Sex differences in English Channel swimming performance: an experiment of nature

Sex differences in endurance performance are well established, but the magnitude of these differences appears smaller in ultraendurance events, particularly swimming. This study examined sex differences in solo English Channel swimming performances and assessed the influence of age and water temperature. A retrospective analysis included 2,593 unique solo English Channel swims (1,724 males and 869 females) recorded between 1875 and 2025. Swims were obtained from the Channel Swimming & Piloting Federation database. To minimize variability from noncompetitive crossings, analyses were restricted to the top 100 swimmers of all time within each age and water temperature subgroup. Sex differences were assessed across all finishers, among the top 100 swimmers, within age and water temperature categories, and over consecutive 5-yr periods. Across all swims, males were 2.2% faster than females (13.54 ± 0.06 h vs. 13.84 ± 0.10 h, <i>P</i> = 0.009). Restricting analysis to the top 100 swimmers, the sex difference was 7.2% (9.00 ± 0.08 h vs. 9.67 ± 0.08 h, <i>P</i> < 0.001). Sex differences increased with age, ranging from 5.2% in the 20- to 29-yr group to 26.5% among swimmers >60 yr. Males outperformed females across all water temperature categories, and the fastest swims for both sexes were observed at 17°C to 17.9°C. Male swimmers are faster than female swimmers in English Channel crossings, with differences most pronounced among top performers and older athletes. However, sex differences are smaller than those reported in ultraendurance running, suggesting that the unique environmental and physiological demands of open-water swimming partially attenuate male performance advantages.<b>NEW & NOTEWORTHY</b> This study leverages an experiment of nature framework to examine sex-based differences in ultraendurance swimming using English Channel performance data. Males were faster than females, with the greatest differences observed among older swimmers and top performers. Performance was optimal in moderately cool water temperatures (17-18°C). These findings highlight how naturally occurring endurance events provide unique opportunities to study physiological limits and the combined influences of sex, age, and environment on human performance.