Sperm competition games between sneaks and guards: a comparative analysis using dimorphic male beetles

Area of Science:

• Evolutionary Biology
• Behavioral Ecology
• Sexual Selection

Background:

• Sperm competition is a significant driver of sexual selection, influencing male reproductive strategies.
• Theoretical models predict increased ejaculate expenditure with higher sperm competition risk.
• Alternative male mating tactics, like sneaking and guarding, are key in sperm competition dynamics.

Purpose of the Study:

• To test predictions of sperm competition games in a comparative context using the beetle genus Onthophagus.
• To investigate the relationship between the frequency of alternative male morphs and ejaculate expenditure across species.
• To examine ejaculate expenditure differences between sneak (minor) and guard (major) males within species.

Main Methods:

• Comparative analysis of testes size across 16 species of dimorphic Onthophagus beetles.
• Examination of testes allometry in two populations of O. taurus with divergent frequencies of minor males.
• Correlation of testes size with the frequency of the minor male phenotype across species and within populations.

Main Results:

• Across species, testes size increased with the frequency of the minor male (sneaker) phenotype.
• Minor males generally exhibited larger testes size compared to major males (guards).
• The disparity in testes size between morphs was independent of the frequency of minor males across species.

Conclusions:

• The relative frequency of sneaker males in a population influences male expenditure on ejaculate.
• Findings support theoretical predictions linking sperm competition risk to male investment in reproduction.
• Onthophagus beetles provide a valuable model system for studying the evolutionary consequences of alternative mating tactics.