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Why Have Twins?

Mothers more likely to have twins have heavier, healthier non-twin babies, possibly explaining why twinning evolved.

By | August 11, 2011

FLICKR, JOELLE INGE-MESSERSCHMIDT

New research shows that a genetic propensity for multiples could result in heavier single babies, which tend to be healthier and more likely to survive, according to a study published Tuesday (August 9) in Biology Letters, possibly explaining why some women have twins, despite the inherent dangers involved.

Bearing two babies at once is risky business—they are often smaller and weaker than single babies, and there’s a greater chance that either the mother or the babies, or both, will not survive. So the fact that some people have twins has long stumped scientists. To see if he could find an answer, evolutionary biologist Ian Rickard of the University of Sheffield in the United Kingdom mined a unique data set of 50 years’ worth of medical data on people in Gambia, collected by the UK Medical Research Council. Examining the birth weights of nearly 2,000 single babies, Rickard and his colleagues found that those born to mothers who had also had twins were 226 grams heavier than mothers who never gave birth to multiples. Thus, bearing twins on occasion might also afford mothers to have healthier single babies, suggesting that the benefits to the mothers might outweigh the costs of twinning.

Indeed, carrying twins is believed to improve blood flow to the uterus, which could benefit future babies by providing better access to nutrients, Rickard told ScienceNOW. But the study found that even single babies born before their younger twin siblings tended to be heavier, suggesting other mechanisms must be at play. The authors suggest that the answer may lie in a protein called IGF-1, which in addition to increasing the chance of twinning (by causing the ovaries to release multiple eggs), regulates fetal growth.

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Avatar of: Brian Hanley

Brian Hanley

Posts: 66

August 11, 2011

More babies in less time = more descendants. This is a very straightforward matter.

Even if 90% of all twins had one twin die before having children it still gives mothers of twins a clear evolutionary advantage. It doesn't take much. Only if twins lowered the number of descendants would twinning be expected to die out.

The explanation above describes a biological mechanism that improves the chances of twins surviving, thus raising the evolutionary advantage of twinning by improving the survival rate of twins to those mothers who have them. This mechanism of higher birth weight babies is not a driver of the evolution of twins.

Evolution would favor the appearance of the ability to provide more nourishment to the uterus in mothers where a mutation favoring twins happened to appear. This article has its understanding of evolution backwards. Evolution is a random process, and what we call "favoring" in evolution is mostly a lack of disfavoring of randomly appearing mutations. Twinning is one of those rare evolutionary traits that directly favors the spreading of those twinning genes because there are more copies of them. But mostly evolution is a negative process where the least deleted gene lines are "favored" by less removal.

Avatar of: micro

Anonymous

August 11, 2011

Another interpretation of the results is that women who tend to produce heavier babies are better able to produce twins which survive. 

In other words it's not that having twins makes one more likely to produce heavier babies.  It's that women who are more likely to have heavier babies (i.e. provide more nutrients in the womb) are more likely to produce surviving twins and thus evolution gives them that opportunity.  Additionally, twins may miscarry in women who do not produce heavy babies, and these would not be counted.

Avatar of:

Posts: 0

August 11, 2011

More babies in less time = more descendants. This is a very straightforward matter.

Even if 90% of all twins had one twin die before having children it still gives mothers of twins a clear evolutionary advantage. It doesn't take much. Only if twins lowered the number of descendants would twinning be expected to die out.

The explanation above describes a biological mechanism that improves the chances of twins surviving, thus raising the evolutionary advantage of twinning by improving the survival rate of twins to those mothers who have them. This mechanism of higher birth weight babies is not a driver of the evolution of twins.

Evolution would favor the appearance of the ability to provide more nourishment to the uterus in mothers where a mutation favoring twins happened to appear. This article has its understanding of evolution backwards. Evolution is a random process, and what we call "favoring" in evolution is mostly a lack of disfavoring of randomly appearing mutations. Twinning is one of those rare evolutionary traits that directly favors the spreading of those twinning genes because there are more copies of them. But mostly evolution is a negative process where the least deleted gene lines are "favored" by less removal.

Avatar of:

Posts: 0

August 11, 2011

Another interpretation of the results is that women who tend to produce heavier babies are better able to produce twins which survive. 

In other words it's not that having twins makes one more likely to produce heavier babies.  It's that women who are more likely to have heavier babies (i.e. provide more nutrients in the womb) are more likely to produce surviving twins and thus evolution gives them that opportunity.  Additionally, twins may miscarry in women who do not produce heavy babies, and these would not be counted.

Avatar of:

Posts: 0

August 11, 2011

More babies in less time = more descendants. This is a very straightforward matter.

Even if 90% of all twins had one twin die before having children it still gives mothers of twins a clear evolutionary advantage. It doesn't take much. Only if twins lowered the number of descendants would twinning be expected to die out.

The explanation above describes a biological mechanism that improves the chances of twins surviving, thus raising the evolutionary advantage of twinning by improving the survival rate of twins to those mothers who have them. This mechanism of higher birth weight babies is not a driver of the evolution of twins.

Evolution would favor the appearance of the ability to provide more nourishment to the uterus in mothers where a mutation favoring twins happened to appear. This article has its understanding of evolution backwards. Evolution is a random process, and what we call "favoring" in evolution is mostly a lack of disfavoring of randomly appearing mutations. Twinning is one of those rare evolutionary traits that directly favors the spreading of those twinning genes because there are more copies of them. But mostly evolution is a negative process where the least deleted gene lines are "favored" by less removal.

Avatar of:

Posts: 0

August 11, 2011

Another interpretation of the results is that women who tend to produce heavier babies are better able to produce twins which survive. 

In other words it's not that having twins makes one more likely to produce heavier babies.  It's that women who are more likely to have heavier babies (i.e. provide more nutrients in the womb) are more likely to produce surviving twins and thus evolution gives them that opportunity.  Additionally, twins may miscarry in women who do not produce heavy babies, and these would not be counted.

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