Did you know that mothers make better milk for sons than daughters?  Or, that first-born daughters get the lowest density milk than sons or later born daughters? Well, neither did I until I attended a talk by Dr. Katie Hinde, Assistant Professor at Harvard University and author of one of my favorite blogs, Mammals Suck…Milk! Dr. Hinde discussed her research on mother’s milk and its impact on the behavioral, psychobiological and physiological development in infants. Her talk was full of incredible surprises. Keep reading and you will be amazed too!

Dr. Hinde studies how complex social mammals interact and how those interactions are shaped by early development. In order to become a proficient adult mammal, you must first survive infancy. During this period, infants are entirely reliant on mother’s milk for the nutrition needed to sustain behavioral activity. As Dr. Hinde pointed out, mother’s milk includes all the energy a developing neonate needs; vitamins, minerals (calcium), hormones (cortisol), critical immuno factors to protect the infant’s developing immune system, and water.

Interestingly, milk composition is incredibly unique and varies among species and individuals due to a variety of factors. For instance, environment is an important factor. An infant born into an arid environment needs more water and so the mother’s milk has a high level of water to keep the infant hydrated. Another factor is diet. An herbivore will produce a different type of milk than a carnivore. How the species develops is a factor. Are they a fast or slow growing species? Do they interact with mothers and nurse often, or do they have long intervals between nursing? All of these factors together influence the amount of energy the mother’s milk must supply to the infant.

Dr. Hinde mainly works with Rhesus Macaque monkeys housed at the California National Primate Research Center (CNPRC). The center, one of eight national primate reserves, houses more than 5,000 monkeys and has 24 enclosures on 300 acres. With such a large population and multitude of enclosures, researchers can study and compare different social groups and their dynamics. Even better, the monkeys live in the exact same way they would in the wild and they display natural behaviors, not the stereotypical behaviors of animals placed in captivity.

Dr. Hinde says the Rhesus Macaque is actually wonderful to study because they

Rhesus Macaque at city gates

are the most common non-human primate model. Their genome has been mapped and we know a lot about their genetics. They are also widely distributed throughout Asia and can thrive in a variety of conditions including urban environments. More importantly for Dr. Hinde’s research, Rhesus Macaques initiate reproduction early in their life. Their first pregnancy takes place in adolescence and continues into their prime and aged stages.

The three stages of motherhood also vary in how they nurse. Young mothers nurse more frequently and for shorter durations. Researchers believed this was because young mothers were less able than mature monkeys to synthesize milk, but Dr. Hinde’s research proved otherwise. After analyzing the milk composition of the three stages, she found that the young mother’s milk is the identical composition as the older females. The only difference was in the amount of milk each stage produced.

Currently, Dr. Hinde has data on the milk composition of 36 primate species, which shows that among these primates, the milk composition is fairly consistent. Dr. Hinde also noted that humans do not show significantly different properties in their milk in comparison to other primates. This surprised researchers because previous research hypothesized that human neonatal brain growth was a result of mother’s milk energy. This proved it was not the case, opening up the question again of what fuels a human infant’s expanding brain.

The Rhesus macaques are seasonal breeders. Their breeding season begins in the fall, they are pregnant for 5 ½ months, they have a birth season in the spring, and then a period of lactation. For the first few weeks of life, the infants are clinging to their moms and suckling milk. At about one month of age, the infants begin to engage in and explore their environment. Peak lactation is at three-four months of age, where the infant experiences maximal growth rates; spending more time away from their mother, but still relying on mother’s milk to fuel their energy needs. The last stage is a weaning process where the mother begins to restrict access to the nipple. And, with less sucking, lactose synthesis declines and there is less milk production.

So, what does all this mean for the different stages of the mother in reproduction and for the infants being raised by these mothers in different stages? For one, young mothers produce significantly less milk than prime and aged mothers. By peak lactation though, the young mothers have caught up, so the belief is that the milk is targeting this critical period of infant growth.

Two, mothers tend to make better milk for sons than for daughters. The milk is higher in fat and protein so the energy density is significantly higher. Daughter’s milk is of lower quality, but there is more of it. According to Dr. Hinde, this is not due to a gender bias by the mother. Her group is currently running a behavioral investigation to test their suspicions that what is happening is that mothers are allowing daughters to nurse more often and sons to spend more time out interacting with other individuals. It’s also possible that, since mothers and daughters form life long bonds, it could be a way to strengthen this bond. There are very important differences in the social behavior of young males and young females, and Dr. Hinde’s team is working on that data right now, so stay tuned.

Three, not only does birth order matter, but the sex of the first offspring matters as well. First born daughters are the most disadvantaged. At peak lactation, mothers make lower density milk for first-born daughters than they do for a first-born son and for later born daughters. And, the daughters weigh significantly less than all the other groups.  I asked Dr. Hinde if this phenomenon extended to humans. She directed me to a 2010 study in the American Journal of Human Biology, which found that in humans, sons get higher fat in milk than daughters. The study did not look at birth order, but 75% of their samples were from first time mothers so the data is very similar to Dr. Hinde’s.

And four, mother’s milk feeds not only the infant, but also the infant’s gut. Animals have 100 trillion microorganisms living in their bodies and these microbial communities are established during infancy following birth. Mother’s milk has known properties that directly contribute to establishing this microbiome. Mother’s milk contains oligosaccharides. Bifidobacteria is the most common bacteria in a healthy infant’s intestinal tract, and it prefers to eat the oligosaccharides in mother’s milk.

One of the most fascinating pieces of information Dr. Hinde revealed is the current Biobehavioral Assessment Program (BBA), a study taking place at the CNPRC under the direction of Dr. John Capitanio. The study, which is assessing over 3,000 infant monkeys, began in 2000 and is the largest behavioral study of any primate to date. The BBA is a 25-hour period in which a 3-½ month old infant is removed from its social group and put through a battery of behavioral, psychological and physiological challenges to see how they respond. The two measures researchers looks at are what do the infants do in a novel situation when separated from their mother and social group. And, what is their temperament at the end of this 25-hour period, which is noted by using 16 trait adjectives (i.e. depressed, aggressive, timid, playful, curious, confident, etc.). What they found is that infants getting a combination of higher energy density milk and lots of it during the first month of life were more behaviorally active and coped better during the novel situations. The more milk the mothers made for the infant, the more active the infant became even when separated from the mother. So the milk the infant gets during the very early development period (the first month) influences later behavioral activities regardless of the milk the infants receive at later stages.

Dr. Hinde ended her talk by reminding the audience that mother’s milk is not one size fits all. It actually contains highly personalized nutrition and immune properties. Researchers do not know everything that is in milk, or why there are variations among individuals, or how they impact infant development. But, they continue to search for answers and with Dr. Katie Hinde in the field, I am sure they will find them soon.

Rhesus Macaque Mother and Child