INTRODUCTION

Social communication and social monogamy

Communication is an integral component of social processes and social evolution (Hahn & Simmel, 1974; Philips & Austad, 1990), and variation in social systems influences patterns of communication (Marler & Mitani, 1988). Amongst other functions, communication may serve to attract, stimulate, defend, or compete for mates, and thus is intimately related to social organization and mating systems (Hahn & Simmel, 1974; Johnstone, 1997). The intensity of signals and displays can serve as a cue to the quality of mates or opponents, and if interindividual variation of signal quality exists, sexual selection can act upon signals and displays through mate choice and intrasexual competition (Johnstone, 1995). Examples are visual and vocal displays in many mammals, birds, and other vertebrates (e.g., Clutton-Brock & Albon, 1979; Ryan, 1983; Hill, 1990). Since in most animals it is usually males who compete for females, and females who are choosy about males, sexual selection results in signals and displays being either exclusive to or exaggerated in males (Andersson, 1994). Nevertheless, mutual sexual selection can lead to signals being expressed to the same degree in both sexes (Jones & Hunter, 1993). This may be the case in monogamous animals, which are often monomorphic, both physically and behaviourally, including their communication patterns (Kleiman, 1977). Deviations from the behavioural monomorphism of signals and displays in socially monogamous animals can be expected if the selective forces driving males and females into a monogamous system differ between the sexes.