Spontaneous synchronization in nature

Mutual synchronization of oscillators is ubiquitous in biology. The author reviews two examples in some detail. Everyone has heard of the phenomenon of synchronized menstrual cycles among women friends or roommates (Anonymous 1977). The first scientific study of menstrual synchrony was carried out by Martha McClintock (1971) while she was an undergraduate psychology major at Radcliffe in the late 60's. She studied 135 women undergraduates and had them keep records of their periods throughout the school year. In October, the cycles of close friends and roommates started an average of 8.5 days apart, but by March, the average spacing was down to 5 days, a statistically significant change. Randomly matched pairs of women showed no such change. In the animal world, groups of Southeast Asian fireflies provide a spectacular example of synchronization. Along the tidal rivers of Malaysia, Thailand and New Guinea, thousands of fireflies congregate in trees at night and flash on and off in unison. When they first arrive, their flickerings are uncoordinated. But as the night goes on, they build up the rhythm until eventually whole treefuls pulsate in silent concert. You can see this display on David Attenborough's (1992) television show The Trials of Life, in the episode called "Talking to Strangers". As he explains, "All those that are flashing are males, and their message, of course, is directed to the females, and it's a very simple one: 'Come hither-mate with me'".