In many species, males can make rapid adjustments to ejaculate performance in response to sperm competition risk; however, the mechanisms behind these changes are not understood. Here, we manipulate male social status in an externally fertilising fish, chinook salmon (Oncorhynchus tshawytscha), and find that in less than 48 hr, males can upregulate sperm velocity when faced with an increased risk of sperm competition. Using a series of in vitro sperm manipulation and competition experiments, we show that rapid changes in sperm velocity are mediated by seminal fluid and the effect of seminal fluid on sperm velocity directly impacts paternity share and therefore reproductive success. These combined findings, completely consistent with sperm competition theory, provide unequivocal evidence that sperm competition risk drives plastic adjustment of ejaculate quality, that seminal fluid harbours the mechanism for the rapid adjustment of sperm velocity and that fitness benefits accrue to males from such adjustment.
Comparing sperm velocity and sperm concentration in males of different social status at each stage of the experiment
This file contains data used to compare ejaculate parameters between dominant and subdominant males at the same stage of the experiment. Sperm concentration and velocity measurements for male chinook salmon taken at both stages of a two-stage social manipulation experiment. Social dominance was determined within each pair using video recordings of aggressive behavior. Sperm concentration was measured using a haemocytometer. Sperm velocity was measured using Computer Assisted Sperm Analysis.
Assessing the change in sperm velocity and sperm concentration in the same males between stage 1 and 2 of the experiment
This file contains data used to assess the change in ejaculate parameters from stage 1 to stage 2 of the experiment within males of the same social status group. Sperm concentration and velocity measurements for male chinook salmon taken at both stages of a two-stage social manipulation experiment. Social dominance was determined within each pair using video recordings of aggressive behavior. Sperm concentration was measured using a haemocytometer. Sperm velocity was measured using Computer Assisted Sperm Analysis.
Measuring change in sperm velocity for sperm incubated in their own seminal fluid vs rivals seminal fluid
Chinook salmon ejaculates were experimentally manipulated, separating sperm from seminal fluid and then recombining sperm with either seminal fluid from the same male or from a rival male. Ejaculates were collected from male chinook salmon taken at both stages of a two-stage social manipulation experiment. Social dominance was determined within each pair using video recordings of aggressive behavior. Sperm velocity was measured using Computer Assisted Sperm Analysis.
Measuring change in VAP for sperm incubated in their own seminal fluid vs rivals seminal fluid.xlsx
Comparing fertilisation success for males of different social status and across seminal fluid treatments
Sperm competition trials were conducted, each trial using a single female and two male chinook salmon from a two-stage social manipulation experiment. Trials were also conducted using two seminal fluid treatments, (1) milt was unmanipulated, (2) seminal fluid was switched between the males in each trial. This data file contains sperm velocity measurements and paternity share data for each male. The paternity share of each male was determined using mircosatellite loci. Social dominance was determined within each pair using video recordings of aggressive behavior. Sperm concentration was measured using a haemocytometer. Sperm velocity was measured using Computer Assisted Sperm Analysis.
