This study looked at different characteristics of butterflies to understand how sperm competition affects their reproduction. The researchers found that male butterflies with larger bodies usually had bigger testes, which produce sperm. They also discovered that in species where males compete more for mating, they tend to have longer fertilizing sperm. This suggests that butterflies invest more in making longer sperm when they need to compete with others for reproductive success. Interestingly, the size of the non-fertilizing sperm was more related to the body size of the butterfly rather than the competition level. This indicates that different types of sperm play different roles in the butterfly reproduction process.