Studie zeigt, dass Spermien im Sommer schneller schwimmen

Must be from all that lemonade and ice cream

I never will forget those nights, I wonder if it was a dream

Well this is just the flow vs concentration. Best times are still in winter months „According to a study published in the American Journal of Obstetrics and Gynecology, the quality of male sperm produced in the winter time is better than sperm produced in the summer months. Researchers found that sperm concentration and percentage of fast motility decrease in the spring, summer, and fall and rebound in the winter and early spring. Sperm cells also have a healthier physical structure in these months“

Maybe this is vestigial functionality left over from pre-human ancestors that had a mating season.