Data from: Black goby territorial males adjust their ejaculate's characteristics in response to the presence of sneakers

In many species males can rapidly adjust their ejaculate performance in response to changing levels of sperm competition, an ability that is probably mediated by seminal fluid adaptive plasticity. In the black goby, Gobius niger, female spawning in the nest of a territorial male can last hours, and males attach a viscous ejaculate to the nest roof, from which sperm are slowly released. During spawning, sneaker males attempt to approach the nest and release their sperm, and territorial males intensify their defensive effort at the increasing of sneaking pressure. We show here that territorial males adjust the characteristics of their seminal fluid in response to the number of competitor sneakers. Territorial males produce an ejaculate from which sperm are released at a rate that decreases as the number of surrounding competitor sneakers increases. Field observations showed that multiple sneaking attempts occurred sequentially, forcing territorials to frequently leave the nest. Overall these results suggest that the adjustment of seminal fluid viscosity may guarantee a continuous release of sperm from the ejaculate, although at a lower rate, when the territorial male is engaged in prolonged nest defense possibly minimizing the postcopulatory cost of an increased precopulatory mate guarding.