Taxes and kineses: how animals orient by moving
Two ways a simple animal finds a better place
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Plants are fixed, so they respond by growing. Animals can move, and simple animals use two very different strategies to end up somewhere favourable.
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A taxis is a directional movement — the animal moves towards or away from the stimulus, and the direction of movement is determined by the direction of the stimulus.
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A kinesis is a non-directional movement — the rate of movement or turning changes with the intensity of the stimulus, but the animal does not steer relative to it.
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The outcome can look identical: both end with the animal concentrated in the favourable area. The mechanism is what the exam tests.
Taxis
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Taxes are named after the stimulus, and labelled positive (towards) or negative (away):
- Phototaxis — light. Many aquatic invertebrates are negatively phototactic, moving into shade.
- Chemotaxis — chemical concentration. A mosquito moves up a CO2 gradient towards a host.
- Geotaxis — gravity.
- Rheotaxis — water current. Īnanga (whitebait) swim against the current — positive rheotaxis — when migrating upstream.
- Thigmotaxis — contact. Wētā move until in contact with surfaces on several sides, so they end up wedged in crevices.
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The mechanism requires the animal to compare stimulus intensity, either:
- between paired receptors on opposite sides of the body at the same moment, or
- at the same receptor over time, as the animal moves or swings its head from side to side.
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The animal then turns until the difference is zero (moving directly towards or away), and continues.
Kinesis
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A kinesis has no steering component at all. The animal cannot tell which direction is better; it only registers how good the current spot is.
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There are two forms, and the distinction is worth knowing:
- Orthokinesis — the speed of movement changes with stimulus intensity. In unfavourable conditions the animal moves fast; in favourable conditions it slows or stops.
- Klinokinesis — the rate of turning changes with stimulus intensity. In unfavourable conditions the animal turns often (a tight, random path); in favourable conditions it turns rarely (long straight runs).
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Both produce the same statistical outcome: the animal spends more time in the favourable area and leaves it slowly, while crossing unfavourable areas quickly. Over time, a population accumulates where conditions are best.
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This is a random process — no individual is aiming anywhere. The distribution is a result of differential time spent, not navigation.
Selective advantage
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Taxis — īnanga migrating upstream (positive rheotaxis).
- Swimming against the current takes juveniles from the sea into freshwater streams to feed and mature.
- The direct route means less time exposed to marine predators and less energy spent than a random search.
- Reaching adult habitat reliably means more individuals survive to spawn.
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Taxis — negative phototaxis in freshwater kōura (crayfish).
- Moving away from light takes them under rocks and overhangs by day.
- This puts them out of sight of visual predators such as shags and trout, and reduces water loss risk in shallows.
- Higher survival means more individuals reach reproductive size.
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Kinesis — wētā and humidity.
- Wētā have a thin cuticle and lose water rapidly in dry air.
- In dry conditions they move faster and turn more (high orthokinesis and klinokinesis); on reaching a humid crevice they slow and turn less, so they stay.
- Remaining in humid refuges by day prevents desiccation and keeps them hidden from predators, so more survive to breed.
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The general trade-off worth noting for Excellence: a taxis is more efficient but requires paired receptors and the neural machinery to compare them. A kinesis is far less efficient but needs only a single receptor and almost no processing. In very small or simple animals, the kinesis is the cheaper solution and is favoured despite being slower.
Worked Example
Worked Example
Twenty invertebrates are placed in the centre of a chamber with a damp half and a dry half. Their movement is tracked for 10 minutes.
- After 10 minutes, 17 of 20 animals are in the damp half.
- In the dry half: mean speed 1.8 cm s−1, mean turns 2 per cm travelled.
- In the damp half: mean speed 0.4 cm s−1, mean turns 11 per cm travelled.
- Individual paths in both halves are irregular and wandering, with no consistent bearing.
Identify the type of response, justify your identification from the data, and explain its selective advantage.
Answer:
Identifying the response. This is a kinesis, not a taxis — and specifically both orthokinesis (speed changes) and klinokinesis (turning rate changes).
Justifying from the data. The 17-of-20 result on its own does not distinguish the two, because a taxis and a kinesis both concentrate animals in the favourable half. The distinction comes from the movement data:
- The paths are irregular with no consistent bearing. In a taxis, animals would travel on a direct path towards the damp half, so the paths would show a consistent direction.
- Speed falls from 1.8 to 0.4 cm s−1 on entering the damp half — a change in rate, which is orthokinesis.
- Turning rate rises from 2 to 11 turns per cm — again a change in rate, which is klinokinesis.
Both measured variables are rates, and neither is a direction. The animals are not steering; they are changing how they move according to how good the current conditions are.
How this produces the distribution. Moving fast and straight in the dry half means an animal crosses it quickly and is likely to leave it. Moving slowly with frequent turns in the damp half means an animal covers little ground and keeps doubling back, so it is unlikely to leave. Animals therefore enter the damp half at a normal rate but leave it very slowly, and over 10 minutes they accumulate there. No individual navigated anywhere — the distribution is entirely a consequence of differential time spent in each half.
Selective advantage. These invertebrates have a permeable cuticle and a high surface-area-to-volume ratio, so in dry air they lose water rapidly by evaporation.
- Accumulating in damp conditions reduces the water potential gradient between the animal and the air, so water loss falls.
- Less water loss means the animal avoids desiccation and avoids diverting energy into osmoregulation.
- Damp microhabitats — leaf litter, under logs — are also dark and enclosed, so the same behaviour reduces exposure to visual predators.
More individuals therefore survive to reproduce, and since the behaviour is heritable, its frequency increases in the population. Note that the kinesis achieves this with no directional receptor at all — an advantage for a small animal in leaf litter, where humidity gradients are patchy and a reliable direction may not exist to be detected.