At the onset of winter, the American wood frog burrows into the soil of its native northern Canada. As the temperature drops, the frog's body begins to freeze, and in response it produces a supply of glucose that effectively lowers the freezing point of water, preventing the formation of ice crystals. For months the frog stops breathing and its heart stops beating. When spring arrives and the land begins to thaw, so does the wood frog.
Animals and plants are able to adapt to the planet's changing seasons by virtue of their internal calendar, write Russell G. Foster and Leon Kreitzman in Seasons of Life: The Biological Rhythms That Enable Living Things to Thrive and Survive (Yale University Press). In their previous book together, Rhythms of Life (Yale University Press), the authors dealt with the circadian clock that allows organisms to track the time of day, even in controlled laboratory settings. The circannual calendar, they note, requires much more patience of the scientists who study it; but although their knowledge of seasonal biological variations remains incomplete, "we have uncovered some of the mechanisms involved."
Those mechanisms include hibernation, migration, and seasonal reproduction, which ensures that animals give birth at the optimum time of year for survival —largely determined by the relative abundance of food. Many nonequatorial animals accomplish this by breeding only at certain times. During the rest of the year, their reproductive organs "regress," or all but disappear, saving much-needed energy as well as preventing conception.
Foster and Kreitzman —respectively a professor of circadian neuroscience at the University of Oxford and a science writer and broadcaster —stress the interconnectivity between species' seasonal adaptations: "The organisms higher up the food chain have to time their activities to the rise and fall in the abundance of those lower down the food chain." Thus interference with one species' population can set off a complex series of reactions throughout an entire ecosystem.
The second half of Seasons of Life focuses on Homo sapiens and the myriad ways our lives —and deaths —are affected by the time of year. The authors examine the seasonal cycles of infectious diseases, as well as seasonal affective disorder and how it has been diagnosed and treated throughout human history. They even suggest that the month in which a person is born has significant statistical correlations. Although predicting people's future based on the month of their birth has long been the province of astrologers, studies now show that, depending on geographical location, date of birth may be linked to incidence of certain diseases, physical characteristics, and even personality traits. Summer babies are more likely to develop the digestive disorder celiac disease, winter babies "show increased novelty and sensation seeking," and, "in the Northern Hemisphere, babies born in the early part of the year are 6 to 8 percent more likely than others to develop schizophrenia later in life."
Noting that "humans, just as chimpanzees and gorillas, are ready to procreate more or less at the drop of a hat, more or less most of the time," the authors look to factors like women's nutritional status at the time of conception to account for variations in offspring. Our past plays a role, too: June is still the most popular month in which to marry, in part because, for our agrarian forebears, "a June or July conception and a subsequent early spring birth meant that the mother had recovered to some extent in time for the busy autumn harvest season the following year."
Climate change has already begun to affect the duration and dispositions of the seasons, threatening all species, warn the authors: "Those that succeed will be those that have the flexibility to change and adapt to the new temporal regime."
