Honest signalling among gametes.

The gametes of many lower eukaryotic organisms emit pheromones that attract gametes of the opposite mating type or sex. Gametes move or grow in the direction of the highest pheromone concentration, suggesting that the strength of the pheromonal signal is used to infer proximity, or that the strongest signal is most likely to be notice. Here I offer a new explanation of pheromonal signalling and chemotaxis in gametes. I show that pheromonal signals can be interpreted as sexually selected traits that honestly advertise variation in quality among gametes, given that signals are costly to produce and that gametes compete; by 'quality' I refer to some aspect of a gamete's fitness. A gamete's preference for a mating partner, then, is predicted to vary with the quality of a prospective partner as inferred from the strength of its signal. This view can explain characteristics of the signalling and mate selection behaviours of gametes that are not predicted by models of mate choice based on proximity or 'passive attraction' to the strongest signal. These include repeated partner exchanges, escalated exchanges of mating pheromones, and rejection of gametes that signal at low levels.