Biological Rhythms

Biological Rhythms • Most organisms show rhythmic activities. Many of these rhythms are linked to cyclical environmental changes. • Exceptions - inhabitants of the ocean depths and deep underground caves,

Cyclical Environmental Changes • Due to the annual orbit and tilt of Earth. Seasonal contrast increases with latitude. • Daily rotation of Earth produces day/night cycle.

Monthly lunar orbit changes night time illumination. • The combined effect of lunar orbit and Earth rotation produces twice-daily ocean tides causing a cycle of exposure and coverage of the intertidal area. • Tidal levels vary due to relative sun-moon positions.

Daily Rhythms of animals are linked to day-night cycle. • Most organisms are active during a portion of a 24 hour cycle and may be nocturnal, diurnal or crepuscular.

Nocturnal = mainly active at night e.g. morepork (ruru), weta, moths, bats

Diurnal - mainly active during the day e.g. hawks, humans, bees. • Human body temp, urine composition and drug sensitivity show daily rhythmic changes

Crepuscular - mainly active in twilight of dawn and duske.g. rabbits, mosquitoes, some geckos

Tidal Rhythms • Tides are due to Earth’s rotation relative to the moon [2x daily] every 12.4hours lunar rhythms) • Biological rhythm  Circatidal

Lunar Rhythms Linked to rotation of Moon around the Earth (29.5 day cycle) e.g. young salmon move to estuaries before their seaward migration. At the new moon, thyroxine output surges allowing regulation of salt excretion.

Semi-lunar Rhythms • Linked to the rotation of the Moon around the Earth but has 2 cycles per lunar month • Spring tides occur ~ every 15 days, just after new and full moon. Therefore some organisms are un/covered by tide every 15 days

These organisms show twice monthly rhythms rather than the twice daily rhythms of mid-shore organisms.

e.g. Inanga spawn in estuaries with the full and new moons in March and April. The eggs are laid amongst vegetation that is not normally submerged. Eggs develop within 2 weeks and hatch on next spring tide as whitebait.

Annual rhythms • Mating and rearing of young is timed to exploit annual periods of abundant food and suitable environmental conditions.

Compound Rhythms The shore environment changes in a more complex way than elsewhere because of the combined effect of several cycles such as day-night, tidal, lunar, semi-lunar and annual each with a different period.

For example: • To avoid desiccation, sandhoppers feed only when low tide occurs at night. • Therefore they are responsive to daily and tidal cycles.

Internal and external rhythms seldom have the same period. • Usually the internal rhythm is longer than the external cue. • circadian = approximately a day (~ 24 hr) • circatidal = approximately each tide (~12.4 hrs) • circalunar = approximately each moon rotation (~ 29.5 days) • circannual = approximately a year(~ 365 days)

Circa-rhythms only occur if the behaviour continues under constant environmental conditions. They are the result of an internal clock that is free-running.

Just because a behaviour is cyclic, does not mean it is due to a biological clock. • Biological rhythms can be driven by either exogenous factors or endogenous factors.

Endogenous rhythms • Environment independent • Can be entrained by exposure to an external cue that synchronises the rhythm to a natural cycle • The behaviour is cyclic because an internal clock acts independently of any cues from the animals external environment • If kept under constant conditions the animal should exhibit the behaviour at the same period each cycle.

Entrainment • The internal clock is ‘reset’ or entrained by the environment to ensure internal and external rhythms are synchronised.

The environmental cue that resets the clock is called the zeitgeber. Circadian rhythms usually have sunrise or sunset as a zeitgeber.

Benefits of Entrainment • Allows organisms to adjust to seasonal changes e.g. day length. • Allows migratory organisms to update their clocks as they move east-west.

In diurnal organisms the zeitgeber is usually dawn. • In nocturnal animals the zeitgeber is usually dusk. • With circatidal rhythms the zeitgeber is usually the water turbulence.

E.g. sheep need to be ready to mate in the autumn so that lambs will be born in the spring when there is plenty of food. So it is important that they are not misled by a brief warm spell.

Entrainment by Pheromone Synchronisation of the menstrual cycle in women living together is brought about by pheromones.

E.g. In continuous darkness a particular plant shows circadian rhythm in CO2 production. • A four hour period of light will delay peaks by 12 hours.

Phase Shifting • Entrainment involves phase shifting in which the rhythmic peaks are advanced or retarded.

Exogenous Rhythms • Externally driven rhythms based on environmental changes. The normal response does not happen in the absence of the environmental cue.

Evidence for Internal Clocks Rhythms continue in a constant environment. E.g. Oxalis shows daily ‘sleep’ movements of their leaflets. When kept in constant light, the rhythms continue.

When a rhythm continues in constant environmental conditions, it is said to be free-running, as it is unaffected by external cues. • Gradually these rhythms when isolated (from the zeitgeber) shift out of synchrony with the external environment.

Relocation Experiments • Bees normally fed at the same time every day, will continue to do so even when shifted to a new place. • The bees will continue to feed according to the time at their place of origin. • If their feeding was determined by an external cue they would have adapted to local time.

Rhythms vs Circa-rhythms • Rhythms may be daily, tidal, lunar, semi-lunar or annual and only occur under natural conditions. • Rhythms may be circadian, circatidal, circalunar, circasemilunar, or circannual.

Exogenous Rhythms • Environment dependent • The behaviour is cyclic because neural mechanisms are responding to cyclic variations in the environmental cues • If kept under constant conditions the rhythm should disappear.

Endogenous Rhythms • Internally driven responses controlled by an internal ‘clock’ usually ‘reset’ by an environmental change.

Biological Rhythms