The main seasonal factors that can influence fertility in a number of mammalian species are photoperiod, temperature and humidity. Although traces of the same kind of regulation are likely to be found in most species, human beings differ from most other species in that they are socialized and are non-seasonal breeders, and it is worth asking whether these seasonal factors influence fertility in humans.
This chapter will review recent observations about circannual variability of a number of measures of human fertility, including sperm quality, menstrual cycle cyclicity and hormonal rhythmicity, in addition to factors influencing birth seasonality. The ways in which climatic seasonality, as mediated through variations in photoperiod, temperature or humidity, can interfere with the physiological processes associated with fertility are also examined, as is the importance of interactions between environmental factors and social life in determining human fertility.
Seasonal variability in fertility parameters
The last decade has seen an increase in medically-assisted conception in developed countries, and the number of sperm banks has risen as a consequence. Studies of sperm quality, carried out across the calendar year, show seasonal fluctuations in semen quality for both fertile and infertile males.
Analyses of semen from fertile volunteer donors in Lille (France) have shown seasonal variability in sperm count, with the highest values recorded in late winter and early spring and the lowest values recorded in late summer (Saint Pol et al., 1989). No circannual pattern was demonstrated either for sperm volume or for percentage of motile spermatozoa. A similar finding has been reported in the United States.