Evolution and Specialization of Fish Hormonal Pheromones

Teleost fish commonly release steroid and prostaglandin hormones and their metabolites to the water, where some function as water-borne odorants that induce specific physiological and/or behavioral reproductive responses in conspecifics. In this paper, we evaluate processes responsible for the evolution and specialization of these hormonal pheromones and give examples. Several new definitions are coined. The topic is complex owing to the evolutionary age and great diversity of fish species. We hypothesize that the evolution of fish hormonal pheromones has been influenced by two types of factors, factors intrinsic to the pheromonal function (e.g. factors which directly determine the nature of pheromones while being themselves influenced by how well the pheromones function), and extrinsic factors which are not subject to conspecific feedback. Both types of factors may influence receivers and/or donors of pheromonal stimuli in independent manners. We further hypothesize that hormonal pheromones originated through’ spying,’ in which the receiver benefits by detecting a hormonal cue and in which neither the donor nor its cue is specialized. However, we also suggest that in many instances the receiver’s response will benefit the donor, thereby driving specialization of pheromone production-a phenomenon we term’ communication.’ Whereas evolutionary pressures to increase the efficiency of cue detection may lead to an increase in the number of stimuli detected, sexual selection acting on the donor may cause species to increase the efficiency of signal production. Donors might achieve this by’ amplifying’ signal output and/or increasing the biological relevance by optimizing its information content through’ signal elaboration.’ In some instances hormonal pheromones also will be modified by extrinsic factors, a subset of which may lead to the evolution of species-specificity.