It turns out reproduction is seasonal across all living organisms, from plants, to insects, to reptiles, to birds and mammals – including human beings. The ultimate explanation for this phenomenon is an evolutionary one.

Earth’s environment is seasonal. Above or below the equator, the year is structured by the winter, spring, summer and fall. In equatorial regions, the wet and dry seasons punctuate the year. Organisms have evolved strategies to reproduce at the time of year that will maximize their lifetime reproductive success.

Humans are no exception and maintain this evolutionary outcome: birth seasonality. Researchers, including us, have recently been working to understand more about why births are seasonal because these patterns can have a big impact on childhood disease outbreaks.