The Berkeley Science Review: Articles

Of Ice and Men
Cold weather shapes male survival (view PDF)
by Susan Young

Choosing your baby's sex is the subject of numerous old wives' tales—eat red meat to have a boy or have sex in the afternoon to have a girl, to name a few. Most of us don't place much stock in these ideas, but there may be more to these tales than just superstition. It was recently shown that the sex of a child can be influenced by whether a mom-to-be eats her Wheaties, as women who ate cereal for breakfast increased their odds of having a boy. Population stressors, including natural disasters and economic and psychological stress, also affect the secondary sex ratio—that is, the ratio of male to female births (the primary sex ratio is the ratio of male to female conceptions). Generally, more stress leads to fewer males. A recent study from UC Berkeley Professor of Public Health Ray Catalano and colleagues found that particularly harsh winters in Scandinavia generated enough stress in the population not only to change the sex ratio, but also increased the lifespan of males in the next generation.

Between the years 1895 and 1914, colder than average winters in Scandinavia were followed by the birth of fewer males. Females, however, weren't as affected; in utero and beyond, males are more sensitive to stress. "Females are particularly robust to stressors and seem more resilient than males are," says Tim Bruckner, a postdoctoral fellow in the Department of Health Policy and researcher involved in the study. For as yet unclear reasons, "males in general do worse in response to stressors than females across the whole spectrum of life."

For those lucky males who survived, cold stress during gestation also had an effect later in life. The researchers found that those males who made it into childhood tended to live longer than males who were not subjected to the cold stress in utero—on average, 14 days longer than males born after a more temperate gestation. This difference, while statistically sound, is a tiny change over a lifetime and could only be detected with a large data set like the one the authors examined. According to Professor of Global Environmental Health and co-author Kirk R. Smith, it was "a fortuitous combination of the longest temperature records and the longest good demographic set in the world" that allowed the researchers to uncover the phenomenon. Before this study, it was unclear whether stresses that lower the sex ratio do so via a random loss of male fetuses or whether weaker males are selectively terminated. The link between cold stress and increased male lifespan "supports the hypothesis that mothers' bodies aren't aborting just any male fetuses, but are instead aborting the weaker ones," says Smith.

How does a colder than normal winter exert its effect upon the mother's body? The earliest populations in the study were born in the late 19th century, so while they might not have had central heating, they were certainly well-insulated from cold shock in their homes. However, poor nutrition due to reduced food production, depression, or increased exposure to indoor air pollution are all potential sources of stress in harsh winters. Moreover, stress-related hormonal responses in pregnant women have been shown to induce the spontaneous abortion of fetuses, particularly males. Whatever the mechanism, this study indicates that an increase of just 1° C (about 2° F) during gestation predicts that a male's lifespan will be shortened by 14 days. "The human body is closely attuned to the environment and relatively small changes do have an impact," says Smith.

Environmental stressors aren't alone in their effects on the secondary sex ratio. Great tragedies, like the September 11, 2001 attack on New York City, can inflict widespread distress on populations. In another study, Catalano, along with Bruckner and others, explored the effect of the trauma of 9/11 on the sex ratios of populations born soon thereafter. In both New York and California, unexpectedly low numbers of males were born in December 2001, three months after the event, associating the stress of the event with the miscarriage of male fetuses. The reduction in the sex ratio in New York was greater than it was in California (chosen because of its distance from the attack), indicating a dosage effect of stress during gestation.

As humans become more comfortable in our modern lives, insulated from disease, cold, and hunger, it might be easy to ignore how subtle changes in the environment can influence the journey of our species as a whole. However, something as banal as a cold winter can actually alter the composition of the next generation. "We often view human evolution as something that happened in the past," says Bruckner. "But even looking at data from as recent as 1915, we can see that we shouldn't view it as only in the rear-view mirror."

Susan Young is a graduate student in molecular and cell biology.

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