Behaviour is the action that occurs in response to a stimulus. Studying animal behaviour can help us discover why animals do what they do, what motivates them and even emotions they may be feeling.
Animal behaviour is essentially the product of two factors; morphology (the physical structure of the animal) and physiology (the function of that morphology). Behaviour is limited by an organisms structure, for example, the presence of opposable thumbs allow us to behave differently to cats. An animals physiology, such as teeth and claws define not only its structure, but also the behaviour it exhibits in order to survive it its environment.
Due to behaviours being directly related to and animals survival, for example predator avoidance and finding food, natural selection tends to ensure the success of animals that practice ‘beneficial behaviours’. Beneficial behaviour is primarily due to the two most important things to the success of a species; eating and reproducing.
From Crash Course: Biology on YouTube presented by Hank Green.