INTRODUCTION

Mating systems are among the central topics in the study of animal social organization. Monogamy is loosely defined and used to refer to the situation in which amating pair stays together exclusively for at least one breeding season (e.g., Fuentes, 1999). Obviously, this definition applies only to social, not genetic, monogamy. In the following discussion, unless otherwise indicated, I use the term monogamy to refer to social monogamy.

Kleiman (1977) was the first to summarize mating systems in mammals and found that fewer than 3% of mammalian species were socially monogamous (also see Rutberg, 1983; Kinzey, 1987). This was in sharp contrast to birds, where about 90% of the species were traditionally believed to be socially monogamous (Lack, 1968). This general impression was prevalent for decades until a recent flurry of data revealed that many of these socially monogamous birds and mammals routinely engage in extra-pair copulations (EPCs) (see Birkhead & Møller, 1995; Fuentes, 2002). Several chapters in this volume provide some fresh data about EPCs in mammals, which are less studied than birds in this respect.

In mammals, parental investment is extremely skewed towards females owing to the time- and energy consuming aspects of mammalian female gestation and lactation. As a result, male mammals are selected for reproducing more polygynously than males in other groups of animals. Males provide parental care in fewer than 10% of mammalian species (Woodroffe & Vincent, 1994). However, even male parental care does not necessarily lead to social monogamy, and based on a few socially monogamous species that have been genetically screened (see Ribble, chapter 5; Fietz, chapter 14), strict genetic monogamy appears to be much rarer in mammals.