1Responding to the environment

A stimulus is any change an organism detects, inside or outside its body; a response is the change it makes as a result. AP wants the link and the benefit, not the nerve pathways or plant hormones behind them: specific mechanisms are not examined.

Behavioural responses are actions:

• Taxis is movement directly towards or away from a stimulus. A flatworm swimming away from a bright light shows negative phototaxis; a moth flying towards a lamp shows positive phototaxis.
• Kinesis is a change in the speed of movement or the rate of turning in response to a stimulus, with no fixed direction. A woodlouse in dry air walks faster; in damp air it slows down and often stops. It does not steer; it ends up in the damp because it slows down there.
• Nocturnal animals are active at night and diurnal animals by day. A desert rodent that forages at night avoids the daytime heat and the hawks that hunt by sight.

Physiological responses are changes in the body:

• Phototropism is a plant growing towards light (in a shoot) by changing how its cells elongate.
• Photoperiodism is a response to the length of day or night, such as a plant flowering only when nights are long enough, or a mammal growing a winter coat as days shorten.
• Shivering, sweating and changing the blood flow to the skin are physiological responses to temperature.

The exam rarely asks you to label a response as one or the other. It asks for the link: what changed, what the organism did, and why that helps here.

A response earns marks only when you name the change, the response and the advantage it gives in that particular environment.

2Exchanging information and signalling

Organisms also respond to each other. Information passes between them in response to internal changes and external cues, and it changes behaviour.

• Internal change. A frightened animal releases adrenaline and prepares to run or fight: the fight-or-flight response. Its posture, smell or call can warn others nearby.
• External cue. A ground squirrel that sees a hawk gives an alarm call, and the whole group dives for cover.
• Plants signal too. A leaf chewed by a caterpillar releases volatile chemicals. Neighbouring plants switch on their defences early, and some of the chemicals attract wasps that parasitise caterpillars.

Communication is a signal from one organism that changes another's behaviour. It can be visual (a lizard flashing a bright throat patch at a rival), audible (birdsong from the edge of a territory), tactile (ants touching antennae on a trail), electrical (a weakly electric fish changing its field near another fish) or chemical (a fox's scent mark on a post).

Animals use signals to show dominance, to find food (a honeybee's dance tells hivemates where flowers are), to establish territory and to ensure reproductive success (courtship displays, songs, colours). Bright petals and scent are a flower's signal to pollinators.

What the exam marks is the consequence. Signals change who gets the food, the territory and the mates, so they cause differential reproductive success: some individuals leave more offspring than others because of how they signal and respond. The details of particular systems (the angles of the bee dance, the chemistry of a scent) are not examined.

3Behaviour and fitness

Natural selection favours any behaviour that increases survival and reproduction. That applies to both kinds of behaviour.

• An innate behaviour is performed correctly the first time, without learning: a newly hatched sea turtle crawls towards the brighter horizon over the sea.
• A learned behaviour changes with experience: a young bird learns to avoid a caterpillar that tasted foul.

Both can be favoured: an innate response needs no risky trial and error, and a learned one adjusts to local conditions.

Cooperative behaviour usually increases the fitness of each individual and the survival of the group. Wolves hunting in a pack can bring down prey too big for one wolf. Fish in a school are harder for a predator to pick out. Bees in a colony share the work of foraging, defence and rearing young.

Helping others at a cost to yourself is explained by kin selection: relatives share many of your alleles, so helping them reproduce passes on copies of alleles you carry. A sterile worker bee spreads its alleles by helping its mother, the queen, raise more sisters and brothers.

Worked example: argue it the AP way. Claim: meerkats that take turns as sentries have higher fitness than meerkats in groups with no sentry. Evidence (invented): in groups with a sentry, 12% of foraging adults were caught by predators over a season; in groups without, 31%. Reasoning: the sentry's alarm call gives the others time to reach a burrow, so more of them survive to breed; survival to reproduce is fitness. Claim, evidence, reasoning: three parts, every time.

4Designing a behaviour experiment

Take the question: do flatworms prefer dark places? A choice chamber is a dish divided into two halves that are identical except for one factor. Here one half is covered and dark, the other lit.

• Independent variable: light (light half or dark half). It is what you change.
• Dependent variable: the number of flatworms in each half after a set time. It is what you measure.
• Controlled variables: temperature, water depth, the number of worms, the release point (the centre), the time before counting. Each could also move the worms, so each is kept the same.
• Control. Run the same chamber with both halves uncovered. If the worms still gather on one side, something other than light (a draught, a warm spot from the lamp, a slope in the bench) is moving them. Justifying a control means saying exactly that: it shows that any difference in the experiment is caused by the independent variable and not by some other feature of the set-up.

Use many animals and repeat the trial: one flatworm's choice tells you nothing.

Worked example: the chi-square test. An invented result: 50 flatworms released in the centre; after 10 minutes, 34 are in the dark half and 16 in the light half.

The null hypothesis is that the worms show no preference, so the expected numbers are 25 and 25.

$$\chi^2 = \sum \frac{(o-e)^2}{e} = \frac{(34-25)^2}{25} + \frac{(16-25)^2}{25} = \frac{81}{25} + \frac{81}{25} = 6.48$$

There are two possible outcomes (dark, light), so the degrees of freedom are $2 - 1 = 1$. The critical value at $p = 0.05$ with 1 degree of freedom, from the formula sheet, is 3.84. Since 6.48 is greater than 3.84, reject the null hypothesis: the difference is unlikely to be chance, and the data support a preference for the dark half.

Rejecting the null hypothesis says the distribution is not random. It does not prove why the worms moved; the control rules out heat and draughts. And if $\chi^2$ had come out below 3.84, the correct wording would be fail to reject the null hypothesis, never "accept" it and never "prove" it.