Seasonal variation of semen quality and fertility

At many locations in the northern hemisphere human sperm counts are reduced during the summer. Experiments with rhesus monkeys suggest that such seaso nal variation may be induced by an endogenous biological clock reset annual ly by changes in the length of daylight. Little is known about seasonal var iation in human female reproductive function, although fecundity may be low er during the summer, as it is for men. Variation in fecundity could help e xplain seasonal patterns of birth in countries such as the United States, w here there is a deficit of spring births. But the relationship to patterns of birth in Canada and Europe, where the birth rate is highest during late winter and spring, is unclear.