Earth's cycles
Our external environment is full of cycles; daily cycles, monthly cycles, yearly cycles... all caused by the behaviour of the Sun, Earth and Moon. Each of these cycles can influence the behaviour of animals and plants, as they cause particular changes to occur in the environment, which they have to respond to. Therefore, animals and plants can display one or some of the following rhythms: daily rhythms, lunar rhythms, and annual rhythms.
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Daily rhythms
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Tidal rhythms
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Lunar rhythms
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Annual rhythms
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On Earth there is a daily night/day cycle that takes 24 hours. This is because it takes 24 hours for the Earth to rotate once on its axis, giving each part of the Earth time facing towards and away from the sun.
Animals display three types of activity linked to this 24 hour cycle: - Diurnal - are active during periods of light - during the day - e.g. humans, honeybees, blackbirds - Nocturnal - are active during periods of dark - during night time - e.g. bats, owls, moths - Crepuscular - are active at dawn and dusk - e.g. mosquitoes, rabbits, fiddler crabs |
The relationship between the Earth, Moon and Sun create our tides - and many animals live in or near the sea and are affected by the changes in tides!
Keep in mind that as the Earth rotates once every day, we get smaller changes in the tidal levels - we get one high tide and one low tide every day. This is because as we rotate, different parts of the Earth are closer to the Moon, where the gravity will pull the water on Earth towards the moon until we spin past it! This is called a tidal rhythm. |
On Earth there is a monthly cycle that occurs, based on the Moon's orbit around Earth.
The monthly orbit of the moon creates different amount of illumination at night, throughout a month. This can influence an animal's rhythms. Rhythms resulting from this are called lunar rhythms.
The Moon is a source of gravity and at particular locations during its orbit, the gravitational pull is slightly stronger and pulls the oceans in a particular direction. This creates spring and neap tides, which are overly high tides, and overly low tides. We get two of each every month. This is an example of a semi-lunar rhythm.
The monthly orbit of the moon creates different amount of illumination at night, throughout a month. This can influence an animal's rhythms. Rhythms resulting from this are called lunar rhythms.
The Moon is a source of gravity and at particular locations during its orbit, the gravitational pull is slightly stronger and pulls the oceans in a particular direction. This creates spring and neap tides, which are overly high tides, and overly low tides. We get two of each every month. This is an example of a semi-lunar rhythm.
The Earth has a slight tilt, so as it orbits the Sun, it orbits on a bit of a lean. During our Summer, the southern hemisphere is 'leaning' towards the Sun, making our half of the Earth warmer. 6 months later, when we are on the opposite side of the Sun, our hemisphere is 'leaning' away from the Sun, making our half slightly cooler - our Winter. We get one Summer every year, it is an annual rhythm. You could say the same for all other seasons - they are an annual rhythm.
During particular times of the year, different parts of the Earth have an abundance of resources (i.e. food, water) whereas during other times of the year these same areas have nothing. These annual cycles trigger behaviours such as migration and hibernation. |
Plant and animal responses
To know what the time is, you need a clock. Many animals (pretty much all of them) have an internal clock, or a biological clock, which helps them keep track of the time of the day. Plants also have biological clocks, so then know when to start flowering or dropping their leaves. Biological clocks are very important in helping an organism keep in time with the external cues, and compensate for any fluctuations of environmental conditions. For example:
Prediction and preparation
for events such as hibernation and migration - knowing when to store food as fat to use as energy later |
Synchronization
of reproduction, social activities or migration. It helps animals group together at breeding sites at the same time of year (rather than at different times of year). |
Synchronization
of some internal processes, such as regulating female menstruation cycles, and pregnancy |
Time compensation
during navigation (migration and homing), and sun compass orientation - as the animal compares the position of the sun to its internal clock to compensate for the sun's movement throughout the day |
Exogenous rhythmThe rhythm is controlled solely by the external environment. A stimuli is detected, and the animal responds. There is no biological clock involved.
If you took away the cue, the behaviour would not occur in the usual pattern. It is often all over the place, scattered, as there is no cue to start it off. |
Endogenous rhythmThe rhythm is synchronized to an external cue (i.e. light or temperature) that is detected by the organism (just like exogenous). The receptors that detect the cue then send a message to the biological clock which then organizes a response.
If you took away the cue, the behaviour pattern would remain similar but the time it starts would differ as the cue is gone. |
Biological clockAn internal timing system that allows behaviours to continue without external time cues.
The biological clock also helps animals time particular behaviours (i.e. mating) so that they occur in the most favourable conditions. Daily rhythms such as sleep, temperature regulation, alertness, metabolic rate, sex drive, blood pressure, etc. are all controlled by the biological clock. They can occur in a regular pattern even without an external cue. |
For example, sheep need to give birth to lambs in the spring time - when it is warm and there is plenty of food. This means they need to mate in autumn so that the lambs will be born in spring. It is important that the sheep are not misled by a brief warm spell during autumn, and 'think' it is still summer and decide not to mate. Their internal clock will help keep the sheep in tune with the seasons (or any other cycle). Sheep respond to changes in photoperiod, rather than temperature.
Click here for some great notes on circadian rhythms
Many rhythms are controlled by both endogenous and exogenous timing mechanisms - in a way we can say that their internal clock can be synchronized to the external timing mechanism (i.e. daylight, temperature getting warmer, etc).
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Click the picture below to be taken to an interactive website that explains circadian rhythms
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We can tell if an plant or animal's rhythm is exogenous or endogenous by placing it in a constant environment. This is an environment that is unchanging - the conditions are kept the same. If, when placed in a constant environment, the animal continues to show the rhythm, we can infer that the animal has some sort of internal clock - an internal timing mechanism - which controls the rhythm (endogenous). If the animal does not continue to show the rhythm, we can infer that the rhythm is solely controlled by external factors and is not linked to a biological clock (exogenous).
A continuing rhythm/pattern of activity that occurs when an animal is placed under constant conditions is called free-running. The time between repetitive peaks of activity during a free-running rhythm is called the free-running period.The length of the free-running period may be longer or shorter than the period of the environmental rhythm.
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Most biological clocks can't keep in exact time with environmental rhythms, and therefore must be reset by a particular cue such as light, temperature, pheromones, etc. The particular cue that resets the internal clock is called the zeitgeber (time-giver).
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The regular re-setting of an internal clock by the zeitgeber is called entrainment. Day length is a reliable cue, and entrainment by daylight helps animals to better exploit the seasonal changes associated with changes in day length (i.e. in summer there is warmer temperatures and an abundance of food - summer is also when days are the longest so having periods activity entrained to day-length means the animal can use all the time where light is available to make the most of these resources! In winter, as day-length gets shorter, nocturnal animals can re-set their clocks with the earlier nights and make the most of the extra night hours.
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When the start of the period of the rhythm is changed, and becomes earlier or later, we call this a phase shift. When we turn our clocks back for daylight savings, the next morning we have a phase-shift. This is when, as well as resetting our clock, we change the starting time of the clock. This also happens when you change time-zones - the cues are the same but the time of the cue is different!
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Below are some seasonal responses in animals that are known to be regulated by photoperiod (day length)
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Breeding time
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Hibernation
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Migration
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Physical changes
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Dipause
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The onset of breeding in many animals such as sheep and deer are triggered by changes in day length
- The adaptive advantage of this is so that when the offspring are born, the conditions are ideal for them to get enough food and be in warm enough temperatures to grow enough before the winter.
- = more likely to survive and reproduce!
Animals that live in close proximity to the poles (i.e. polar bears) will often hibernate during the winter
- The adaptive advantage of this is that the animals are not spending heaps of energy during the winter trying to find food and shelter and regulate their temperature (etc.) - instead they go into 'low energy mode' and become inactive while the conditions are extremely unfavourable.
- = more likely to survive and reproduce!
Many birds (e.g. cuckoos, godwits), some mammals and other animals will migrate during winter to areas that are warmer
- The adaptive advantage of this is the animals spend their winter in a location that is not enduring such an extreme change in environmental conditions - the animals move to a place where it is warmer and there is plenty of food, and then return to their original location during its summer, where there is plenty of food etc at that time.
- = more likely to survive and reproduce!
Physical changes occur, such as the growth of a winter coat (e.g. arctic hare) or the colour change of their coat (e.g. plumage (feathers) of birds are white in winter)
- The adaptive advantage of this is so the animals are well-equipped to deal with the changes associated with the winter seasons and can stay warm and/or hide from predators.
- = more likely to survive and reproduce!
The onset of dipause in many insects is triggered by changes in day length
- Dipause is a period of 'arrested development' (or halted growth), common for insects and other arthropods that occupy niches in cold climates. Shorter days signal that colder temperatures are coming, so the insects 'press pause' basically! After being exposed to a period of cold conditions (when development is halted), development is resumed (e.g. in the black field cricket, the eggs require several weeks of chilling before they will hatch).
- = more likely to survive and reproduce!
Try and think of an example for each of these responses (for you to remember), and explain in more depth how the response occurs and why it is advantageous for that particular animal to respond in that way.
Scientists have done a lot of work on how plants change with the seasons, and what causes it. This research into photoperiodism has allowed scientists to group plants into one of three categories:
Long-day plants
LDP will only begin to produce flowers when the photoperiod (daylength) is longer than a particular value - the critical day length (CDL). When days begin to get longer (i.e. in Spring and early Summer), the CDL is reached and plants begin to flower. You can also think of these plants as having a short night |
Short-day plants
SDP will only begin to grow flowers when the photoperiod is shorter than the CDL. This is often when days begin to get shorter, like during Autumn. You can also think of these plants as having a long night |
Day-neutral plants
DNP are not sensitive to photoperiod (some plants don't live long enough to endure several changing seasons). Other plants include the tomato and dandelion. |
Factors such as temperature, nutrient availability and the age of the plant can influence the photoperiodic requirements of a plant. Scientists have also found that what's really important is the night-length rather than the day-length.
Photoperiod is detected by the plant's leaves and the signal must travel to the apical meristem so that it knows to stop making leaves and to start making flowers! The signal that is sent is identical in SDP and LDP.
Detection is done by a pigment in the leaves called phytochrome. |
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Phytochrome is a pigment (blue-green colour in leaves) that acts as the plant's photoreceptors. It detects the presence of light (day) and the absence of light (night). There are two forms of phytochrome - Pr (red light) and Pfr (far-red light). Pr can be converted into Pfr, and vice versa. Pr is able to absorb red-light (found in high amounts in sunlight). Pfr absorbs far-red-light, which is barely visible to the human eye (found in low amounts in sunlight). In natural light (daylight), Pr is readily converted to Pfr. In the absence of natural light (night time), Pfr is converted back into Pr, but slower than the previous conversion. This can also happen if far-red light is shone onto the leaves. To 'know' the daylength, the pleant measures the amount of phytochrome in each form. If days are longer than nights, there will be a large amount of Pfr, and the plant will detect this and know that it is spring or summer. A sufficient amount of Pfr must be present in the morning for LDP to flower. When days are long, lots of Pr is converted into Pfr during the day, and at night, not very much Pfr is converted back to Pr. This leaves plenty left over in the morning! Phytochrome interacts with particular 'clock genes' of a plant (the plant's internal clock). These genes will be expressed in the form of a hormone (scientists aren't sure which yet), which promotes flowering. The hormone travels from the leaf to the stem and to the apical meristem, where flowering is initiated.
Example: Cherry trees These are Long Day Plants - they flower in spring. When the night is shorter than their critical length (i.e. spring = shorter nights) NOT all of the Pfr is converted back in to Pr, meaning there IS AN accumulation of Pfr overnight. This stimulates LDP's to flower in spring. There must be little to no Pfr present in the morning for SDP to flower. This means that days need to be shorter and nights need to be longer, so that there is plenty of time for Pfr to be converted into Pr. The near-100% conversion of Pfr to Pr stimulates flowering in SDP!
Example: Christmas Lillies from the Northern Hemisphere
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Further tests:
Interruption of the long, dark period (needed by a SDP to convert all the Pfr to Pr) will inhibit flowering in SDP!
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Adaptive advantage of photoperiodism
Plants are stuck in the ground! They can't just gather up their roots and move away from unfavourable conditions... they must endure them! Their rate of growth is too slow for them to 'grow away' from unfavourable climatic conditions. To give themselves the best chance of survival, plants have become very good at responding to changes in photoperiod in anticipation of an environmental change. Photoperiodism in plants allows them to:
Photosynthesis efficiency |
Likelihood of urvival |
Reproductive success |
Responding to longer photoperiod
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Plants may drop their leaves as the days get shorter (to prepare for winter), because
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Other plant timing responses:
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Abscistic acid (ABA) promotes seeds to become dormant, stopping the germination process until conditions are favourable enough to promote germination. When conditions are right for germination, the seed produces and releases gibberellins to break the dormancy period and to promote growth of the embryo. Gibberellins stimulates the division and elongation of cells (helped by auxin) to allow roots and shoots to grow in their correct directions. In the plant-growing industry, gibberellins is given to pre-germinating plants to speed up the process and to synchronise all the seeds so they germinate at the same time. This is when dormant seeds require a period of extreme coldness for seed dormancy to be broken. Immediately after the time of the year where it is extremely cold, conditions become quite favourable quite quickly! Vernalisation allows seeds to begin germinating when the time is right for them to most likely be successful and be big enough to survive the following winter! |
Ponder this:
Why do plants and animals respond to photoperiod more than any other environmental change?
Why do plants and animals respond to photoperiod more than any other environmental change?