Orientation in Animals

Part 1: Taxis and Kinesis

Orientation Mechanisms

• Organisms detect environmental stimuli (e.g. light intensity, gravity, temperature...) using receptors or organs (e.g. eyes, statoliths, ears...)
• They can therefore orientate themselves to move out of unfavourable conditions (e.g. extreme temp, moisture or light intensity) and into favourable ones
• There are two quite different kinds of orientation mechanisms:
• Taxes (singular taxis)
• Kineses (singular kinesis)
• All simple orientation responses are innate (inborn; natural)

Taxes

Directional movement

Taxes (singular taxis)

• A taxis is a directional movement of the whole animal towards or away from a stimulus.

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• Naming taxis:
• We start by naming the direction:
• We use 'positive' (towards) the stimulus and 'negative' (away) from the stimulus
• We use the suffix taxis to show its a directional movement
• The prefix denotes what the stimulus is
• Photo=light, chemo=chemical, geo=gravity, hydro=water, thigmo=touch, thermo=temperature, rheo=current (water)

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• Positive thermotaxis: movement towards heat, e.g. mosquitos move towards body heat
• Negative phototaxis: movement away from light, e.g. earthworms burrow away from sunlight.

Examples of Taxis

• Flatworms moving towards a piece of raw meat show positive chemotaxis.
• Land snails moving away from gravity (upwards) show negative geotaxis
• Trout will line themselves up in an upstream direction so they are positively rheotactic.

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• Moths flying towards light show what kind of orientation?

Perception of direction

• The precision of the response depends on the type and position of the sense organs.

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• Tropotaxis: If there are 2 organs, such as eyes, then the animal can simultaneously compare impulse frequency and move fairly directly towards or away from the stimulus.
• E.g. Movement of terrestrial slugs away from light
• Klinotaxis: If there is only 1 organ, then the animal must move about to get more information over time.
• Maggots have a light detecting organ on their heads. As it moves forward it swings its head from side to side. The stronger the light source when it bends in a particular direction, the more in bends the other way, so it moves in a rather zigzag line with a mean path away from the light.

Kineses

Non Directional (random) movement

Kinesis

• This is a non-directional response to a stimulus.
• I.e. The direction of movement is random and has no relation to the direction of the stimulus.

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• It is a change in activity rate in response to a change in the intensity of the stimulus.

Types of Kineses

• Orthokinesis
• Stimulus intensity determines the speed of the movement.
• E.g. Slaters rate of movement is inversely proportional to humidity. Since the rate of movement decreases in damper air, the animals spend more time in damp areas.
• Klinokinesis
• Stimulus intensity determines the rate of turning.
• E.g. Flatworms turn more quickly in light, so when they leave darkened areas they turn more often, increasing the likelihood to crawl back into the dark.