How and to what extent do sneakers gain proximity to females in an externally fertilizing fish?

Recent studies have shown that fertilization success in external fertilization systems is influenced by differences in the temporal and spatial positions of competing males at ejaculation, as is the case for internal fertilization systems. Little is known about how these positions differ between competitors. We tackled this issue in the triplefin fish Enneapterygius etheostoma, which spawns in turbulent flow environments, and which employs external fertilization and alternative reproductive tactics (i.e. sneakers and territorial males). A frame-by-frame analysis of video images of sneaking behaviours recorded in the wild revealed that sneakers ejaculated later, and further from the spawning female, than territorial males. The majority of sneaking attempts failed because of attacks by territorial males. Successful sneakers mostly hid behind shelters around the spawning sites and then rushed into the site (i.e. were undetected), or were detected and attacked as they approached the site or hid near it, but rushed in anyway (attack-initiated sneaking). Of the undetected successful sneakers, a minority rushed in when the spawning female was being ignored by a territorial male that was attacking other sneakers (induced sneaking) and the others rushed in when there was no territorial attack (attack-free sneaking). Attack-free sneaking was carried out by sneakers that hid closer to the spawning female for longer, and were spatially and temporally closer to territorial males at ejaculation. Altogether, we conclude that the abilities to get past territorial vigilance and observe gamete release of spawning pairs at a hiding place are key factors that helped sneakers to improve their position at ejaculation, and thus perhaps their fertilization success.