Behaviour is all the observable actions of an animal. Anything that we see, hear or smell an animal doing is part of its behaviour. We can eavesdrop on an animal's actions. We can use electronic devices, to gather sounds above or below the limits of human hearing, to view wavelengths of light not resolved by the human eye, or to slow down actions too fast to interpret. The end result is that the study of behaviour has a vast subject matter drawn from the huge diversity of animals, and their equally diverse repertoires of behaviour. Furthermore, behavioural acts may vary from one instance to the next, between individuals at some moment in time, and across an individual's lifetime.
Behaviour is generated by the neuromuscular system. It is not confined to the somatic nervous system and striated muscles since autonomic responses (e.g. piloerection) and glandular secretion at the periphery are important components of behaviour. Behaviour plays the pivotal role in an animal's biology because it is through behaviour that the animal solves ecological problems. No matter how elegant its form and inner workings may be, nor how complex its external environment is, an animal's survival and reproductive success depend on finding a place to live, getting energy and nutrients, avoiding injury and predation, and finding a suitable mate and rearing offspring. Behaviour is the key to achieving all these goals; behaviour is at the cutting edge between the animal and its environment.