Abstract
Data from Rosengrave, P. C., Lymbery, R. A., & Evans, J. P. (2024). Patterns of sperm swimming behaviour depend on male mating tactic and spawning environment in chinook salmon. Scientific Reports, 14(1). https://doi.org/10.1038/s41598-024-76115-4.
Many species exhibit alternative mating tactics (ARTs), with larger, socially dominant males competing for females and smaller males adopting “sneaker” strategies to exploit fertilisation opportunities without competition or courtship. Females typically prefer larger socially dominant males, but their ability to manipulate mating or fertilisation outcomes is largely unknown. Here, using chinook salmon Oncorhynchus tshawytscha, we ask whether the female’s ovarian fluid (OF) differentially influences the temporal patterns of sperm swimming in ejaculates from non-preferred sneaker (‘parr’) and preferred (dominant) males. We demonstrate that OF improves sperm swimming speed and linearity compared to river water, regardless of male mating tactic, but also report a novel tactic-specific difference in sperm linearity; parr male sperm initially maintain straighter trajectories in river water, compared to dominant males, but then rapidly change to less linear and more circular paths over time. Intriguingly, we show that OF counteracts this change in sperm linearity in parr males so that patterns become indistinguishable from dominants when parr sperm swim in OF. Together, these results lead us to predict that male chinook salmon exhibit differential sperm trait investment strategies that depend on reproductive tactic.