e.g. jet lag is the body’s slow altering of the circadian rhythm to fit the new cycle.

Biological Timing Responses

Governed by internal clocks Annual cycles Daily cycles Lunar cycles

NOT necessarily governed by environment: organisms can anticipate changes cycles continue in constant lab conditions

Geophysical cycle

length of cycle

biological rhythm

known zeitgebers

solar year 365.2 days circannual photoperiod

lunar month 29.5 days circamonthly light of full moon

solar day 24 hours circadian light, temp., humidity

tidal 12.4 hours circatidal tidal activity

• Biological clock – internal

• Period of rhythm – length of cycle

• Phase shift – change in onset of period

• Free-running period – independent of external cues

• Entrainment – regular resetting of clock (zeitgeber)

• Zeitgeber (time giver) – environmental cue to reset clock

• Circa (about) – cycles not in synch with cues

• Photoperiod – responses to length of day / night

• Exogenous rhythm – controlled by external cues

• Endogenous rhythm – controlled by internal clock

Biological Clocks

• Endogenous, free-running

• May use exogenous cues to reset clock

• May predict onset of periodic change

• Cycle often longer than associated exogenous cycle

• Entrained by exposure to external cues.

• May involve chemical signals.

• May involve a phase-shift e.g. jet lag

Biological clocks used for:

• Control daily rhythms – sleep, pulse, blood pressure, temperature, sex drive.....

• Reproduction timing – coordinate courtship rituals, release of sperm and eggs into water, sexual preparedness ......

• Migration preparedness – eating lots

• Winter preparedness – storing food, coat thickness, hibernation

• Navigation – solar & stellar

Biological RhythmsDaily Rhythms – patterns of behaviour linked to the day-night cycle • nocturnal = night-active e.g. possums• diurnal = day-active e.g. bees• crepuscular = active in twilight (dawn and dusk) e.g. rabbits Annual Rhythms – linked to Earth rotation around the sun and seasonal

changes due to Earth’s axial tilt (such as photoperiod) e.g. change in coat colour of Arctic fox

Tidal Rhythms – patterns of behaviour linked to the tides, a result of the

gravitational pull of the Moon and the Earth’s rotation e.g. mud crabs Lunar Rhythms – (much less common) patterns of behaviour linked to

the rotation of the Moon around the Earth e.g. Whitebait spawning

Geophysical Rhythms

• Biological rhythms mirror geophysical rhythms.

• ‘All living organisms are subjected to a diverse set of geophysical oscillations due to the ceaseless rotation of the moon round the earth, and the earth round the sun and on its own axis’ (D. S. Saunders, 1977).

Circadian rhythm

De Mairan (1729) showed mimosa leaf movements –circadian rhythm - in absence of external cues.

Absence of rhythmic environments?

Rhythmicity persists in animals in the absence of environmental cues.

Eg organisms in deep sea, caves and polar regions.

Honey bees are chronobiological

• Very accurate time perception / time sense

• Clear circadian rhythms and memory.

• Exhibit distinct behaviours in their everyday life.

• Complex communication.

• Return from a good food source and perform a dance to communicate position of the food.

Testing internal clock hypothesis

Timing controlled by internal biological clocks, which continue to function in the absence of external time cues.

Recording animal activity

We expect to see a recognisable rhythm associated with a geophysical rhythm.

Location A Location B Location C

Waggle dance (food distant)

A

CB