Woodlice may look like a type of millipede or beetle, but they are in fact related to crabs and other crustaceans! These small and abundant creatures (they live everywhere except for Antarctica) are ideal for observing animal movement using techniques such as the choice chamber experiment. All living beings constantly adjust to their environment to survive. This adaptation happens through responses to external and internal stimuli and ensures homeostasis. Woodlice change their movement pattern when responding to stimuli such as temperature or food sources in their environment, it is therefore easy to recreate their ideal habitat to observe their behaviour due to them being tiny minibeasts!
Homeostasis describes the regulation and maintenance of various biological systems to support a stable internal environment.
Taxis and kinesis
A taxis is a simple type of movement response by a motile organism. The direction of the stimulus determines the direction of the movement. Taxes are very similar to another kind of response by plants called tropisms. In a tropism response, plants can only change the direction of their growth either towards or opposite the direction of the stimulus because plants can’t move. In taxis, however, a motile organism can move away from unfavourable stimuli (negative taxis) or towards a favourable stimulus (positive taxis).
The type of taxis is defined according to its originating stimulus.
Algae that move towards the light to perform photosynthesis more effectively have a bigger chance of survival. They have positive (moving towards) phototaxis (taxis induced by light stimuli).
Fig. 1: Positive phototaxis of insects being attracted to a light source
Other examples include the positive chemotaxis of some bacteria that will move into regions of higher glucose concentrations, an essential source of food.
A motile organism refers to an organism that can move around its environment.
Both taxis and kinesis are interlinked behavioural responses, they respond to stimulus and are found in living motile organisms like animals and insects. Kinesis is more dependent on taxis, see below to understand why.
Difference between taxis and kinesis
A kinesis is a distinct type of behavioural response from a motile organism. Unlike taxis, a kinesis response does not involve changing the direction of movement according to a stimulus direction but instead changing the organism’s speed of movement and the rate of direction change. This response can be important when reacting to less directional stimuli like temperature or humidity and doesn’t vary in space in clear gradients.
Taxis is a more specific and directed movement, whereas kinesis is undirected and random.
In a kinesis-type response, when an organism senses that it has temporarily entered a more unfavourable location, it will increase its speed and rate of turning to exit the space. However, if the organism continues in such adverse surroundings, it will decrease its turning rate (direction change); thus, moving in long straight lines and increasing the chances of reaching a more suitable environment, where the temperature might be milder.
Another interesting difference between taxis and kinesis is that with:
- taxis there are several types like phototaxis (light), chemotaxis (concentration gradients), geotaxis (gravity), thermotaxis (thermal source) etc...
- whereas with kinesis there are only two types; klinokinesis (frequency or rate of turning) and orthokinesis (dependence of stimulus to the movement).
The movement of woodlice
Woodlice are crustaceans often used as examples when investigating animal movement responses. These animals can display both taxis and kinesis-type responses when searching for ideal living conditions in their habitat.
For instance, woodlice display negative phototaxis, moving away from light sources and preferring dark environments.
Woodlice also display kinesis-type responses when searching for ideal temperature and humidity conditions. Unlike light sources, these stimuli can be harder to pinpoint and may not vary in clear gradients, making it harder to inform a directional response. These stimuli lead to random movement responses from woodlice seeking better environmental temperature and humidity conditions to increase their survival chances.
In a kinesis response, woodlice move faster if the temperature drops below or rises above a specific optimal range in their current location. The higher speed increases the chance of the woodlice exiting the unfavourable area more quickly to another area where the temperature is more favourable and in the optimal living range.
The same happens when woodlice encounter a dry environment. Woodlice prefer damp areas because they lose water content from their bodies in dry conditions. When moving from a damp area to a dry one, woodlice moves faster and changes direction more often to return to a damp area.
However, if this strategy is unsuccessful, woodlice will change direction less often to move in longer stretches.
All these behavioural strategies will increase the chances of woodlice or any other animal spending more time in favourable conditions (damp and mild temperature) than unfavourable ones (dry, extreme temperatures).
Woodlice also display thigmokinesis, a movement or immobilisation response to contact stimuli. Since woodlice are highly attracted to solid objects, and thus, to each other. Once woodlice come into contact with each other they stop and tend to clump together.
Fig. 2: Woodlice kinesis response to humidity conditions
What is a choice chamber?
Choice chambers are artificial structures used to investigate animal behaviour. Any small motile organism can be stimulated by different environmental conditions, and their movement response is analysed using this method. These structures allow us to know what environmental conditions animals prefer when looking for suitable habitats.
Woodlice are often used in these experiments because they are easy to find in nature and can be used to display taxis and kinesis-type responses to different stimuli.
Choice chambers are enclosed in designed environments with several linked compartments within a large plastic petri dish, simulating desired environmental conditions. Animals like woodlice are placed in these structures, and their behaviour is assessed by analysing their movement through these compartments. A woodlice’s preference for certain habitat conditions reflects in their choice of chamber division that most resembles their ideal natural environment.
For example, light and humidity preference are two environmental factors often tested using choice chambers. An animal’s preference over light/dark and dry/damp areas can be checked using these structures because these conditions are easy to replicate in an artificial setting, like a choice chamber.
A choice chamber can be made with both dry and damp compartments to investigate the influence of humidity on woodlice movement. The same can be achieved for light stimulation simply by covering part of the dish from any light source, thus creating a dark compartment within the chamber. The animals, like woodlice, are then dropped into the structure and start moving around. After experiencing the different stimuli, the organisms will gravitate towards the more suitable environment through taxes and kineses responses.
Silica gel beads can simulate a dry area because they absorb moisture from the air, while wet cotton in another compartment can replicate damp conditions.
Fig. 3: An example of a choice chamber
Woodlice choice chamber experiment control
It is possible to replicate several different combinations of compartments in a choice chamber depending on which environmental factors and how many are being assessed. However, an empty control chamber must always be present to ensure that the stimuli influence animals’ movements and that woodlice are evenly distributed in an empty chamber.
Woodlice choice chamber experiment results
Counting the number of woodlice within each chamber determines the choice chamber results. The compartment with the highest number of animals likely simulates that animal’s most desirable condition in the wild for that particular environmental factor.
Following the woodlice example, in a choice chamber displaying a dark and a light compartment, woodlice display negative phototaxis and accumulate in the dark compartments.
- A choice chamber with a dry and damp area would result in woodlice accumulating in the damp area through movement responses.
- In a choice chamber with dark damp, dark dry, light damp, and light dry areas, woodlice accumulate largely in the dark, damp compartment.
The humid and dark environment prevents them from desiccating (through water content loss from their skin surface) and allows them to hide from possible predators.
Woodlice Choice Chamber Experiment - Key takeaways
Kinesis and taxis are two types of simple movement responses by motile organisms to various environmental stimuli like temperature, light, or humidity. These responses ensure that these organisms seek and move towards areas in their habitat with more favourable conditions.
Taxes are movement responses from motile organisms towards favourable stimuli (positive) or away from unfavourable stimuli (negative). Kineses are random movement responses consisting of changes in the speed of movement and rate of a direction change from motile organisms in unfavourable environmental conditions.
In a kinesis response, animals that enter unfavourable environments increase their speed and rate of direction change but will decrease that rate and move in long straight lines if they persistently remain in the same unfavourable spot.
Choice chambers are artificial compartments replicating environmental conditions used to investigate animal behaviour.
Woodlice display negative phototaxis, preferring dark as well as damp environments as seen in the woodlice choice chamber experiment.
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