A study by the University of Manchester and The Silber Infertility Center of St. Louis has found that human sperm quality peaks in the summer and decreases in winter. The research, published in the journal journal The Journal of Science, compared semen samples from over 15,000 sperm donors in Denmark and the U.S. across both populations. The researchers found a consistent pattern, with the highest levels of progressively motile sperm appearing in June and July, while the lowest levels occurred in December and January. The study suggests that this seasonal pattern may be an evolutionary vestige since reproduction is timed in temperate climates to occur in spring, which could increase the likelihood of spring births. However, this seasonal effect is likely muted as humans have adapted to survive well in the winter.