The ENVIRONMENT in which any organism lives is always changing, for example, light to dark or warm to cool. Organisms need to respond to these changes in order to survive. The response of the organism is called 'BEHAVIOUR'. These behavioural responses may be simple (e.g. moving away from light), or more complex (e.g. calling and displaying for a mate).

ADAPTIVE BEHAVIOURS are behavioural adaptations that increase an organism's chance of survival and reproductive success (i.e. FITNESS - its ability to survive and pass its alleles to the next generation).

Behaviours (usually called 'responses' in plants) are subject to NATURAL SELECTION. Those responses/behaviours that increase fitness are retained in the population, because they give that organism an ADAPTIVE ADVANTAGE. Whereas those responses/behaviours that decrease fitness are eventually lost.

Are you ready to get your mind blown?

Why do animals behave in the way they do? This simple question can have two different meanings, one about the past, and one about the future.

1. What were the events that caused the behaviour?
   This is a physiological question, and is about the past. It relates to the process going on inside the animal that gave rise to the behaviour. The answer would be in terms of hormones, nerve impulses, muscle contractions, and so on.
   

2. What is the effect of the behaviour, or how does it benefit the animal?
   This is an ecological question, and concerns the future.

The physiological events that are the immediate cause of the response/behaviour are under the control of genes that were selected for over previous generations. An animal responds the way it does because the circumstances it meets in its lifetime are not greatly different from those experienced by previous generations. So, the ultimate cause of the behaviour is the advantage of that behaviour to the previous generations.