Sperm competition risk drives rapid ejaculate adjustments mediated by seminal fluid

Area of Science:

• Reproductive Biology
• Behavioral Ecology
• Evolutionary Biology

Background:

• Males in many species adjust ejaculate performance based on sperm competition risk.
• The underlying mechanisms for these rapid adjustments remain largely unknown.
• Understanding these mechanisms is crucial for comprehending reproductive strategies.

Purpose of the Study:

• To investigate the mechanisms behind rapid ejaculate performance adjustments in response to sperm competition.
• To determine if social status manipulation influences sperm velocity in chinook salmon.
• To assess the role of seminal fluid in mediating changes in sperm velocity and reproductive success.

Main Methods:

• Social status of male chinook salmon (Oncorhynchus tshawytscha) was manipulated.
• Sperm velocity was measured following social status changes.
• In vitro sperm manipulation and competition experiments were conducted to analyze seminal fluid effects.

Main Results:

• Males upregulated sperm velocity in less than 48 hours when exposed to increased sperm competition risk.
• Rapid changes in sperm velocity were mediated by seminal fluid.
• Seminal fluid's effect on sperm velocity directly impacted paternity share and reproductive success.

Conclusions:

• Sperm competition risk drives plastic adjustments in ejaculate quality.
• Seminal fluid contains the mechanism for rapid adjustment of sperm velocity.
• Males gain fitness benefits from plastic adjustments in ejaculate quality.