This book is about the survival value of behaviour. It describes how ecological selection pressures associated with acquiring resources and reproducing, influence behaviour. The book uses up-to-date examples to show precise hypotheses may be formulated and tested experimentally. Theoretical ideas and empirical results are closely integrated throughout and in this new edition, the material which was covered in one chapter has been divided into three parts to clarify and simplify the presentation. Another new feature of the second edition is the inclusion, at the end of each chapter, of a number of topics for discussion. The book is suitable for second and third year undergraduate courses in animal behaviour or behavioural ecology.