Sperm competition, mating rate and the evolution of testis and ejaculate sizes: a population model

Area of Science:

• Evolutionary biology
• Reproductive strategies
• Sperm competition dynamics

Background:

• Estimating ejaculate expenditure involves absolute sperm number (s) and relative ejaculate expenditure (E).
• Relative testis size (sigma) is a proxy for mating rate (M) and ejaculate mass (s).

Purpose of the Study:

• To develop a new model integrating mating rate, sperm competition, and sperm allocation.
• To investigate the relationship between relative testis size, ejaculate expenditure, and sperm competition across species.

Main Methods:

• A novel theoretical model was constructed.
• The model assumes female inter-clutch interval is independent of sperm number (s) or mating rate (M).

Main Results:

• Relative testis size (sigma) is directly proportional to mean relative ejaculate expenditure (E) for a species.
• Across species, both sigma and E increase with sperm competition.
• The relationship between sperm number (s) and sperm competition varies: s increases at low risk but decreases at high intensity.

Conclusions:

• Sperm competition influences ejaculate expenditure and testis size.
• The complex interplay between ejaculate expenditure (E) and mating rate (M) drives the non-linear relationship between sperm number (s) and sperm competition intensity.