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The Best Science Writing Online 2012
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Mother's milk may be the first food, but it is not created equal. In humans and other mammals, researchers have found that milk composition changes depending on the infant's gender and on whether conditions are good or bad. Understanding those differences can give scientists insights into human evolution.
Researchers at Michigan State University and other institutions found that among 72 mothers in rural Kenya, women with sons generally gave richer milk (2.8 percent fat compared with 0.6 percent for daughters). Poor women, however, favored daughters with creamier milk (2.6 versus 2.3 percent). These findings, published in the American Journal of Physical Anthropology in September, echo previous work that showed milk composition varying with infant gender in gray seals and red deer and with infant gender and the mother's condition in rhesus macaques. The new study also follows findings that affluent, well-nourished moms in Massachusetts produced more energy-dense milk for male infants.
Together the studies provide support for a 40-year-old theory in evolutionary biology. The Trivers-Willard hypothesis states that natural selection favors parental investment in daughters when times are hard and in sons when times are easy. The imbalance should be greatest in polygamous societies, in which men can father offspring with multiple wives, such as the Kenyan villages. In those societies, a son can grow to be a strong, popular male with many wives and children, or he can end up with neither. Well-off parents who can afford to invest in sons should do so because their gamble could give them many grandchildren. Conversely, poor parents should not heavily invest in sons because it is unlikely to pay off—their offspring start at the bottom of the socioeconomic ladder. For those families, daughters are a safer bet because as long as they survive to adulthood, they are likely to produce young.
The new study is “exciting and enthralling,” says Robert Trivers, an evolutionary biologist at Rutgers University and co-author of the hypothesis, who was not involved in the recent work. “It is a Trivers-Willard effect I wouldn't have the guts to predict.”
Even beyond fat and protein, other milk components might vary in humans, says Katie Hinde, an assistant professor in human evolutionary biology at Harvard University. She has found higher levels of cortisol, a hormone that regulates metabolism, in rhesus macaque milk for male infants. Her work shows that milk differences could change infant behavior and might affect growth and development. “Only half the story is what the mom's producing,” Hinde says. “The other [half] is how the infant uses the milk.” These findings could have implications for formula, which could be tweaked to optimize development for both boys and girls.
This article was originally published with the title His Milk, Her Milk.
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16 Comments
Add CommentI'm sorry, but no these findings should not have implications for formula. What these finding should do is further prove that mother nature knows best and will produce the perfect milk for baby's needs (something we can never recreate). We should be concentrating on helping women to breastfeed and making donor milk, instead of formula, the norm for those that can't.
Reply | Report Abuse | Link to thisTotally agree with Gillis5683. COME ON! Fascinating article, fascinating implications and the author ends with a way for formula companies to further destroy the breastfeeding relationship? This definitely SHOULD NOT have implications for formula. It has none whatsoever. Human babies should drink human milk. If their own mothers are unable for whatever reason, there is plenty of donor milk available. I have donated for 12 months total -- 8 months with my 2nd child, and 4 months with my 3rd child. It is what babies DESERVE. I can't believe a publication as outstanding as Scientific American would allow such an uninformed, ridiculous statement to conclude an otherwise informative article. I am appalled.
Reply | Report Abuse | Link to thisYes, these findings could have implications for formula; as in get rid of the stuff and encourage mothers to breastfeed; don't allow formula anywhere near a newborn. This is a most fascinating article but you sure messed it up with that totally totally ridiculous final sentence.
Reply | Report Abuse | Link to thisThis article states that the social benefits of having males or females is the cause of the difference in nutrition of the milk. I'm not quite sure I follow the logic. Wouldn't it be more probable that in a time of hardship the mother doesn't have a long lifespan so the milk would favor girls since a replacement might be necessary for the mothering position? In a healthy environment the mother might be alive for a lot longer so a female replacement won't be needed as early, more benefits lies with having a male child.
Reply | Report Abuse | Link to thisI don't see mention of human trials; just human implications. This seems relatively benign and non invasive. What happens to the milk when a mother has offspring of each gender - a brother and a sister. Do babies of different genders prefer one breast over another?
Reply | Report Abuse | Link to this"This article states that the social benefits of having males or females is the cause of the difference in nutrition of the milk."
Reply | Report Abuse | Link to thisMy interpretation of the article was that the cause of the difference was reproductive benefits, not social ones. In good times, male children have the best chance of having lots of offspring, but in tough times, the odds favor the girls. The difference in milk content is a way for mom to optimize the chance that her investment will pay off with her genes being passed to a generation of grandkids.
@ gillis5683, lilyalayne & GraceD -
Reply | Report Abuse | Link to thisBreastfeeding should obviously be the first choice for infant nutrition. And donor milk would be preferable to formula, but not every mom that is unable to breastfeed has access to milk donors. For those who can't get donor milk, high-quality formula is ESSENTIAL. Therefore, any research that can make formula more like real breastmilk is a good thing.
"What happens to the milk when a mother has offspring of each gender - a brother and a sister."
Reply | Report Abuse | Link to thisI'm not a reproductive biologist, but my guess would be that mom would produce higher-fat milk in that case, regardless of socioeconomic conditions. In good times, the boy would get the designed boost & the girl would just be "along for the ride". In tough times, the roles would be reversed. It's also possible that some other mechanism could kick in & mom would simply devote more time to feeding the child with the best chance of providing grandkids.
Now I understand why I've had the urge to moo ever since I was a child.... Must have been all that cows milk I was fed as an infant.
Reply | Report Abuse | Link to thisActually, I wish I had be nursed. Wonder how it would have changed my life.... But not many women nursed back in those days.
Just an interesting thought: Female IQ's are on the rise and have closed the gap with male IQ's... I wonder if this could be due to the "formula" generation. Perhaps formula companies should NOT make 2 different types of formula. Maybe the differences in breast milk increase survival rates for the general population in poor countries but cause intelligence inequities between the sexes in richer countries where lack of food is not an issue for many people. Maybe females have been getting breast milk that is inferior for mental development.
Reply | Report Abuse | Link to thisI'm not against breast feeding. I breast fed one child for 3 1/2 years (the little "sucker" did not want to ween). But what if breast milk differences caused a disadvantage for girls? I would suggest that if this were the case perhaps it could be remedied by nutritional supplements not by forgoing breast feeding (which I firmly believe is the natural way a baby should be fed.).
Just some food for thought.
Really? Some consider this article a forum for debate of formula vs breast milk. My interpretation is this is looking at what makes breast milk what it is. Further, like any basic science finding, these data can be applied to make the "artifical" better. Who knows if formula will ever be better than the real thing? Who cares? The issue is what can be done to make it better than it is now.
Reply | Report Abuse | Link to thisDo the critics think that research should stop because it doesn't agree with their views?
I was wondering the same thing! I tandem breastfed my boy-girl twins for a year and not only did they nurse at the same time, but often would switch breasts halfway through the feed. I wonder how my body decided to respond to THAT with its milk content!
Reply | Report Abuse | Link to thisYou got that right. Bottle/formula fed children don't get as much closeness and have much weaker immune systems. This can have all kinds of mental and physical drawbacks.
Reply | Report Abuse | Link to thisQuestionable Milk
Reply | Report Abuse | Link to thisThe Scientific American (SA) article “His Milk, Her Milk” (Advances) by Marissa Fessendon in the December 2012 issue provides a popular account of an article published by Masako Fujita and colleagues in the American Journal of Physical Anthropology (AJPA). The Fujita article supports the prediction that mothers favor daughters in bad conditions and sons in good conditions, an intriguing idea based on sociobiology theory. However, the numbers in the SA and AJPA articles don’t add up.
The SA reports that rural Kenyan mothers produce milk with a 2.8 % fat content for sons and 0.6% for daughters. The SA article then reports that poor mothers give daughters milk with 2.6% fat versus 2.3% fat for sons. To maintain the average value of 0.6 % fat content for all daughters, assuming roughly equal number of rich and poor mothers, rich mothers would have to give their daughters milk with a negative fat content. There is clearly a mistake, and it is in the SA article; based on the AJPA article, the values of 2.8 % fat content for sons and 0.6% for daughters should be just for rich mothers (and not for all mothers).
The SA article implies that these values are the average fat content of the milk. However, as reported in the original article, these values are estimates based on a statistical model. The average values of fat content in each of these categories are not given in either article. Fujita and colleagues should have presented these numbers in their article but they did not. Without these numbers, the results cannot be accepted; the values in both articles are apparently only estimates based on a statistical model.
Also, common sense suggests that mothers cannot adjust the fat content of their milk by a factor of more than 4 (from 0.6% fat content for daughters to 2.8% fat content for sons in the case of rich mothers). This strange result is almost certainly an artifact of their model; to prove their point, we need to see the actual data.
In addition, the rich daughters would not survive on this low calorie diet of 0.6% fat content, which is almost like water. Again, this does not make sense. Part of the problem in understanding their results is that the AJPA authors were only measuring and reporting on the low-fat foremilk that comes out when a baby begins to feed, and not the subsequent hindmilk which constitutes the bulk of the milk supply and is higher in fat content. If the goal was to measure fat content, the authors should have measured just hindmilk, or hindmilk in addition to foremilk.
This example of mother’s milk from Kenya should be regarded as very preliminary, and more careful work needs to be done by the authors and others before this example can be accepted.
Richard B. Primack
Biology Department
Boston University
Boston, MA
Fact about girls. Must check http://www.stumbleupon.com/su/2JkINk/www.gay3ty.com/smaller-the-better/
Reply | Report Abuse | Link to thisi am wondering how is this new research when this thing has already been told about 1400 years ago in the holy book (quran) of muslims that "the share of a male child is twice the share of female child what they inherit from parents". where mother milk is also a natural inheritance to the child from the mother. there is also well known event in the early era of islam when an infant boy and girl from 2 mothers were mixed and there was a confusion to whome the baby boy/girl belongs to. so in the guideline of "Quran" the decision was made on the basis of milk from the 2 mothers. Quran is a rich source of knowledge and wisdom not only in social science but also in scientific knowledge.
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