Small, Variable Sperm in a Barn Swallow Population with Low Extra-Pair Paternity.

Area of Science:

• Evolutionary Biology
• Behavioral Ecology
• Sexual Selection

Background:

• Sperm competition is a theoretical driver of sperm morphology evolution.
• Barn swallows (Hirundo rustica) exhibit population-specific variation in extra-pair paternity, a key indicator of sperm competition intensity.
• Limited data exists on how varying sperm competition levels shape sperm morphology within a species.

Purpose of the Study:

• To investigate the relationship between sperm competition intensity and sperm morphology in barn swallows.
• To compare sperm morphology in a barn swallow population with minimal extra-pair paternity (Joetsu) to populations with high extra-pair paternity.

Main Methods:

• Comparative analysis of sperm morphology (head, midpiece, total length, and length variability) in barn swallows from the Joetsu population (low sperm competition) versus other populations (high sperm competition).
• Quantification of sperm length variation using the coefficient of variation.

Main Results:

• Barn swallows in the Joetsu population exhibited significantly shorter sperm head, midpiece, and total length compared to those in populations with frequent extra-pair paternity.
• The Joetsu population displayed approximately twice the variability (coefficient of variation) in total sperm length compared to populations with high extra-pair paternity.
• Sperm morphology differences align with theoretical predictions of sperm competition's effects.

Conclusions:

• Sperm competition intensity is linked to both the mean and variance of sperm morphology in barn swallows.
• Findings support a positive, directional, and stabilizing effect of sperm competition on sperm traits.
• This study provides crucial evidence for the adaptive significance of sperm morphology in response to varying sexual selection pressures in a wild bird species.