From brains to behaviour: hormonal cascades and alternative mating tactics in teleost fishes

Abstract

Each alternative male reproductive morph in teleosts has a distinct suite of secondary sex characteristics including behavioural, endocrinological, somatic and brain traits. Accepting that the central nervous system (CNS) is the ‘seat’ of behaviour, then male dimorphisms in the CNS (whether e.g. morphological, chemical or biophysical) must determine the production of distinct male reproductive tactics. The available data suggest that the preoptic-pituitary/GnRH-gonadotropin axis provides a proximate mechanism that initiates testicular maturation, subsequent changes in titers of circulating sex steroids, and the expression of secondary sex characteristics. Individuals may be predisposed, developmentally, to express one male morph or the other. Thus, irrespective of the time during an individual male's postembryonic life that a GnRH cascade is initiated, he would express only one male morph. (Developmental predisposition may depend on genetic or epigenetic events.) Alternatively, the temporal relationship of a GnRH cascade to an individual's age or body size may be critical to determining which morph is expressed. Thus, if the GnRH ‘trigger’ were to occur at 1 year of age or 6 cm body size, a sneak-spawn tactic would be followed; at 2 years of age or 12 cm, a parental, nest-guarding tactic is followed. The latter mechanism is akin to that for sequential hermaphrodites, where an individual has the potential to express both reproductive tactics, although for species such as midshipman, only one morph is ever manifested. Irrespective of which developmental mechanism is operative, the result is a mating system with two male reproductive morphs, each of which has a distinct suite of traits subject to sexual and natural selection.