Ovarian fluid allows directional cryptic female choice despite external fertilization

Area of Science:

• Reproductive Biology
• Evolutionary Ecology
• Animal Behavior

Background:

• Cryptic female mate choice, where females influence fertilization post-mating, is well-documented in internally fertilizing species.
• The potential for such post-mating choice in externally fertilizing species has remained largely unexplored.
• Understanding female control in external fertilization is crucial for evolutionary insights into reproductive strategies.

Purpose of the Study:

• To investigate whether cryptic female mate choice occurs in species with external fertilization.
• To determine the role of female ovarian fluid in modulating sperm competition outcomes.
• To assess the impact of ovarian fluid on sperm performance and paternity.

Main Methods:

• Conducted in vitro sperm competition experiments using fish ovarian fluid.
• Assessed the effects of ovarian fluid on sperm number, velocity, motility, straightness, and chemoattraction.
• Compared sperm performance between different male phenotypes in the presence and absence of ovarian fluid.

Main Results:

• Female ovarian fluid significantly altered sperm competition by reducing the importance of sperm number and increasing the importance of sperm velocity.
• Ovarian fluid generally enhanced sperm velocity, motility, straightness, and chemoattraction, but did not differentially affect sperm from alternative male phenotypes.
• Males releasing fewer but faster sperm likely gain higher paternity under natural conditions due to ovarian fluid's influence.

Conclusions:

• Females in externally fertilizing species possess greater control over fertilization than previously assumed.
• Ovarian fluid plays a key role in mediating post-mating sexual selection in external fertilizers.
• These findings suggest females can exert significant selective pressure on male reproductive traits even with external fertilization.