3. Animal Orientation in Time

Orientation in Space

Orientation in Time

Plant Orientation in Time

Species Interactions

Concept 11: Relationship between Astronomical, Environmental, & Biological Rhythms

Success Criteria & Vocabulary

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Adaptive advantage: Any trait that results in an organism having a greater chance of surviving to an age where it can reproduce.

Breeding: The mating and production of offspring by animals.

Circadian: A rhythm that cycles over an approximately 24-hour period.

Circalunar: A rhythm that cycles over an approximately 29.5 day period.

Circannual: An annual rhythm (one year).

Circatidal: A rhythm that matches the movement of the tides and has a period of ~12.5 hours.

Crepuscular: Animals most active in dawn and dusk.

Diurnal: Animals most active in daytime.

Environment: All the factors in an organism's surroundings that can potentially affect it.

Environmental rhythm: Predictable cycles in abiotic factors due to the motions of the Earth, Moon, and Sun.

Foraging: Searching widely for food.

Migration: The long distance movement of animals from one region to another, usually seasonally.

Nocturnal: Animals most active at night. 

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Concept 11: Support Notes

What are the various astronomical cycles?

The motions of the Earth, Moon, and Sun result in complex, interdependent cycles, creating ENVIRONMENTAL changes that range from short term (a few hours) to long term (many hundreds of days). 

Cosmic forces, such as the movement of the Moon around the Earth and the Earth and planets around the Sun generate predictable environmental cycles such as day and night, and the seasons. 

Most organisms are adapted to take advantage of such ENVIRONMENTAL RHYTHMS by having internal timing mechanisms. These cycles provide cues that enable organisms (both plants and animals), to time important events in their lives, such as FORAGING activity, BREEDING, and MIGRATION. 

The image below summarises these various astronomical cycles that influence environmental rhythms. The tidal cycle is not shown on the diagram below, but involves the gravitational pull of the moon as well as centrifugal forces on the oceans. 

Types of environmental rhythms

Annual rhythms

Rhythms with peaks a year apart are linked to the seasons. 

Seasons are the result of the Earth's orbit around the Sun and Earth's 23.5° tilt from its orbital plane with the Sun. 

Daily rhythms

Daily rhythms are linked to the rotation of the Earth on its axis every 24 hours, resulting in the cycle of day and night. 

Most animals are active for only part of each 24-hour cycle, and fall into three groups, according to when they are most active:

Tidal rhythms

The rise and fall of the tides is caused mainly by the gravitational attraction between Earth and Moon, together with the rotation of the Earth on its axis. The gravitational pull of the moon causes water to pile up beneath it. 

Each day, high tide is about 50 minutes later than on the previous day. This is because the moon is also rotating around the Earth. 

Organisms living on the shore regularly experience two environments:

Types of Biological Rhythms

The activity patterns of animals often occur around the predictable ENVIRONMENTAL RHYTHMS, such as the light-dark cycle (day/night), and the changing of the tides and the seasons. 

It is important that animals are able to synchronise their activities with other animals and their ENVIRONMENT. This synchronisation is ADAPTIVE because it ensures the animal is active at the appropriate time. For example, bats in caves need to be able to predict when it is dark before they come out of the cave to feed as they are nocturnal. 

Read the examples of  biological rhythms below. The length of time it takes to complete the entire cycle is termed the rhythm's period, e.g. 24 hours. 

Rhythm: CIRCADIAN (daily)

Period: ~24 h


Example: Weta are generally active during the night (nocturnal) when they forage in leaf litter or trees. Being active at night makes them less vulnerable to daytime predators. 

Rhythm: CIRCATIDAL (tidal)

Period: ~12.4 h (coincides with tidal flows)


Example: In the New Zealand tunnelling mud crab (Helice crassa) locomotion and feeding occurs at low tide when food items (bacteria and algae) are easy to collect. They are more active during the day than at night. 

Rhythm: CIRCALUNAR (lunar)

Period: ~29.5 days (a month)


Example: Inanga spawn during the months of March and April during the spring tides (new and full moons). Activity peaks 2-3 days after the new or full moon. During the spring tides they are able to lay eggs higher up river banks, which protects the eggs from aquatic predators.

Rhythm: CIRCANNUAL (annual)

Period: ~a year


Example: NZ long-tailed bats hibernate for 4-5 months during autumn and winter when temperatures are low and insect food is scarce. 

Rhythm: CIRCANNUAL (annual)

Period: ~a year


Example: In many domestic livestock species, like sheep, the reproductive cycle is timed so that young are born in spring when the weather is warmer and food is more plentiful. 

Circadian rhythms in NZ birds

In animals, the circadian rhythm can be further described by the pattern of activity exhibited:

Exogenous vs Endogenous Rhythms

Circadian, circatidal, and circannual rhythms can either be:

Exogenous (externally driven)

Exogenous rhythms are controlled by environmental factors outside the body - they are externally driven. 

Changes in environmental factors could be light availability in a day-night cycle, tide length in a circatidal rhythm, or the changing day length in circannual rhythms. 

There are NOT many rhythms controlled only by the environment - many are driven by an internal clock AS WELL AS changes in the environment. 

Endogenous (internally driven)

Endogenous rhythms are internally driven by an internal clock, called a biological clock.

Since they are controlled internally, they are not affected by changes in the environment and can continue when the environmental conditions remain constant (e.g. constant light).

For example, the biological clock controls the production of cortisol and melatonin which control sleep-wake cycles.

Concept 12: Entrainment of the Biological Clock by a Zeitgeber

Success Criteria & Vocabulary

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Adaptive advantage: Any trait that results in an organism having a greater chance of surviving to an age where it can reproduce.

Biological clock: An endogenous timing system an organism uses to synchronise its activities with the environment and keep track of time. 

Endogenous: A stimulus originating within the organism itself, e.g. hormonal changes, is this. 

Endogenous rhythm: A rhythm that continues even in the absence of any external cues.

Entrainment: The synchronisation of an endogenous rhythm with an external cycle or cue such as light and dark. 

Hypothalamus: A region in the brain that is the location of the biological clock in mammals. 

Period: Length of time/duration

Physiology : Chemical or physical functions in an organism.

Pineal gland: A tiny organ in the brain that is the location of the biological clock in birds, reptiles, and amphibians. 

Suprachiasmatic nucleus (SCN): A group of cells in the hypothalamus, just behind the eyes, responsible for the biological clock.

Zeitgeber: An exogenous cue that synchronises/entrains an organism's endogenous rhythms to the rhythms of the environment, e.g. the Sun or the light-dark cycle. 

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Concept 12: Support Notes

The ENDOGENOUS BIOLOGICAL RHYTHMS we observe in animals is the result of an EXOGENOUS (external) factor called a ZEITGEBER, interacting with an ENDOGENOUS (internal) factor called the BIOLOGICAL CLOCK. 

Basically, the zeitgeber (environmental cue or rhythm) resets the biological clock so that the animal's internal timing system (biological clock) can be synchronised with the environment.  

What is a Biological Clock?

A BIOLOGICAL CLOCK is an ENDOGENOUS (internal) timing system that helps to control the PHYSIOLOGICAL responses and activities of an animal. 

The biological clock helps to control ENDOGEOUS RHYTHMS such as heart activity, hormone secretion, blood pressure, oxygen consumption, and metabolic rate. These endogenous rhythms established by the biological clock  will continue even in the absence of environmental cues, although the PERIOD (duration) of the rhythm may be slightly different to the environmental rhythm. However, when the endogenous rhythms controlled by the biological clock become out-of-sync with the environment, various short or long term disorders can occur, e.g. jet lag. 

Biological clocks have an ADAPTIVE function, such as helping anticipate environmental changes and preparing the body for the activities that will predictably follow. Here are some functions of the biological clock:

To predict and prepare for events in the environment. 

For example: Hedgehogs store food reserves as fat for periods of torpor (slowing down) or hibernation during cold months.

To synchronise migration, reproduction, or social activities. 

For example: Gannets congregate (gather around) at breeding grounds at the same time of year. 

To synchronise circadian and annual rhythms with changes in the environment. 

For example: Tuataras and other reptiles synchronise with day-night cycles to be able to bask in the sun to warm up.

To know the time and use this for navigation and sun compass orientation.

For example: Honeybees must use the time of day and position of the Sun to orient themselves in space. 

Where is a Biological Clock located?

The location of the BIOLOGICAL CLOCK varies between organisms. In birds, reptiles, and amphibians, it is located in the PINEAL GLAND (in the brain). In insects, each cell has its own biological clock. In mammals, the biological clock is located in the HYPOTHALAMUS. 

For most humans, the biological clock runs at about a 25 ½ hour day. To keep it synchronised with the 24 hour-day cycle, it needs to be reset each day, reacting to outside stimuli such as light and dark, and meal times. The clock is made up of a collection of cells in the hypothalamus, called the SUPRACHIASMATIC NUCLEUS (SCN), just behind the eyes. Light from the eyes stimulates the nerve pathways to the SCN and regulates its activity. 


What is Entrainment?

Biological clocks stay synchronised with the environment because they are regularly reset by a ZEITGEBER. The process of resetting the biological clock is known as ENTRAINMENT. 

Entrainment of the biological clock is adaptive, because it contributes to fitness by ensuring the success of critical activities such as mating, birth, germination, foraging, and periods of torpor and dormancy. 

Concept 13: Interpreting Actograms

Success Criteria & Vocabulary

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Actogram: A special graph that shows the activity of an animal or plant during the day and over many successive days. 

Biological clock: An endogenous timing system an organism uses to synchronise its activities with the environment and keep track of time. 

Biological rhythm: Physiological changes or changes in activity in living organisms occurring in a cyclic manner. Most often associated with predictable daily, monthly, or annual environmental changes. 

Circadian: A rhythm that cycles over an approximately 24-hour period.

Circannual: An annual rhythm (one year).

Circatidal: A rhythm that matches the movement of the tides and has a period of ~12.5 hours

Crepuscular: Animals most active in dawn and dusk.

Diurnal: Animals most active in daytime.

Endogenous rhythm: A rhythm that continues even in the absence of any external cues.

Entrainment: The synchronisation of an endogenous rhythm with an external cycle or cue such as light and dark. 

Exogenous rhythm: A pattern that occurs only in response to external cues and which disappears when cues are removed. 

Free running period: The period of an endogenous rhythm when given no external environmental cue by which to synchronise the rhythm (i.e. conditions are constant).

Nocturnal: Animals most active at night. 

Period: Length of time/duration

Phase shift: The amount of time difference between an entrained endogenous biological rhythm and the same rhythm when the zeitgeber(s) have been removed. 

Zeitgeber: An exogenous cue that synchronises/entrains an organism's endogenous rhythms to the rhythms of the environment, e.g. the Sun or the light-dark cycle. 

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Concept 13: Support Notes

What is an Actogram?

An ACTOGRARM is a special graph that shows the activity of an animal or plant during the day and over many successive days. This gives a good picture of when an animal is active at any time. 

Periods of activity are shown as black bars, and periods of rest are white spaces where there are no bars. Each line on an actogram sometimes shows activity over 24 hours (a day), but more often each line is double-plotted, showing activity over 48 hours. Double plotting allows us to see patterns in activity more easily.

Experiments are usually carried out that change the light cycles the organism experiences. They are shown as LD cycles, e.g. LD 12:12 means 12 hours light then 12 hours dark. Total darkness is often labelled as DD, and total light as LL.

When an animal is exposed to a LD 12:12 cycle for numerous days, the black bars (representing activity) stack/align on top of each other. The biological clock is said to be ENTRAINED because it has been reset by the ZEITGEBER.

However, when the environmental cues are kept constant (e.g. constant dark, DD) the black bars become misaligned and no longer stack on top of each other. In the actogram to the right, activity begins later and later each day in a regular pattern.

The biological clock is said to be in FREE RUNNING when the animal is kept in constant conditions. 

The length of time it takes for the cycle to repeat under constant conditions is the free running period. The video below explains free running period really well. 

From actograms, you can work out the following:

A PHASE SHIFT occurs when an organism is entrained to a new regime of environmental cues. 

How to analyse actograms

When analysing actograms, ask yourself the following questions:

How to calculate the phase shift and free running period:

Example:

The actogram below shows the activity of a cockroach in laboratory conditions, initially given a cycle of 12 hours light and 12 hours dark (LD 12:12). Light was removed after day 5 leaving the cockroach in complete darkness. Calculate the phase shift and free running period. 

Actogram of human activity

Activity pattern in animals

A free-running endogenous rhythm usually has a slightly different period from the environmental rhythm that entrains it. The rhythm can be recorded as an actogram. In the absence of an external zeitgeber (such as light) an endogenous rhythm will adopt a free-running period, which is usually lightly different from the cyclic environmental variable to which it is usually entrained. 

Below, the activity patterns of two species were recorded in the absence of environmental cues to determine the free running period of the rhythm. The results are displayed below as actograms. On each actogram, the first day of recording is only shown on the left (day two is outlined and shaded in blue). 

The house mouse is a nocturnal animal. The chart above recorded the activity patterns of two different mice. Their activity was recorded for 7 days, using a running wheel with sensors connected to a computer. During the course of the experiment, the mice were kept in dim light. 

Toebiters are small marine isopod crustaceans that scavenge for food in the surf zone of sandy beaches in New Zealand, foraging with the incoming tide and retreating to burrows with the outgoing tide. The chart records the activity pattern over 14 days. During the experiment, the freshly collected toebiter was kept in total darkness at a constant temperature of 20°C 

Cockroach activity

Cockroaches are nocturnal insects. The experiments below investigated the periodicity of their behaviour under controlled conditions.

Experiment 1: Free-running period

The first experiment determined the free-running period. The following charts record the activity rhythm of a cockroach kept for 20 days in a running wheel actogram. There are activity records shown for each of the 20 days, with each day's record presented in succession down the page. The periods of activity are shown as grey rectangular blocks and periods of inactivity shown as no rectangle. 

The onset of constant darkness on day 11 exposed the free-running period and produced a phase shift. 

Light regime is the term used to describe the cycles of light and darkness. It is indicated by bars of 'light' and 'darkness' at the bottom of each table:

Experiment 2: Entrainment

Remember that the process by which the endogenous rhythm is synchronised to an environmental cue (or zeitgeber), such as a 12 hour light/12 hour dark cycle, is termed entrainment. The results of the second experiment show the entrainment of the rhythm of a cockroach. It is being entrained to a new light cycle, which occurs nine hours earlier than the one it had been experiencing previously. 

Entrainment usually has the following features: