There are two main approaches to animal science — the physiological and the 'whole animal'. Physiologists are mainly interested in how the body works, that is, in how the nerves, muscles and sense organs are coordinated to produce complex behaviour. Those taking the 'whole animal' approach, although they are often interested in the mechanisms of behaviour, study the behaviour of the intact animal and the factors that affect it. Within the 'whole animal' approach, a distinction is often made between psychologists and ethologists. Psychologists have traditionally worked in laboratories on the learning abilities of a restricted range of species, mainly rats and pigeons.

Ethologists have been more concerned with the naturally occurring, unlearnt behaviour of animals, often in their wild habitats. Although this distinction still exists to some extent, there is now a fruitful coming together of the two. Among these types, it is the physiologists who like to emphasise that their methods are the more fundamental.

However, even if we knew how every nerve cell operated in the performance of some pattern of behaviour, this would not remove the need for us to study it at a behavioural level also. Behaviour has its own organisation and its own units that we must use for its study. Trying to describe the nest-building behaviour of a bird in terms of the actions of individual nerve cells would be like trying to read a page of a book with a high-powered microscope.