THE EVOLUTION OF SPERM SIZE IN BIRDS

Area of Science:

• Evolutionary biology
• Reproductive strategies
• Animal behavior

Background:

• Sperm size variation across species is significant but poorly understood.
• Previous research linked longer sperm to faster swimming velocity, suggesting an advantage in sperm competition.
• Earlier studies faced limitations due to indirect measures of sperm competition intensity.

Purpose of the Study:

• To investigate the relationship between sperm length and sperm competition intensity using direct measures.
• To explore the indirect mechanisms, such as female reproductive tract morphology, influencing this relationship.
• To re-evaluate existing theories on sperm size evolution in light of new data.

Main Methods:

• Utilized DNA profiling to directly assess extrapair paternity, a measure of sperm competition intensity.
• Analyzed data from 21 passerine bird species.
• Employed path analysis to determine direct and indirect effects between sperm length, extrapair paternity, and sperm storage tubule (SST) length.

Main Results:

• A positive correlation was found between sperm length and the degree of extrapair paternity.
• Path analysis revealed this relationship is indirect, mediated by the length of female sperm storage tubules (SSTs).
• Increased sperm competition intensity positively correlated with SST length, which in turn correlated with sperm length.

Conclusions:

• Sperm length evolves in response to sperm competition intensity, but primarily through female control over SST length.
• Female SST length may be influenced by sperm competition, potentially enhancing female control over fertilization.
• Male responses in sperm length to SST size suggest an evolutionary arms race.
• Findings contradict previous models predicting decreased sperm size with increased competition and highlight the role of female manipulation in sperm morphology evolution.