My cousin recently sent me a link to a report that the nutritional content of milk provided to male and female offspring differs. Very interesting and definitely a “cool” result but to paraphrase Janet Mock, we are born as babies. On the first day of life, it seems to me that the needs of a baby have little to do with their sex. Puberty is far enough in the future that I just don’t see a great deal of selection pressure for different nutritional provisions to boy babies and girl babies in the early days. In addition, tailoring milk to the sex of an offspring really only works for moms that have one baby. Not so useful for rodents with their 5-15 pups. The more general principle that milk content can vary, perhaps according to internal state or environmental conditions, appears evolutionarily robust, meaning applicable to many mammalian species in many situations and not just to one-offspring-at-a-time moms.

My point in this post is, however, to talk about the importance of suckling to social development and adult eating behavior. Of course, milk is important for its nutritional content. No question. Milk is also important as a source of antibodies to babies whose immune systems are not firing on all pistons until about 6 months of age. Mammalian young require milk. If they don’t get milk, they die. And if the milk is of poor nutritional quality, the babies are likely to fail to thrive.

Now that we have paid due respect to the nutritional benefits of milk, let’s turn our attention to the less appreciated aspects of the suckling-nursing interaction. The young suckle and mom nurses and this interaction is about so much more than transferring milk. Consider the experience from the baby’s side: warm, skin contact coupled with sucking actions and yummy milk intake. All of these components – warmth, touch, sucking, and ingestion – make us feel good. A coincidence? I doubt it. I would say that this is nature’s way to show babies what happy and content should feel like and to motivate the babies to crave this feeling. As would be expected if suckling-nursing served more than nutritional purposes, young mammals often suckle without drawing milk. Human babies fall asleep at the breast. Rat pups latch on but don’t draw milk; this happens more and more frequently as the pups get older.

Young mammalian offspring excel at sucking. The young have to suck and they have to suck hard to get milk. They have special motor circuits (called central pattern generators) for sucking that are re-purposed to support other orofacial movements in adults. So adults don’t have the same brain circuitry and actually don’t have the ability to suck as hard as do babies. Parents know this; put your finger in a baby’s mouth and be prepared to be impressed. And sucking is all part of the baby’s warm, soft, yummy experience of suckling. Now does it make more sense that babies and kids suck their thumbs?

Consider the experience from the mom’s side. Once breastfeeding is working well – baby learns how to get a good latch on – many moms describe the experience in dreamy terms: happy, in love with the baby, relaxing, sleepy. Isn’t nature marvelous?

Finally let’s think about the interaction as a complex negotiation revolving around food. Towards the end of a suckling bout, when milk is less plentiful, the baby has to suck harder to get any milk. The baby may also be getting full signals from her/his belly at this point. So is it worth it to continue sucking? If the baby is no longer that hungry, maybe falling asleep is more compelling than continuing to suckle. Thus the suckling experience is a way for babies to learn how to pay attention to how they feel, how full they are, how hungry they are. The young mammals can learn to recognize satiety and hunger through breast-feeding.

How about bottle-feeding? Well it is far easier for the baby because not as much suction is required. And the supply of formula is typically unlimited. Consequently, studies have consistently found that bottle-fed babies ingest more than do breast-fed babies and weigh more as infants and young children. One can imagine that the lessons learned before weaning may influence how one eats as an adult. Overweight people are less influenced by signals of fullness (satiety) than are thinner people. So it is very possible that what we learn during our first year or so influences our later eating habits. This simple question has been difficult to address from an epidemiological point of view as the demographics of bottle-fed and breast-fed babies are significantly different to start with. But suffice it to say that these considerations probably played a role in NYC Mayor Michael Bloomberg’s campaign to urge women to breast-feed and to recommend that hospitals not give out free samples of formula to every new mother. As it is basically impossible to switch a baby from bottle-feeding to breast-feeding, “trying” formula on the day of birth is tantamount to a final decision to bottle-feed. The movement of hospitals away from providing free formula bags recognizes that using the bottle initially is indeed a point of no return.

I really am going to wrap this up but I can just hear you saying, “if breast-feeding is so great, then wouldn’t babies prefer it and if they prefer it, why wouldn’t a newborn switch from bottle to breast?” Well, first of all, learning to latch on to a nipple and suck milk from the breast is way harder than learning to get milk from a bottle. So the learning bit is really night and day in the two situations. The other answer is that babies will reject the bottle if they have learned to breast feed. In today’s world, many women work and have their baby bottle-fed while they are away from the home. Then when at home, these mothers breast feed their babies. In these situations, the babies may revolt if given a bottle when mom is around. The babies know what’s going on and they apparently want the whole good, warm experience if mom is there to give it to them. They weren’t born yesterday.

Question for you: As I was perusing my photo albums (yes that dates me), I noticed that girls were sucking their thumbs and boys were sucking their fingers. For all you parents out there, is this something that you have noticed or just the vagaries of a small sample?