Allo-Mother’s Milk
Wait, what?
Yes, women otherthan the mother provide breast milk for babies. Throughout human history and across cultures there are numerous documented practices of allo-maternal nursing.
Within Islamic culture, there is the practice of “milk kinship.” This is a cultural construct of a familial relationship, analogous in many ways to “god-parents” among Catholics. In such instances infants are nursed by a woman not their mother, and consider her biological children “milk brothers” and “milk sisters.” In this way life-long social bonds are established and maintained throughout childhood, adolescence, and adulthood. And these relationships are in place in the event of the death of the biological parents (Parkes 2005).
Wet-nursing was a prevalent practice before the advent of commercial formula. Throughout the Renaissance and into the early 20thCentury, poor women were hired to nurse the infants of wealthy women. Physiologically the period of lactation, especially early and peak lactation, requires the mobilization of maternal body fat and skeletal minerals. When mothers are losing weight, ovarian function is suppressed and women do not experience a menstrual cycle (Valeggia & Ellison 2009). Once mothers recover from this “depletion” of their bodily stores, they can conceive. In this way inter-birth intervals are often correlated with the duration of exclusive breastfeeding (but not always).
By hiring wet-nurses, wealthy women were able to shorten inter-birth intervals and produce large families by forgoing the somatic costs of lactation. In contrast, the poor women would often neglect their own infants and bias nursing behavior toward paying customers. The mortality rate for the biological infants of wet nurses was estimated to be quite high. Disentangling infant mortality due to wet-nursing from the immuno-socio-politico-ecomonic context in which it occurred becomes damned tricky, especially when relying on historical records. Take home message; wet-nursing was contingent on wealth disparity and generally had bad outcomes for oh, let’s randomly say, these guys.
By hiring wet-nurses, wealthy women were able to shorten inter-birth intervals and produce large families by forgoing the somatic costs of lactation. In contrast, the poor women would often neglect their own infants and bias nursing behavior toward paying customers. The mortality rate for the biological infants of wet nurses was estimated to be quite high. Disentangling infant mortality due to wet-nursing from the immuno-socio-politico-ecomonic context in which it occurred becomes damned tricky, especially when relying on historical records. Take home message; wet-nursing was contingent on wealth disparity and generally had bad outcomes for oh, let’s randomly say, these guys.
Allo-maternal nursing is not just a cultural invention of humans, it has been observed in many other mammals (>70 at last count). A review by Roulin (2002) discussed the prevalence of the behavior and considered several evolutionary explanations. Identifying the functions of allo-maternal nursing is quite important. Synthesizing milk is costly- a repeated theme on Mammals Suck…Milk!- so why would natural selection favor mothers who nourished young not their own?
Here are the most compelling hypotheses, to my mind, in playlist form:
1. U Can’t Touch This
(aka milk stealing)
In species that give birth in large colonies, infants may be able to “steal” milk and nimbly evade females that would otherwise not nurse them. This is observed in elephant seal rookeries (Reiter et al. 1978). Weaned pups will travel the beach suckling from sun-basking females who are slow to notice that they are nursing some other kiddo. Upon the realization, females will bark and attempt to bite, but by then the pup has scampered away. Elephant seal pups that use milk stealing tactics can grow to twice the size of normal weanlings and are colloquially called “super-weaners.” From the female’s perspective, the small amount of milk stolen is likely less costly than the energy it would take to be super vigilant and punishing sooooo… we can infer that its cheaper from a selective perspective to just take the hit.
2. Nothing’s Gonna Stop Us Now
(aka misplaced maternal care)
The gestation and delivery of a mammalian baby triggers the release of hormones, coursing through a mother’s brain and body, sending an overwhelming signal to “love” & nourish the little bundle of joy. For example, oxytocin is an ancient mammalian hormone that is important for milk let down and the establishment of the mother-infant bond. There are numerous overlapping neurobiological, behavioral, and physiological systems for mothers to take care of infants, because infant survival among mammals requires maternal investment to some extent.
Next time you are at the airport, look at the number of rivets that hold the wing to the plane. Does any one rivet hold the wing on? No. All those rivets are there so that if one, or two, or five fail- the plane KEEPS FLYING.
Selection did the same thing with mutations that enhanced maternal effort- favoring redundancies and fail-safes. Selection is not effing around on this one. Because maternal effort is so critical for female reproductive success among mammals, there are a motherloadof adaptations for motherhood.
But sometimes the infant dies when hormones are still compelling the mother to bond (Thierry and Anderson 1986). In such circumstances some primate mothers will continue to nurture the dead corpse for hours, days, or even weeks. Alternatively, high-ranking females will occasionally kidnap the infant of a lower-ranking female. Sometimes these kidnapping events stick and the adoptive mother nurses the infant, sometimes the biological mother gets her infant back, and sometimes the infant gets sick, hurt, or dead. And every now and then there is a glitch in the matrix- females are too mothering and nurse other females’ babies like the junk lady in Labyrinth collects dolls. Such rare and less common events are when the dial of functional adaptations are turned all the way to “11.” Although these behavioral manifestations aren’t adaptive for those particular individuals, the underlying architecture from which they originate is adaptive.
3. Cooperation
(aka cooperative breeding)
Some species are characterized by cooperative breeding in which related or unrelated adults contribute to rearing infants of a dominant breeding pair. Contributions can include provisioning breeding females (and pups as they are weaned), baby-sitting, territorial defense, and in some species females will nurse the dominant female's babies. Among meerkats, pups will suckle from allo-mothers who have lost their litter or who have spontaneously started lactating (Scantlebury et al. 2002). This allo-nursing is unidirectional from subordinate females toward the pups of a dominant female. The benefit to the pup is clear, but for the allo-mother is less clear. The evolution of cooperative breeding social systems are complicated, but are often attributed to kin selection or reproductive queuing.
4. Tit for Tat
(aka reciprocal altruism)
Cross-nursing occurs when two females nurse each other’s infants. Among some species of mice, two females will share a nest and take turns going on foraging expeditions and staying behind and nursing all the pups at the nest. The females nurse their pups and the other pups equivalently. About half the time the two females are sisters, but these reciprocal arrangements are just as often between unrelated females. Moreover the arrangement can be stable across multiple birthing seasons (Weidt et al 2008). Such an arrangement likely involves the dual benefit to the pups of better protection from predators and shorter inter-nursing intervals (which may improve growth).
5. Rockin’ Pneumonia
(aka enhanced immune function)
Mother’s milk is not only nutritive, but is integral component to defending the infant against pathogens and entrains the infant’s developing immune system. Milk includes maternal antibodies, commensal bacteria, and special sugars for beneficial bacteria to consume (future post topic). Infants who suckle from multiple females may be boosting their immune system from the diverse exposures they get from allo-maternal milk.This may technically be a sub-category of reciprocal altruism or milk stealing because while the benefit to the infant is clear, the benefit for a female nursing a non-biological infant is less straight-forward without minimal costs or a tit-for-tat exchange.
Part I: Conclusions
Although allo-maternal nursing occurs in many mammalian species, most of our understanding of it comes from opportunistic studies and arm chair hypothesizing. There is so much that is not known about the advantages and disadvantages of drinking allo-mother’s milk (Milligan et al. 2008). Our superficial understanding is particularly alarming because human milk is becoming a multi-million dollar cottage industry involving millions of units annually.
Citations
Milligan LA, Gibson SV, Williams LE, Power ML. 2008. The composition of milk from Bolivian squirrel monkeys (Samiri boliviensis boliviensis). Am J Primatol. 70:35-43.
Parkes. 2005. Milk Kinship in Islam. Substance, Structure, History. Social Anthropology 13: 307-329.
Reiter, Stinson, & Le Boeuf. 1978. Northern Elephant Seal Development: The Transition from Weaning to Nutritional Independence. Behav Ecol Biol 3: 337-367.
Roulin A. 2002. Why do lactating females nurse alien offspring? A review of hypotheses and empirical evidence. Animal Behaviour. 63:201-208.
Scantlebury M, Russell AF, McIlrath GM, Speakman JR, Clutton-Brock TH. 2002. The energetics of lactation in cooperatively breeding meerkats Suricata suricatta. Proc Biol Sci. 269:2147-53.
Thierry & Anderson. 1986. Adoption in Anthropoid Primates. Int J Primatol. 7: 191-216.
Valeggia C, Ellison PT. 2009. Interactions between metabolic and reproductive functions in the resumption of postpartum fecundity. Am J Hum Biol. 21(4):559-66.
Weidt, Hofmann, and König. (2008). Not only mate choice matters: Fitness consequences of social partner choice in female house mice. Animal Behaviour, 75:801-808.
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