Can seminal fluid proteins be honest signals of sperm availability?

Abstract Seminal fluid proteins (Sfps) are produced by males, yet they have significant effects on female physiology and behaviour. Sfps are often viewed as a mechanism for males to manipulate female reproductive behaviours, but despite many studies identifying their varied effects and their molecular underpinnings, the ways males and females use Sfps remain unclear. In the most studied case of Drosophila melanogaster particular peptides within the seminal fluid have been identified to guide female reproduction: sex peptide (SP, Acp70A) is known to induce changes in egg production, oviposition and remating and is attached to sperm and continuously supplied for days after mating, while ovulation inducing peptide, ovulin (Acp26Aa), is a free peptide with only short term effects after mating. Using a biologically informed model we show how these peptides can synchronize sperm and egg release, hence reducing the number of unfertilized eggs a female lays. We further show that the exhaustion of SP might be the key signal of this synchronization. Finally, we demonstrate that sexual conflict over the regulation of female reproductive physiology by Sfps should be limited, with the primary conflict probably centring around the regulation of remating behaviour.