Seasons of Life

Seasons of Life: The Biological Rhythms that Living Things Need to Thrive and Survive
By Russell Foster and Leon Kreitzman
Profile Books £20, 303 pages
FT Bookshop price: £16

Every autumn, on October 23, a swarm of swallows leaves the mission of San Juan Capistrano in California to migrate 9,600km south to Argentina, and every spring, on March 19, the birds return. At the end of summer, monarch butterflies, born and bred in Canada, know exactly when to fly thousands of miles south to their wintering grounds in the pine forests of Mexico.

Great migrations have always fascinated people. Scientists have mostly paid attention to how migrating animals find their way – navigating by the sun, moon and stars, by Earth’s magnetic field and by landmarks such as rivers and coastlines.

Russell Foster and Leon Kreitzman, however, focus on the amazing timekeeping of migrations and other seasonal phenomena of nature, from breeding animals to flowering plants. Until now the emerging science of chronobiology has concentrated on the circadian rhythm. This is governed by an internal clock that co-ordinates the biological functions of almost every living creature with the daily cycle of night and day.

Scientists recently unravelled the complex workings of our circadian clock: a series of feedback loops between genes and proteins creates a molecular oscillation with a period of 24 hours, controlled by a pacemaker in the brain called the suprachiasmic nuclei.

The circannual cycle, the subject of Seasons of Life, has been relatively neglected, partly because scientists can obtain results far more quickly studying a daily clock than one with a period 365 times longer. The equivalent of a two-week experiment for a circadian biologist would take 14 years for a circannual researcher. As Foster and Kreitzman say, it makes for a tough research regime.

Yet a few patient scientists have persevered, proving that animals have an intrinsic circannual clock that continues to “free run” in the absence of any external clues. The pioneer was the late Eberhard Gwinner, a German biologist who monitored birds in indoor aviaries with a constant day length of 12 hours and a steady temperature, for several years; they continued to show all the physiological signs of reproduction, moulting and migratory restlessness on an annual cycle. No one has yet located this seasonal timekeeper within the body, let alone discovered how it works.

Like its daily counterpart, the annual clock interacts with signals from the outside world that keep it running on time. Most important is changing day-length over the seasons, although weather has an effect too. These interactions tell animals when to mate, migrate or hibernate – and plants when to flower and shed their leaves.

Foster, a neuroscience professor at Oxford University, and Kreitzman, a science writer, collaborated to produce Rhythms of Life, an excellent book on chrono-biology that focuses on the circadian cycle. Seasons of Life is just as good.

One of the authors’ pressing concerns is the varying pace at which the seasonal cycles of plants and animals are responding to climate change. Species with an annual clock that is rigidly tuned to day-length continue to migrate and procreate at the same time every year. Others are more sensitive to temperature and are adjusting their activities to a warmer world.

Seasons of Life gives many examples where the resulting mismatch is causing ecological stress.

For example, caribou deer in western Greenland migrate inland as their birthing season approaches, to the edge of the ice cap where new summer vegetation is emerging on the tundra. Nowadays the plants reach their peak nutritional value much earlier than they did just 15 years ago, in response to rising temperatures, but the animals have not changed their timing. As a result the pregnant caribou are too late to catch the most nutritious food, and fewer healthy calves are born.

Foster and Kreitzman also explore the impact of the seasons on human health and disease. Although our ancestors evolved mainly in tropical Africa, where seasonal change is less important than in temperate and arctic regions, it nonetheless exerts a strong physical, psychological and social influence on us – as anyone suffering from seasonal affective disorder knows only too well.

As Seasons of Life shows, in its vivid analysis of annual patterns of human birth, life and death, we can mask the impact of the seasons with artificial heat and light – and preserved and imported food supplies – but the circannual cycle still frames our lives.

Clive Cookson is the FT’s science editor