Taxes and kineses
These forms of behaviour are responses shown by simple motile organisms
, i.e. those that can move from place to place. This sort of movement is called locomotion
These may be small animals, bacteria or simple unicellular organisms - including photosynthetic protoctistans and algae.
is a directional movement
of a (whole) organism in response to a stimulus. It often serves to cause movement towards favourable stimuli or away from unfavourable ones.
, and chemotaxis
are common examples.
It is known that phytoplankton in sea and freshwater consist largely of motile species (dinoflagellates). These move using their flagella, rising to higher levels in the water when exposed to sunlight, and sink or swim to lower levels when light levels fall, or when ultraviolet levels rise. This behaviour optimises exposure to light for photosynthesis.
And zooplankton, which feed on them, show similar behaviour. This phototactic behaviour
underlies productivity in aquatic ecosystems.
Fine-tuning the response
These responses are generally innate (unlearned) but they may be modified in certain circumstances. This is called behavioural plasticity.
It has been found that the small nematode worm Caenorhabditis elegans
can learn to associate a particular temperature with the presence or absence of food.
If the worms have recently been well fed in an environment at a constant temperature they form an association between this temperature and food.
When deprived of food for a short time and placed on a surface with a temperature gradient, they will migrate to a region with the same temperature to which they had become accustomed.
This diagram shows normal thermotactic behaviour
('isothermal tracking') - movement towards the same temperature in which the worms have been cultured and fed - in this case 17 °C.
A mutant variety shows 'athermotactic behaviour' - with no preference for any temperature.
The normal response has the advantage that the worms will tend to stay in the optimal position combining the availability of food with this temperature. In the absence of food they will respond to temperature, which may coincidentally also lead them to more food.
Some of the behaviour patterns mentioned in this section may be modified under certain circumstances.
For instance, some animals may show a different response when fed to their normal hungry response!
And insects may show different behaviour at different stages in their life cycle.
is also a movement of a (whole) organism in response to a stimulus, but it is non-directional
In this case the stimulus causes movement to vary in a different way (i.e. not towards/away from the stimulus):
is speeding up or slowing down of movement. This is often accompanied by
, which is (randomly) turning in different directions, changing in response to the strength of the stimulus.
The frequency or rate of turning is proportional to the stimulus intensity.
As a result the organism tends to remain in a favourable environment
, although they will probably keep moving slowly.
Although a kinesis is not actually oriented with the direction of the stimulus, it can still result in migration which has a general directional component, so it is possible to describe the effect as positive or negative.
A positive photokinesis
is defined as an increase in the speed of movement in the presence of light, and negative photokinesis
covers the slower movement of an organism upon entering an illuminated area relative to its movement in the dark or in dim light.
is movement in response to (air) humidity.
This is sometimes linked with thigmokinesis - movement in response to surfaces - ruts in the ground surface (and sometimes edges of containers in the lab).
These types of locomotory behaviour can be tested using choice chambers or mazes.
Experimental animals might be
- woodlice (although it is often noted that different species - see opposite - show slightly different responses to environmental factors so it is best to check their identity first!)
- flour beetles (Tribolium spp.)
- maggots (larvae of the blowfly Calliphora vomitoria which can be obtained from anglers' supply shops)
An acrylic choice chamber with 4 compartments
Choice chambers can be made up from cardboard or wooden boxes divided into compartments, or specially made plastic structures.
The environmental conditions within each section can be varied: water (soaked into cotton wool) for high humidity, silica gel for low humidity. Other chemicals can be chosen to give intermediate values of relative humidity.
Alternatively sections may be shaded or covered with different coloured filters on the outside.
Inside the container there should be a layer of plastic mesh to prevent animals from falling (or burrowing) into the contents of the sections below.
A number of experimental animals are placed into the apparatus via the central hole. They are then given some time to migrate to their chosen area within the apparatus, then the numbers in each section are counted. Counts can be taken again after suitable intervals.
It is usual to perform a χ2
(chi-squared) test on the results.
A null hypothesis
might be that the animals will not distinguish between the environments in the sections, so they will be evenly distributed (equal numbers in each section).
See Practical suggestions below
These are not lice - which are insects.
They are in fact crustaceans: with a chalky segmented exoskeleton, and belong to the order isopoda.
Many isopods are aquatic, but woodlice tend to inhabit soil and woodland, especially under stones and logs.
They have a flattened body with 7 pairs of legs and a respiratory surface on the underside.
There are over 45 species of woodlouse in the British Isles, and they have been given a number of common names, e.g. cheesy bobs, granny pigs, pill bugs etc.
Three of the most common species are shown below.
is the main species that rolls into a ball.
In choice chamber experiments it is often found that woodlice favour humid environments, and that they move quickly into these areas and then slow down.
What sort of behaviour is shown by the woodlice?
> (negative) hygrokinesis
What survival advantages would that give to the woodlice in their normal habitats?
> minimises water loss from respiratory surface
> stay in places where food might be
In addition to humidity, it is seen that woodlice move directly to the dark side of the choice chamber.
What name is given to this behaviour?
> negative phototaxis
What other survival advantage would that give to the woodlice, in their normal habitats?
> More likely to find shade beneath trees, and hide under logs/stones - so less visible to predators, and reduce drying effects of sun
Unlike woodlice, Tribolium
flour beetles move faster in the more humid environment. They slow down and tend to remain in a drier area.
What survival advantage would that give to the beetles?
> Less likely to get caught in a sticky goo, and easier to hide in dry flour
Different species of bacteria have different metabolic requirements, especially in relation to oxygen and specific chemicals which they use as respiratory substrates. Not all bacteria are capable of locomotion, but some have flagella which enable them to 'swim' at any position within liquid culture media.
When grown in the lab, some bacteria form a distinct layer called a pellicle at the liquid surface where the availabilty of oxygen is greatest.
It has been found that flagellum-based motility, chemotaxis and oxygen sensing are important for pellicle formation by Bacillus subtilis
and Pseudomonas aeruginosa
By time-lapse imaging it was shown that a turbulent flow forms in the tube, and a zone of clearing appears below the air/liquid interface just before the formation of the pellicle, but only in strains that have flagella. The movement of bacteria is also important in the formation of biofilms, which are significant in certain bacterial infections.
Here these small freshwater crustaceans can be seen swimming towards the beam of light at the surface of the water.
What sort of behaviour is shown by the Daphnia?
[may also be aerotaxis]
What survival advantages would that give to the Daphnia in their normal habitat?
move to areas where plant food organisms (algae, phytoflagellates) may be found
Going flat out?
Planarians are flatworms: simple soft-bodied animals that move by beating cilia on their underside, allowing them to glide along on a film of mucus.
The diagram below shows the movements of an aquatic
flatworm in light and in shade. The path followed by the flatworm over a period of three
minutes was traced on the side of a tank.
What sort of behaviour is shown by the flatworm?
> (photo)kinesis [positive in the light, negative in the shade]
Reasons for this description?
> movement is quite random - not directly towards or away from light
> rate of movement changes - faster speed in light, so more environment visited
> rate of turning changes - more frequent in shade, so less environment visited
Suggest some advantages of the behaviour shown by the flatworm
> remains in shady conditions/ hurries out of light
> avoids being seen by predators?
Blowfly larvae (maggots)
When placed (individually) on a surface lit from one side, blowfly larvae generally crawl directly away from the light, and make a sharp deviation if a second light is switched on from another side.
What sort of behaviour is shown by the blowfly larvae?
What survival advantages would that give to the blowfly larvae in their normal habitat?
makes them more likely to burrow into their food (dead animal body or animal droppings)
> reduces the risk of dehydration and predation at the surface