Any attempt to understand the behaviour of a complex organism such as a bird or a mammal is no easy task; trying to do so across dozens of species in a comparative perspective is rather more difficult, but not impossible. Moreover, it is necessary if an understanding of the evolutionary aspects of the behaviour is to be achieved. This chapter introduces the comparative techniques, methods and sources of empirical data used in this book. The major techniques and sources of phylogenies will be described in some detail for the benefit of those who want to know precisely how the analyses were carried out. Non-specialists may skip those sections if they wish. Some especially important terms are also defined here, the chapter ending with an analysis of the effects of captivity and wild conditions upon the expression of same-sex sexual behaviour in birds and mammals. Let us start, however, with some epistemological caveats.

Epistemological caveats

There are very few guiding principles in modern science that can compete in degree of consensus regarding usefulness and importance, with the so called Ockham’s Razor. This principle states that ‘one should not multiply entities beyond necessity’. The Ockham’s Razor principle was apparently never stated as such by its alleged author, the English medieval philosopher William of Ockham (sometimes also spelt ‘Occam’, a Latinized form), but it is certainly consistent with his philosophical views. Ockham’s Razor reflects a widespread view – also encapsulated in similar principles such as that of Simplicity, or of Parsimony or of Economy of Thought – that when we are seeking a scientific explanation for any phenomenon our hypotheses should avoid unnecessary complications whenever simpler alternatives are available. The principle has been the subject of an extremely illuminating analysis published more than 50 years ago by Lewis S. Feuer (Feuer 1957).