For every hundred babies born in Europe, one or two of them are quickly followed by a twin. From a cold evolutionary point of view, twins look like a win for their mother. History, after all, is written by individuals who are best at passing on their genes, so having more children at once seems like a good strategy. So why are twins so rare?
The standard answer is that giving birth to twins, and raising them, is difficult. Complications during pregnancy and delivery could kill both mother and children. In a classic 1990 paper, a bird specialist called David Anderson expanded on this idea. He suggested that twins are the result of an evolutionary bet-hedging strategy gone wrong.
Anderson knew that birds will commonly lay several eggs as insurance, to make sure that they get at least one strong chick. After hatching, the parents (or the strongest chick) will kill the weaker babies. In humans, a similar (but less brutal) competition happens in the womb. Mothers will often produce several eggs, with the others acting as insurance in case the first doesn’t make it. If two eggs are fertilised, one embryo is often lost – this is why many more twins are conceived than are born, the so-called “vanishing twin” effect. If this process fails, you get twins. That’s a problem, because having twins exacts a physical toll upon the mother.
According to this fairly bleak view, mums with twins have taken an evolutionary gamble that has backfired. But Shannen Robson and Ken Smith don’t agree. In a new study, the two scientists from the University of Utah have painted the birth of twins in a more positive light. To them, the very fact that some mothers can bring twins to term is a sign that they are strong and fit. The presence of twins singles out mums who can bear the extra cost of having twins.
Robson and Smith used the Utah Population Database, which collects the family trees of people who migrated to Utah in the early 1800s and their descendants. It’s one of the most comprehensive sets of family records in the world, and includes data on over 1.6 million people, right up to the 1970s. From these records, the duo pulled out 4,603 women, all of whom lived in the 19th century, survived till at least the age of 50, and had twins. They compared these women to 54,183 mums of similar characteristics who never had twins.
The duo found that women who gave birth to twins “outperformed” their peers who only ever had one child at a time. On average, they lived longer after menopause. They gave birth more frequently, and over the course of their lives, they raised two more children. They had children later on into the lives, and over a longer part of it. These advantages held even after Robson and Smith adjusted their figures to account for things like the women’s age, when they gave birth to their first child, whether their husbands or children died, and their religious affiliations.
The Utah records contradict the idea that twins harm the health of the mother. Instead, it was quite the opposite. The twin-bearers lived longer and raised more children. They even tended to have more singletons than other mums, which means that even excluding their twins, they would still have ended up with larger families.
This isn’t always the case. In 1998, Virpi Lummaa used historical records from Finland to show that mothers did better if they had twins on islands where food was reliable and abundant. However, on the mainland, where crop failures and famines were more common, women who stuck to singletons fared better. This suggests that benefits and costs of having twins depend on one’s environment.
Nonetheless, other scientists have found similar trends to Robson and Smith in other places and times, from 18th-19th century Germany to 18th-20th century Norway to modern Gambia. Others have found that mothers of twins are taller and in better health than those of singletons. However, all of these studies were based on very small numbers of records, no more than 250. By contrast, Robson and Smith’s conclusions are based on over 18 times as many twin mothers.
This doesn’t mean that the act of having twins makes women more fertile or increases their lifespan. Rather, it means that women who are more fertile and long-lived are more likely to “successfully incur the cost of twins.” In this light, twinning isn’t the failed bet-hedging strategy proposed by Anderson. Instead, it’s an opportunistic strategy – an evolutionary ace card, played by mothers in good condition, when times are plentiful.
Reference: Robson & Smith. 2011. Twinning in humans: maternal heterogeneity in reproduction and survival. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2011.0573
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