Why Breast-Fed Babies Cry More
“Breast-fed babies cry more!” is currently rocketing around the news and compelled my return from holiday hiatus. Lauzon-Guillain and colleagues reported in PLoS One this week that formula-fed babies cry less, sleep better, and smile more at three months of age than do breast-fed babies. The study was quite well-done, the only limitations to it were that mothers subjectively rated infant temperament (rather than having an objective technician do temperament ratings) and that infant growth or body mass were not included as covariates. But these are minor points and likely would only have strengthened their results, not changed them.
But why would formula-fed babies cry less and breast-fed babies cry more?
Just as the goat goddess Amalthea nursed Zeus with an inexhaustible supply of milk, formula is instant and plentiful, and overfed babies have little need to signal their non-existent hunger (indeed they are more likely to develop obesity). In contrast, the synthesis of milk by the mammary gland is constrained by real-time physiology. So infants get hungry. And then cry.
As we all know, a baby crying is an excruciating sound. Why would natural selection produce such an unpleasant adaptation? Noxious adaptations are usually to get predators away from you (e.g. skunk spray), but this one is supposed to trigger your mother to cuddle you close and shower you with nourishment. Why don’t infants signal hunger quietly with spirit fingers or jazz hands? The answer is found, as is often the case, in evolutionary theory: specifically Hamilton’s rule, parent-offspring conflict, and costly signaling.
Everything in milk- i.e. fats, sugars, protein, iron, immunoglobulins, vitamin A- are taken from mom’s body for the infant. For example, during lactation mothers mobilize calcium from their skeleton which in the short-term significantly reduces their bone mineral density (Prentice 2000). In evolutionary terms we would say that this is costly- it has the potential to reduce survival or limit reproduction in the future. But those costs are balanced against the benefits- insuring the survival, growth, and development of the infant.
But the benefits are discounted by genetic relatedness (known as Hamilton’s Rule). Infants get 50% of their genetic material from dad and 50% from mom. This means that an infant is 100% related to itself, but only half as much related to mom. And moms are equally related to all of their infants dividing investment among them over time. This is the crux of parent-offspring conflict, babies “want” moms to give twice as much as moms “want” to give. So babies signal that they are soooooo hungry to compel mom into giving more milk.
But why should mom believe that? Talk is cheap and her milk is expensive (replacing skeletal calcium doesn’t happen overnight!). So the signal can’t just be cheap talk. Infant crying takes a lot of energy- the behavior is metabolically costly- suggesting that the infant is honestly signaling hunger. For an infant that is already full there is no benefit to crying* but for an infant that needs milk, the benefits are great so the cost is worth paying. In other words the infant is SOOOOO HUNGRY!!!
This is of course a simplified presentation of a very complex system. For example, infant crying may also have been selected because it is easily and unambiguously detected. Knowing the exact evolutionary pathway of each behavior can be very difficult.
The take home message is that parent-offspring conflict and signaling theory suggest that infant crying may at times be honest and may at times be a manipulation, but that unlike bottle-fed infants that can be frequently fed to satiety, breast-fed infants are more likely to cry because they have a more dynamic behavioral & physiological back and forth with their mothers. One thing worth noting is that women often begin to supplement with formula because they think that the milk they produce is not enough, that the baby is always so hungry. But that is only possibly the case. Instead, the most reliable indicator of whether or not an infant is getting enough nourishment is growth trajectory.
Lauzon-Guillain and colleagues hope these results better prepare mothers for the challenges of breast-feeding. Indeed they cite the signaling literature in their paper (but not parent-offspring conflict theory) saying that dynamic communication between the mother and infant is natural and expected. Breast-feeding advocates in the US, however, fear that these data will scare women away from breast-feeding, but accurate contextualization of the research should prevent that.
Of course accurate contextualization of the research is contingent of an appreciation of natural selection and evolution. As my esteemed colleague Bruce German is fond of pointing out “Milk is the only biomaterial that has evolved under the Darwinian selective pressure for the specific and sole purpose of nourishing growing mammals.”
*No benefit to crying unless you’re chilly or sick or have a dirty diaper or need a snuggle or you just happen to feel like crying.
Blandine de Lauzon-Guillain, Katrien Wijndaele, Matthew Clark, Carlo L. Acerini, Ieuan A. Hughes, David B. Dunger, Jonathan C. Wells, Ken K. Ong. 2012. Breastfeeding and Infant Temperament at Age Three Months. PLoS One. 7(1): e29326. doi:10.1371/journal.pone.0029326
Prentice A. 2000. Maternal calcium metabolism and bone mineral status. Am J Clin Nutr 71:1312S- 1316S.
But why would formula-fed babies cry less and breast-fed babies cry more?
Just as the goat goddess Amalthea nursed Zeus with an inexhaustible supply of milk, formula is instant and plentiful, and overfed babies have little need to signal their non-existent hunger (indeed they are more likely to develop obesity). In contrast, the synthesis of milk by the mammary gland is constrained by real-time physiology. So infants get hungry. And then cry.
As we all know, a baby crying is an excruciating sound. Why would natural selection produce such an unpleasant adaptation? Noxious adaptations are usually to get predators away from you (e.g. skunk spray), but this one is supposed to trigger your mother to cuddle you close and shower you with nourishment. Why don’t infants signal hunger quietly with spirit fingers or jazz hands? The answer is found, as is often the case, in evolutionary theory: specifically Hamilton’s rule, parent-offspring conflict, and costly signaling.
Everything in milk- i.e. fats, sugars, protein, iron, immunoglobulins, vitamin A- are taken from mom’s body for the infant. For example, during lactation mothers mobilize calcium from their skeleton which in the short-term significantly reduces their bone mineral density (Prentice 2000). In evolutionary terms we would say that this is costly- it has the potential to reduce survival or limit reproduction in the future. But those costs are balanced against the benefits- insuring the survival, growth, and development of the infant.
But the benefits are discounted by genetic relatedness (known as Hamilton’s Rule). Infants get 50% of their genetic material from dad and 50% from mom. This means that an infant is 100% related to itself, but only half as much related to mom. And moms are equally related to all of their infants dividing investment among them over time. This is the crux of parent-offspring conflict, babies “want” moms to give twice as much as moms “want” to give. So babies signal that they are soooooo hungry to compel mom into giving more milk.
But why should mom believe that? Talk is cheap and her milk is expensive (replacing skeletal calcium doesn’t happen overnight!). So the signal can’t just be cheap talk. Infant crying takes a lot of energy- the behavior is metabolically costly- suggesting that the infant is honestly signaling hunger. For an infant that is already full there is no benefit to crying* but for an infant that needs milk, the benefits are great so the cost is worth paying. In other words the infant is SOOOOO HUNGRY!!!
creepy vintage card
The take home message is that parent-offspring conflict and signaling theory suggest that infant crying may at times be honest and may at times be a manipulation, but that unlike bottle-fed infants that can be frequently fed to satiety, breast-fed infants are more likely to cry because they have a more dynamic behavioral & physiological back and forth with their mothers. One thing worth noting is that women often begin to supplement with formula because they think that the milk they produce is not enough, that the baby is always so hungry. But that is only possibly the case. Instead, the most reliable indicator of whether or not an infant is getting enough nourishment is growth trajectory.
Lauzon-Guillain and colleagues hope these results better prepare mothers for the challenges of breast-feeding. Indeed they cite the signaling literature in their paper (but not parent-offspring conflict theory) saying that dynamic communication between the mother and infant is natural and expected. Breast-feeding advocates in the US, however, fear that these data will scare women away from breast-feeding, but accurate contextualization of the research should prevent that.
Of course accurate contextualization of the research is contingent of an appreciation of natural selection and evolution. As my esteemed colleague Bruce German is fond of pointing out “Milk is the only biomaterial that has evolved under the Darwinian selective pressure for the specific and sole purpose of nourishing growing mammals.”
Blandine de Lauzon-Guillain, Katrien Wijndaele, Matthew Clark, Carlo L. Acerini, Ieuan A. Hughes, David B. Dunger, Jonathan C. Wells, Ken K. Ong. 2012. Breastfeeding and Infant Temperament at Age Three Months. PLoS One. 7(1): e29326. doi:10.1371/journal.pone.0029326
Prentice A. 2000. Maternal calcium metabolism and bone mineral status. Am J Clin Nutr 71:1312S- 1316S.
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