Sons are tough on their mothers. Whether it is heavier birth weights, amplified testosterone levels or simple, hair-raising high jinks, boys seem to take an extra toll on the women who gave birth to them. And by poring over Finnish church records from two centuries ago, Virpi Lummaa of the University of Sheffield in England can prove it: sons reduce a mother’s life span by an average of 34 weeks.
The 33-year-old Finnish evolutionary biologist, aided by genealogists, has scoured centuries-old tomes (and decades-old microfiche) for birth, marriage and death records—and clues about the influence of evolution on human reproduction. Historians, economists and even sociologists have long used such tactics to explore their fields, but Lummaa is among the first biologists to enlist Homo sapiens as an animal whose population can be followed over time.
After all, humans are relatively easy to track and offer the signal advantage of occasionally keeping detailed records. “I always wanted to work on primates,” Lummaa says. “But if I wanted to collect a similar data set on wild chimps, I would be struggling. I’ve decided to study another primate in the end.” Of late, one of her subjects has been premodern mothers among the Sami people of Finland, who are famous for their reindeer herding.
Among this group, she found that those who bore sons had shorter life spans than those who gave birth to daughters. This discrepancy has to do with birth weight—male babies are typically larger—as well as testosterone. “Testosterone can compromise your immune system; it can affect your health,” Lummaa says, and the mothers of sons proved especially susceptible to endemic infectious disease, such as tuberculosis. “Boys are a little bit more costly” to raise than girls as well, because they drain more physical resources from their mothers, she adds, as has been seen in other mammals, such as the red deer. Sons also are not as likely as daughters to stick around to help their mothers out later in life.
More recently, Lummaa and her colleagues have been studying how sons are not just tough on their mothers but also hard on their siblings. Those born after a son were physically slighter, had smaller families and generally had a greater chance of dying from an infectious disease. The effects held up whether the elder brother died in childhood or not, suggesting that the negative outcome is not a result of some direct sibling interaction, such as competition for food, regular beatings or the practice of primogeniture, in which the eldest brother inherits everything. “Big brothers are bad for you,” Lummaa explains. “If the fifth-born was a male, then the sixth-born is doing worse.”
This phenomenon is particularly evident in twins where one is male and the other is female. Of 754 twins born between 1734 and 1888 in five towns in rural Finland, girls from mixed-gender pairs proved 25 percent less likely to have children, had at least two fewer children, and were about 15 percent less likely to marry than those born with a sister. This brotherly influence remained the same regardless of social class or other cultural factors and even endured if the male twin died within three months of birth, leaving the female twin to be reared as an only child.
The reason that the female half suffers, Lummaa speculates, is because of testosterone exposure in the womb. Researchers have seen such hormonal influence in other animals, including lab rats and cows. When a cow has mixed-sex twins, the female is occasionally born sterile because of testosterone influence.
Whatever the cause, there is no question of the outcome: mothers of opposite-sex twins end up with 19 percent fewer grandchildren than moms of same-sex twins, meaning evolution would seem to favor the latter. “Biological differences between male and female are not determined [simply] by the chromosomes one inherits at birth,” says anthropologist Christopher Kuzawa of Northwestern University. The sibling effects “impact reproductive success and thus have evolutionary significance.”
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2 Comments
Add CommentIf there is a grandmother advantage, and grandmothers tend to focus their efforts on daughters (letting their sons mothers-in-law handle with help for their own daughters offspring) perhaps the longevity "drawback" of having sons instead of daughters can be summed up thusly: I'm not needed anymore, what is the point in living longer?
Reply | Report Abuse | Link to this@ jimthornton : I agree with your comment, been there, done that myself. I let my own mother and my MIL take over the care of my first born (and their first grandchild) so much that I felt I wasn't needed and that they thought I wasn't fit to be a mother. Instead of talking to them about it I slinked back into the shadows and sort of "ran away from home" so to speak... Luckily I came to my senses and all is well. That was twenty years ago and both my kids are really awesome.
Reply | Report Abuse | Link to thisIt's kind of funny that both my kids are opposite of what this article says the "boy-girl" thing would be. My son was Mr. Compliant and my daughter was (is) the strong-willed, larger birth weight and difficult child who is making me grey-haired and bald. lol
I guess I probably did the same to my own mother. LOL