Longer Pregnancy + Breast-Feeding = Bigger Brains, Longer Life

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Advice around breast-feeding can drive new mothers mad, but a new study suggests that the long pregnancies and lactation periods of our prehistoric mamas are responsible for the relatively big brains that differentiate humans from other animals.

By comparing 128 species of mammals, researchers at the University of Durham in England sought to answer this question: do large brains make species live longer by making them smarter in cheating death, or is the longer lifespan of big-brained animals simply the result of the fact that big brains require more care and time to grow?

“We already know that large-brained species develop slowly, mature later and have longer lifespans but what has not always been clear is why brains and life histories are related,” said Robert Barton, lead author of the research published in the Proceedings of the National Academy of Sciences, in a press release.

If the main issue is the time that brains take to develop, this would mean that lifespan should be connected with how long a mother remains pregnant and breast-feeds (which determines how big a baby’s brain grows). Pregnancy and breast-feeding are “expensive” — the more time you spend gestating and nursing just one baby, the less time you have to bear others. The “cost” of a large brain applies to the limits on having more offspring. That cost would be worth it only if it provided other benefits, like having one smart baby that survives and has more, compared with many babies that do not successfully reproduce.

On the other hand, if it’s simply that being smart just allows you to live longer, lifespan and adult brain size should be directly linked.

So what is the actual relationship? The study found that when you take into account the time a mother spends pregnant and breast-feeding, adult brain size is no longer correlated with lifespan. “Our findings suggest that the slow-down in life histories is directly related to the costs rather than the benefits of growing a large brain,” Barton said.

This doesn’t totally rule out the idea that smartness itself increases lifespan, but it does show that a mother’s care really matters in creating intelligence and longevity.

Across species, the study found, brain size relative to body size is closely associated with a mother’s “investment” in pregnancy and breast-feeding: the length of pregnancy governs brain size at birth, and how long a mother nurses determines how much the brain grows after birth.

For example, the fallow deer — which is about the same size physically as a human — is pregnant for just seven months and lactates for six. Its brain is six times smaller than a human brain, and it does not score well on IQ tests. In contrast, humans are pregnant for nine months and have historically breast-fed for three to five years.

Of course, other animals never invented formula — so this study can’t add anything to the contentious debate over the optimal amount of time to breast-feed or its specific benefits.

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