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Do Now

2 reasons why an animal migrates

What is the difference between homing and migration?

Migration example?

What initiates migration?

What wakes you up every morning?

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Biological Rhythms

Most organisms show rhythmic activities.

Many biological rhythms are linked to cyclical changes in the environment due to movements of the Earth and Moon.

Exceptions - inhabitants of the ocean depths and deep underground caves do not exhibit these biological rhythms

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Cyclical Environmental Changes

Daily rotation of Earth produces day-night cycle.
Annual orbit of tilted Earth around the sun leads to seasonal changes in daylength and temperature
Gravitational pull of the Moon causes tides on Earth. Water piles up both towards and away from the moon, creating a high tide. The combined effect of

lunar orbit and Earth rotation

produces twice-daily tides.

Lunar orbit every 27.3 days

causes the visible phases of

the moon.

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Environmentally linked Biological rhythms

Examples of environmentally linked biorhythms:

Daily

(circadian)

Tidal

(circatidal)

Lunar

(circalunar)

Semi-lunar

(circasemilunar)

Annual

(cirannual)

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Daily Rhythms - (Circadian)

Daily rotation of Earth relative to the sun produces day-night cycle.

Daily Rhythms of animals are linked to this day-night cycle.

Most animals are active during a portion of the day night cycle and may be nocturnal, diurnal or crepuscular.

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Daily Rhythms - Nocturnal

Nocturnal = mainly active at night e.g. morepork, weta, moths, bats, kiwis, crickets.

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Daily Rhythms - Diurnal

Diurnal - mainly active during the day e.g. hawks, humans, bees, cicadas, grasshoppers.

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Daily Rhythms - Crepuscular

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

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Tidal Rhythms - (Circatidal)

Tides form due to Earth’s rotation relative to the moon every 12.4hours (2x daily)

Organisms living near the middle of the tidal zone alternately inhabits two environments - land and sea.

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Lunar Rhythms - (Circalunar)

Linked to rotation of Moon around the Earth (27.3 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.

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Annual rhythms - (Circannual)

Seasonal rhythms are often parallelled by biological rhythms, as certain activities are restricted to particular times of the year.

e.g. mating and rearing of young is timed to exploit annual periods of abundant food and suitable environmental conditions.

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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.
E.g. To avoid desiccation, sandhoppers feed only when low tide occurs at night.

Therefore they are

responsive to daily and

tidal cycles.

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Endogenous or Exogenous

Biological rhythms can be of two kinds:

Internally driven, or endogenous rhythms. These involve an internal 'clock', which keeps rhythm even in absence of environmental changes,

The 'clock' is usually adjusted/reset by environmental changes
For example Clover shows daily 'sleep' movements of their leaflets. When the plant is kept in constant light the rhythm continues, though they gradually get out of synchrony with the external environment.

Externally driven, or exogenous rhythms.

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Biological (internal) rhythms

We use the term circa meaning about when describing biological rhythms

Circadian - free-running rhythm exhibits a period of about 24 hours / day
Circannual - free-running rhythm exhibits a period of about 365 days / year
Circatidal - free-running rhythm exhibits a period of about 12 hours / tide
Circalunar - free-running rhythm exhibits a period of about 30 days / month
Circasemilunar - free-running rhythm exhibits a period of about 15 days

A free running period is the length of a period in the absence of environmental cues

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Internal and external rhythms seldom have same period

When a biological (internal) rhythm is free-running, its period (the time between successive peaks of activity) does not correspond exactly with the period of the environmental (external) rhythm

The above actogram shows that when kept in constant conditions, each day the onset of activity is about 2 hours later than the previous one.
The free running period is thus about 26 hours.
Hence the term circadian (circa = about)

When a rhythm is continuing in constant environmental conditions, it is said to be free-running, as it is unaffected by external time cues.

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Actogram = Activity record

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Entrainment

In natural conditions the internal clock must be continually 're-set' or entrained by the environment.

Otherwise the biological rhythm and external rhythm would become unsynchronised.

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