What are taxes and kineses?
Animals can respond to stimuli from their abiotic environment, moving to
more favourable conditions. Unlike plants, which usually have to grow to
respond, animals are able to move quickly.
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If snails are placed in the bottom of a bucket with gladwrap over the top
they all end up clinging to the gladwrap. Let's investigate this movement
a little more closely.
If a snail is placed on a glass sheet lying horizontally on a table the snail
moves at random over the glass. Each time a new snail is placed on the
glass the direction taken is random.
If you now lift the sheet of glass so that it is vertical the snail will always
turn and move upwards as though it is a negative response to gravity.
This would seem to explain why the snails ended up on the gladwrap. But
can we show that gravity was the stimulus? If a snail is placed on a
horizontal surface with a weight tied to its back (shell) it will always move
away from the weight force by pulling the weight along (unless it is too
heavy!). This suggests that the shell is detecting gravity and the response
is to move away from gravity.
Why do they do it? There are two possible reasons. Snails prefer the
young tender new leaves that are found at the tips of plants. Moving up
would be an advantage for feeding. Snails cannot withstand waterlogged
soil as they are air breathers with a lung. Once again, moving up surfaces
would be an advantage to avoid drowning.
We use the word taxis (plural = taxes) to describe the snail's movement.
A taxis occurs when
'animals respond to a stimulus by moving towards or away from the stimulus'.
The snail is showing a negative gravitaxis (orgeotaxis).
What other animals show taxes?
There are many advantages in moving towards or away from a stimulus.
They include:
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avoiding dessication (drying out)
avoiding predation (being eaten)
finding food
finding a suitable habitat (home)
avoiding poisons
finding suitable photosynthetic conditions
1. Phototaxis
This is common among unicellular algae like Euglena or Chlamydomonas,
which must move to optimum light for photosynthesis. They swim by
beating a tail-like flagellum.
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Used with permission.
Some other unicellular protista (or protoctista) that are not
photosynthetic also respond positively to light. They often have tiny hairs
called cilia to help them move.
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Insects (e.g. moths) will move toward the light, often to our annoyance.
They are positively phototactic.
Other animals show negative phototaxis, such as the earthworm and
blowfly larvae. This is an adaptation to avoiding dessication.
2. Gravitaxis
The land snail shows negative gravitaxis as we have already discovered.
Many bivalve shellfish (e.g. pipis and clams) burrow down into the sand
when disturbed. This helps prevent predation and is an example of
positive gravitaxis.
Animal response to gravity is difficult to research as gravity cannot be
turned off. However, experiments with rotating clinostats and
microgravity experiments onboard the space shuttle have shown that
gravity is an important stimulus in the orientation of many unicellular
organisms.
3. Thermotaxis
Some external parasites such as lice, fleas and mosquitos are able to
detect and move towards the body heat of their host animal. This is an
example ofpositive thermotaxis.
4. Rheotaxis
Living in a stream or river poses a problem for animals. They need to be
able to hold a position in thecurrent. Many animals, such as trout and
mayfly nymphs will align themselves in an upstream direction. This is an
example of positive rheotaxis.
5. Chemotaxis
There is a great deal of evidence for animals being able to orientate
according to a chemical gradient.
Our body's immune cells (white blood cells) are attracted to, and move
towards, chemical factors released at the site of infection or inflammation.
They can also 'smell' chemicals released by the invading bacteria as
shown by the images below. A gradient of bacterial chemicals was set up
and the blood cells moved toward it. This is an example of positive
chemotaxis.
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Used with permission.
Once located all that remains is the 'eating'.
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Flatworms can locate and move towards a piece of raw meat. This
positive chemotaxis allows them to find food.
The human egg sends a message to the sperm that aproach it prior to
fertilisation. The chemicals seem to select for sperm with the ability to
penetrate the egg. This increases the chances of successful fertilisation.
Many female insects (e.g. moths) can release chemicals
called pheromones that attract a male, sometimes from many kilometres
away. This ensures successful reproduction, and is an example of positive
chemotaxis. Ants leave a chemical trail which is followed by other
members of the nest to food.
6. Magnetotaxis
Several species of bacterium have been found to swim towards magnetic
lines of force. One such bacterium is shown below with its iron oxide
(magnetite) particles arranged in a row. They act like a compass needle
and in the Northern Hemisphere the bacterium orientates toward the
north pole.
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Because of the inclination of the earth's magnetic lines of force, this
behavior causes the bacterium to swim downward and thus to return to
the sediments in which it lives. Gravity would be no use as a stimulus
because the bacterium is so tiny. So this is an alternate mechanism by
which dislodged bacteria can find their way back into their normal habitat.
Bacteria in the Southern Hemisphere orient toward the south pole. These
are examples of positive magnetotaxis.
Making Choices
When we are trying to find an animal's preference for any
particular environmental factor we usually give them a choice between
two conditions. Such an investigation would probably use a 'choice
chamber'.
Experiments have been carried out to determine the humidity preference
of the common slater, or woodlouse (Porcellio scaber). Humidity is a
measure of the relative moisture content of the air. This can vary
considerably between habitats.
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A typical study of the slater responses would involve a choice chamber of
some sort such as the one below.
Animals are placed in the chamber through either of two holes in the lid
and they cannot fall through to the base because of a platform made of
fine netting.
Used with permission (Modified).
Similarly, the slater's preference for light intensity can be investigated
with a simple choice chamber such as the tube shown below. The animals
are placed in either end by removing a foam stopper.
When these experiments are carried out we find that slaters prefer dim
light and humid conditions.
Can we explain these behaviours? Woodlice fall into a category of animal
called crustacea because they have a hard shell. They evolved from sea
based life forms as opposed to insects for example, which evolved from
land based life forms. A woodlice's skin is therefore permeable - it lets
water in and out. If they did not live in damp places they would dry out as
the water left their bodies through their permeable skin.
Their respiratory surfaces are simple pseudo-lungs with a pore opening to
the outside to allow exchange of gases in and out of the lung. This pore is
unable to be closed and so water is able to continuously diffusefrom the
inner surface of the psuedo-lung out into the surroundings.
So, we would expect to find the slater in cool, shaded and damp places.
This is where we do find them and their choices in the experiments
outlined here support this.
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But how do they make the choices?
What is a kinesis?
If we look carefully at how the slaters (woodlice) move when they are
faced with a choice betweenhumid and dry air we see that they do not
carry out a taxis. Their movement is not toward or away from a stimulus.
Rather, they seem to make lots of random movements that end up
getting them to the favoured damp side of the chamber.
We name their movement a kinesis. This means,
'when an animal moves in response to a stimulus but the movement is not related to
the direction of the stimulus.'
There are two ways this can happen:
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the animal can change the rate of turning.
the animal can change its rate of movement (speed).
Rapid turning is likely to take an animal to different conditions, hopefully
better conditions. Going faster will take an animal away from
unfavourable conditions to a different area with possibly better conditions.
When slaters are observed closely, they show an increase in speed if
placed in dry air. This moves them away from the dry air to the damp
side of the container below. They move much more slowly in the damp
air, which means they remain in the favoured side.
dry air
damp air
Slaters also show a change in the rate of turning, making far more turns
in the dry side, which should lead them away to new conditions. When
they reach damper air they turn less so stay in these conditions.
dry air
damp air
In fact, slater response to humidity is a mixture of these two forms of
kinesis.
Human lice show kinesis towards host mositure. They are more active
when skin is dry, moving randomly. The more they move the greater the
chance they will find damp areas, which are more suitable for their
survival.
Flatworms change direction by turning rapidly in high light. This serves to
move them into darker conditions which are more suitable. This is an
example ofphotokinesis. Another example is the cockroach, which is
more active in low light intensity.
Paramecium use gravikinesis to move upwards in the water towards their
food source and oxygen.
Slaters stop moving when they come into contact with a surface such as a
crevice. This would be a good place to hide from unfavourable conditions
and is an example of thigmokinesis. In fact, most soil or leaf
litter animals show thigmokinesis. The more their body is in contact with
a substrate the less active they are. This leads to slaters grouping under
bark, wood and between grass stems. Here they get better protection
from dessication and predators.
Bacteria show chemokinesis as well as chemotaxis. A common bacterium,
E. coli, propels itself from place to place by rotating its flagella. To move
forward, the flagella rotate anticlockwise and the organism "swims". But
when flagellar rotation abruptly changes to clockwise, the bacterium
"tumbles" in place and seems incapable of going anywhere. Then the
bacterium begins swimming again in some new, random direction.
Swimming is more frequent as the bacterium approaches a chemical
(food). Tumbling, hence direction change, is more frequent as the
bacterium moves away from the food.
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So it is a complex combination of swimming and tumbling that keeps
them in areas of higher food concentrations.
Summary of Taxes and Kineses
So far we have seen that animals respond to stimuli from their abiotic
environment by movements that are either called taxes or kineses. This
depends on whether the movement is directed or random.
Of course, animal behaviour is more complex than so far suggested. For
instance slaters show negative phototaxis by moving directly away from
the light. But their humidity response overrides the light response. This
can be confusing when conducting choice chamber experiments. Great
care must be taken to control all variables.
Slaters show thigmokinesis as discussed previously but they also
show thigmotaxis when they touch the side walls of the petri dish. This
leads to them following the wall around the dish.
The behaviours discussed so far are not as rigid as it might seem. They
can depend upon previous exposure (e.g. hunger, desiccation prior to
testing) and it is important for them not to be locked into a behaviour
that would kill them. Slaters clustered under bark and in humid areas to
avoid desiccation must come out to feed at some time. As the
temperature drops in the evening the humidity response weakens
allowing them to emerge.
Honeybees are negatively phototactic below 16oC. Above 16oC they
become positively phototactic. Some moths are positively phototactic
while flying but they are negatively phototactic when feeding. This
modification of the taxis allows a greater range of behaviour more
appropriate to the immediate needs of the animal.
What appears to be simple behaviour is, in fact, quite complex.