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Breastfeeding (i.e., sucking the mammary gland to ingest milk) is a defining feature of mammalian infant care. Human mother-infant dyads are unique from other primates with regard to the difficulty engaging in this practice. Unlike most other mammals that breastfeed their young fairly automatically, primate mothers need support and teaching in order to achieve breastfeeding success. Humans are not the only primates that need to be taught how to breastfeed their offspring, yet humans require more learning than our closest primate relatives. However, the learning necessary for successful lactation in human mothers seems to be greater in comparison with other primates.
There are multiple potential explanations for this human difference in breastfeeding difficulty. A human neonate can use smell and reflexive motor movements to instinctually reach the mother’s breast after birth, but cannot do this successfully unless placed on the mother’s chest. This relative immaturity of motor development in comparison with other primates seems to be an evolutionary tradeoff for the benefits of an extended period of development. Human infants also have innate reflexes to gape their mouth open when something is near their mouth and to suck when something is in their mouth. However, successfully withdrawing enough milk from the breast requires proper positioning of the breast in the infant’s mouth (i.e., the nipple at the very back of the mouth), which is difficult for human infants without direct help from the mother. If the mother lacks knowledge of how to help her infant engage in this proper technique, the infant will not get enough milk.
Even with proper education and support, many human mothers cease breastfeeding early or choose not to breastfeed their infant for a variety of social, psychological, and biological complications. Another human-unique complication is sucking technique. Though other mammals use negative suction to extract milk from the breast, human infants engage pressure on the areola – using an intro-oral vacuum in combination with a lowering of their tongue – to remove milk from the breast. This sucking technique is novel in the mammalian world, which likely contributes to the difficulty of human mother-infant dyads in effectively breastfeeding. Another human challenge to successful lactation is the psychological condition of perceived insufficient milk supply (PIM), which is when mothers falsely think they don’t have enough milk for their infant and is the explanation given by 35% of mothers who discontinue breastfeeding. This is not solely a Western phenomenon, as studies have reported equivalent prevalence of cases of PIM in Mexico, Hong Kong, Thailand, and Turkey. More broadly, social support (i.e., from partners, parents, friends, and communities) is also critical to increasing the likelihood of breastfeeding success. However, this is most problematic in the Western world, where the availability and convenience of infant formula provides a simple and widely-accepted alternative to making the effort to learn proper breastfeeding techniques.
Lactation was evolutionarily beneficial by decreasing risk of infant mortality while reinforcing altriciality and the mother-infant bond, so it is unclear why humans evolved to have such a challenging time breastfeeding offspring. One theory is that the learning required to achieve breastfeeding success was a tradeoff between having a very successful innate sucking mechanism in infancy versus having a more flexible, adaptive brain. Breastfeeding was also clearly related to the increased importance of social bonds in human history, as social learning and support from other mothers seems to have been necessary for infant survival via breastfeeding competency. Another potential evolutionary mechanism leading to the unexplained difficulty of human lactation is sexual selection pressure. Human breasts are significantly larger than those of other primates, and they serve as a cue to fertility and sexual desirability. Human males reliably look longest at a woman’s breasts when viewing a nude woman. The pressure from sexual selection may have favored larger breasts despite their inefficiency for nursing. In modern humans, breast size and breastfeeding success are not related, as breast size is attributed to fat pads rather than mammary gland branching.
Humans differ from our closest primate relatives with regard to the degree of learning that must occur in order for breastfeeding to be successful. Even with proper education and support, many human mothers cease breastfeeding early or choose not to breastfeed their infant for a variety of social, psychological, and biological complications.
There are multiple social and biological implications of breastfeeding difficulty in modern humans. Given that humans have co-evolved with the bacteria that colonizes the gut microbiome during vaginal birth and extended nursing, the prevalence of formula feeding over exclusive breastfeeding in many human societies has negatively impacted the healthy establishment of the infant gut microbiome. Breastfeeding for at least the first year of life keeps infants’ gut microbiome immature, rather than forcing an early onset of maturation. Breastfeeding is associated with greater concentration of sialic acid in the developing infant’s brain. Oxytocin production due to nipple stimulation has profound effects on the mother, and lactation has been shown to increase maternal sensitivity to infant cues. Breastfeeding is also associated with longer birth intervals. Lactation was also evolutionarily beneficial with regard to the reinforcement of infant altriciality and the mother-infant bond. Infants who were nursed were more dependent on adult care, allowing them to retain neotenous features and retain their altriciality for longer. In humans, the uniquely long period of altriciality is hypothesized to have implications for many human-specific capacities, including our advanced social cognition, complex social systems, and increased parental investment and parent-infant bonding.
Humans are not the only primates that need to be taught how to breastfeed their offspring, yet humans seem to require much more learning than our closest primate relatives. Among captive great apes (gorillas, chimpanzees, bonobos, and orangutans), one study reported that 18% of infant deaths (or near deaths) were attributed to lack of successful breastfeeding. For these apes, being around other ape mothers and being able to observe mothering behavior was highly correlated with lactation success. Learning was also demonstrated to be an important factor in cases where humans had to intervene to help with nursing success (i.e., placing the infant on an orangutan mother’s chest to be able to reach the nipple), and the mother learned the behavior on her own after a few successive interventions, leading to her eventually achieving breastfeeding success on her own.
This is a universal feature of the human species, though may vary in degree of difficulty among different populations.
The rate of breastfeeding initiation in the United States is 64%, and this drops off by about 25% every three months. This is in contrast to traditional cultures that maintain a breastfeeding rate of about 98% throughout the first six months of infancy. This Western difference demonstrates the importance of learning, given that mothers in traditional cultures who have been exposed to breastfeeding their entire life have less difficulty initiating and maintaining breastfeeding. This is in contrast to the West – where historically women have avoided breastfeeding by using wet nurses, bottle feeding, or formula feeding – lack the knowledge of proper breastfeeding techniques due to low exposure to breastfeeding and decreased availability of learning opportunities.
The extreme altriciality of human infants in comparison with great apes means that nursing is relatively impossible without the help of the mother. Though a neonate can use smell and reflexive motor movements to instinctually reach the breast if placed on the mother’s chest while she is laying down, this is impossible if the mother is not in the right position or if the infant is not placed on her chest. In addition, once reaching the breast, the infant requires adult help to properly insert the nipple far enough into the mouth for successful nursing. This extreme immaturity of motor development in comparison with other mammals seems to be an evolutionary tradeoff for the benefits of an extended period of development.
Given the many benefits of breastfeeding, it is unclear why humans evolved to have such a challenging time breastfeeding offspring. One theory suggests that the learning required to achieve breastfeeding success was a tradeoff between having a very successful innate sucking mechanism in infancy versus having a more flexible, adaptive brain. Breastfeeding was also clearly related to the increased importance of social bonds in human history, as social learning and support from other mothers seems to have been necessary for infant survival via breastfeeding competency.
Another potential evolutionary mechanism leading to the unexplained difficulty of human lactation is sexual selection pressure. Human breasts are significantly larger than those of other primates, and they serve as a clear cue to fertility and sexual desirability. Human males reliably look longest at a woman’s breasts when viewing a nude woman. The pressure from sexual selection may have favored larger breasts despite their relative inefficiency for nursing. In modern humans, however, breast size and breastfeeding success do not seem to be correlated.
Difficulty breastfeeding, in combination with modern marketing pressure of buying and using infant formula, has created a culture in Western society that does not facilitate knowledge sharing for successful breastfeeding. This recent change in infant feeding practices in WEIRD cultures has changed every aspect of human development, including social bonding processes, maturation of the gut microbiome, and patterns of gene expression in the brain of modern humans
Outside of other primates, there is no evidence of occurrence of breastfeeding difficulty in other mammals who nurse their infants. In fact, most mammalian young (e.g., kangaroos, pigs) are far more proactive during the latching and sucking process, requiring little or no help from the mother to instinctually reach the nipple and successfully begin suckling.
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