ABSTRACT
The unmistakable vocalization of an estrous cat is a well known example of sexual signalling, but analogous steroid-dependent mechanisms for advertising reproductive status to conspecifics are ubiquitous in the vertebrates. Even in teleost fish, which have recejved relatively little attention, gonadal steroids are known to regulate the development or expression of sexually dimorphic features such as morphology and color (Borg, 1995), and a variety of behavior patterns that generate visual (Liley and Stacey, 1983; Stacey, 1987), electrical (Keller et al., 1986; Zakon, 1995), or acoustic signals (Bass, 1995). A feature common to thes< nd analogous signalling systems in other vertebrate groups is that the information carried by changing steroid hormone levels is conducted only indirectly to conspecifics, through slowly responding somatic effectors ( CNS, muscles, etc.) which may require days or weeks to become functional, and therefore are capable of transmitting only relatively tonic information about the signaller's past hormonal status. Many fish species, however, solve the pro~ lem of assessing the reproductive condition of conspecifics in a much more elegant fashion, simply by detecting and responding to the hormones and hormone metabolites released by conspecifics (Sorensen, 1992a; Stacey and Sorensen, 1991; Stacey et al., 1994a) (Fig. I) . Because they directly conduct information about ongoing hormonal change, these hm UJnal pheromone systems operate in a fundamentally different manner from traditional steroid-driven signalling systems (Fig. I).
