OVERVIEW

Segregation of the sexes within a species is a widespread behavioural phenomenon in both terrestrial and aquatic animals. In the marine realm, sexual segregation is exhibited by many taxa including whales, seals, seabirds and fish. Of the latter group, sharks may be particularly appropriate model animals to test theories on the mechanisms underlying sexual segregation, because sexual segregation is a general characteristic of shark populations, with both sexually dimorphic and monomorphic species being well represented among the approximately 400 extant species (Springer, 1967; Compagno, 1999). The reproductive modes of sharks are diverse ranging from egg-laying (oviparity) to placental live-bearing (viviparity) (Wourms & Demski, 1993). Among sexually dimorphic, viviparous shark species it is generally the female that is larger than the male, whilst in some oviparous species males are larger than females. Sexually monomorphic species also occur. Therefore, sharks possess a number of characteristics that make them an interesting alternative to terrestrial animal models for investigating the causes of sexual segregation.

In this chapter the prevalence and nature of sexual segregation in sharks is described and the relationship with reproductive modes is explored. Hypotheses suggested to account for sexual segregation in sharks are examined with respect to new field and laboratory behaviour studies of males and females of a monomorphic species, the lesser spotted dogfish (Scyliorhinus canicula). The chapter concludes by drawing together the main points from all shark studies to date, and suggests future directions for research in this area.