Written by: Lauren Milligan Newmark, Ph.D. | Issue # 115 | 2023
- In mammals, reproductive experience is associated with improvements in lactation performance.
- A new study on rhesus macaque monkeys found that firstborn daughters had slower growth as juveniles, lower body mass throughout their reproductive career, and produced milk with less available energy at peak lactation compared with laterborn daughters.
- These findings are some of the first to demonstrate that early life conditions, such as reduced maternal investment via lactation, can affect milk synthesis in adulthood.
Being a first-time mother is challenging. Without prior experience, navigating the baby’s sleep and feeding schedules, mastering the swaddle, and knowing when to introduce solid foods can be daunting tasks. But it isn’t just maternal behaviors that benefit from experience—reproductive experience also is associated with improvements in milk production. Across mammals, mothers with more than one pregnancy (multipara) are more effective at transferring milk to their offspring than first-time mothers (primipara) .
This difference in lactation performance can be explained, in part, by changes in the mammary gland after the first pregnancy. For example, in mice , the mammary gland had a stronger response to hormones produced during second pregnancies compared with the first pregnancy . And in grey seals , mammary glands of multiparous mothers had a greater capacity for secreting and storing milk compared with mammary glands of primiparous mothers.
Primiparous mothers can also differ from multiparous mothers in their ability to support the energetic cost of lactation. In rhesus macaque monkeys, mothers often had their first offspring while they were still growing. This meant primiparous macaques had to meet the caloric requirements of body growth at the same time as meeting the energetic costs associated with pregnancy and lactation. And because they were still growing, primiparous macaque mothers had fewer fat stores than multiparous mothers to support those additional costs. With higher energetic costs and fewer stored calories, it isn’t surprising that primiparous macaque mothers were found to produce milk for their daughters that was lower in energy content than milk from multiparous macaque mothers with daughters [4, 5].
Mammal mothers may love all their offspring equally but could be physiologically constrained in their ability to invest equally through milk production. But what this reduced investment might mean for firstborn daughters is not well understood. Nutrition during growth and development is known to affect life history traits such as adult body mass and age at first reproduction. Milk is a very specific type of nutrition, containing not only macro- and micronutrients to support infant growth and development but also hormones and bioactive peptides that play a role in organizing metabolic, reproductive, neuroendocrine, and biobehavioral systems during growth and development . In dairy cows, daughters of primiparous dams produced significantly less milk when they were first-time mothers than did daughters of multiparous dams . Could deficits in milk composition or milk yield experienced by other mammals during a firstborn daughter’s infancy shape her own capacity to synthesize milk throughout her reproductive career?
This intriguing research question was investigated in a new study  on a well-studied population of captive rhesus macaques housed at the California National Primate Research Center in Davis, CA. Researchers Pittet and Hinde tested the effect of birth order on milk characteristics, including composition and yield, as well as early life growth rates, age at first reproduction, body mass of offspring, and body mass as a lactating mother, among firstborn (n = 72) and laterborn daughters (n = 202).
Being the daughter of a first-time mother did indeed leave a signature on growth and lactation performance in rhesus macaques. Firstborn daughters grew more quickly during infancy (4 months–1 year of age) than laterborn daughters, but this trend was reversed during the juvenile period (1–2 years of age). By 2 years old, laterborn daughters were approximately 100 grams heavier than firstborn daughters. These differences in body size persisted through adulthood and laterborns were heavier than firstborns throughout their reproductive careers.
Having more body mass meant more energetic resources for milk production. At peak lactation, higher body mass was associated with higher milk yield . As primipara, firstborn daughters produced a similar milk composition to laterborns but made approximately 23% less milk. Milk output improved with subsequent pregnancies, but firstborns still produced 5% less milk at peak lactation, and their milk was on average 0.67% lower in fat concentration compared with laterborns .
These small differences in yield and fat content resulted in a large difference in available milk energy (AME), the product of milk energy density (kilocalories/gram) and milk yield (grams). On average, AME from firstborns was 16% lower than AME from laterborns. Importantly, this difference was only partly explained by the larger body mass of laterborns. After controlling for maternal mass in the statistical models, the study found firstborns still produced approximately 10% lower AME than laterborns .
Primiparous mothers transferred fewer calories and essential nutrients for somatic maintenance, tissue development, and behavioral activity to firstborn daughters . This “meager milk” not only resulted in a smaller adult body mass for firstborn daughters but could also have influenced their mammary gland morphology, how they used the nutrients they consumed (e.g., are they stored or used immediately?) or even cellular functions (e.g., glucose metabolism). Together, these factors could constrain a firstborn’s ability to match laterborns in AME.
To understand the connection between infant milk intake and adult milk production, the study authors offer a brilliant analogy to the construction of a building . The building itself is the infant’s phenotype—traits influenced by genes and their environment, such as body size, hormone levels, age at first reproduction, and even milk production. The construction materials that help build this house include nutrients from the mother’s milk. But the overall design of the building is also influenced by bioactive signals in milk, such as growth hormones . Maybe the genetic blueprint called for a second story, but the construction materials lacked the necessary building blocks or were too costly to manufacture, so a tiny home was built instead.
It is not yet known how reduced AME from firstborns influenced their daughter’s own milk production (the granddaughters of the current study’s primiparous mothers). But the researchers did find, somewhat surprisingly considering the difference in AME, that infants from firstborns and laterborns were similar in body mass at peak lactation. There’s a long-term cost to cutting corners with the framing and foundation of a building. Pittet and Hinde suggest that infants of firstborns could be devoting a higher proportion of milk energy to growth than infants of laterborns, but they are doing so at a cost. Among this population of rhesus macaques, lower AME is associated with a less confident temperament, suggesting a lack of energy available for play or exploratory activities . Continuing to follow these macaque infants throughout growth and development, including when they become mothers themselves, will help elucidate the intergenerational impacts of constrained milk production among primipara. And ultimately, this type of comparative research can help us understood potential predictors for poor lactation performance among human mothers. Pittet and Hinde rightly emphasize that there is a lack of support for women who experience challenges with breastfeeding, such as low milk supply, while at the same time there is a stigma directed toward mothers who choose to feed their infants formula. Identifying potential intergenerational effects limiting a woman’s capacity for milk production can hopefully limit these dismissive attitudes and pave the way for interventions during infancy to improve lactation outcomes across generations.
1. Pittet F, Hinde K. Meager milk: Lasting consequences for adult daughters of primiparous mothers among thesus macaques (Macaca mulatta). Integrative and Comparative Biology. 2023 May 11: icad022.
2. Dos Santos CO, Dolzhenko E, Hodges E, Smith AD, Hannon GJ. An epigenetic memory of pregnancy in the mouse mammary gland. Cell Reports. 2015 May 19;11(7): 1102-1109.
3. Lang SL, Iverson SJ, Bowen WD. The influence of reproductive experience on milk energy output and lactation performance in the grey seal (Halichoerus grypus). PloS one. 2011 May 11;6(5): e19487.
4. Hinde K. First-time macaque mothers bias milk composition in favor of sons. Current Biology. 2007 Nov 20;17(22):R958-9.
5. Hinde K. Richer milk for sons but more milk for daughters: Sex‐biased investment during lactation varies with maternal life history in rhesus macaques. American Journal of Human Biology: The Official Journal of the Human Biology Association. 2009 Jul;21(4): 512-519.
6. Poczynek M, Nogueira LD, Carrari IF, Carneiro JH, Almeida RD. Associations of body condition score at calving, parity, and calving season on the performance of dairy cows and their offspring. Animals. 2023 Feb 8;13(4): 596.