Thursday, 12 June 2014

Is breastfeeding altruistic?

A skua steals milk from an elephant seal's teat

It certainly feels like it, at times. Like at 4am when papa is snoring and you’ve got a biro wedged under each eye lid, willing baby darling to suck its last so you can sink back into oblivion. Or, several months down the line, when the little monkey gets a mischievous glint in his eye and goes in for an experimental chew. But we must distinguish psychological from biological altruism here.
A behaviour is psychologically altruistic if it feels self-sacrificial: acting out of concern for someone other than yourself. Many mothers in fact get something selfish out of breastfeeding – they enjoy the closeness it involves, or perhaps the boost they get from feeling like they have done something good. Nonetheless, this is not usually the primary motive in feeding the child. Most would agree that breastfeeding fits the bill of being nice, doing something at least a little bit arduous because you put someone else before yourself.

A trait is usually called biologically altruistic, on the other hand, only if it generates a benefit for some recipient, while also incurring a cost to the bearer. Crucially, these benefits and costs are measured out in terms of fitness effects, rather than in simple metabolic or energetic terms.

So is breastfeeding biologically altruistic? Well it almost certainly increases Orson's fitness, by boosting his immunity to disease, providing nutrients and emotional security, and so on. And I can think of ways that breastfeeding might be thought to incur a cost to my direct fitness: my chance of surviving and bearing more offspring. For example, there is an opportunity cost, because time I spend breastfeeding is time I can’t spend doing other, fitness-affecting things – gathering food, competing in sexual competitions, and so on. There is a direct impact on my health just in terms of calorie loss. In a nutrient limited environment where I wasn’t able to cram cake into my mouth at every possibility things would have to be rather different. And I’m trying not to think about how all the *ahem* wear and tear will end up affecting my future quality as a mate. Have you ever seen one of those old fashioned cloth icing bags, where you pour the icing in the bag and squeeze it out through a nozzle? My mum has one of them. I’ll leave it there.

So Orson wins, I lose, a bit: paradigmatically altruistic, you might think? Well, we used to think to.  Orson is my son and shares 50% of my genome. Kin selection theory tells us that anything that boosts the fitness of my son, therefore, boosts my fitness also, indirectly. Breastfeeding my son ultimately increases the probability of my genes replicating themselves. Mother and child are one in seeking only the best for the baby. "The classical view of lactation is one (in which) the mother devotes all her energies, except only those necessary to maintain her own life, to care for her young in order for the young to survive and reproduce. Mother bountiful gives her young the best of everything until they are old enough not to want any more milk. Although lactation is recognized as an enormous drain on the mother, the emphasis is one of unstinting physiological and behavioural altruism" (Peaker 1989).

This image of breastfeeding as representing a total harmony of interests between infant and babe was shattered by Robert Trivers’ parent-offspring conflict theory (Trivers 1985). While I share 50% of my genes with Orson, he shares 100% of his genes with himself. This sets up a conflict of interest. Mama has an interest in maximising the number of humans who carry 50% of her genome, and to invest equally in all of them. She wants darling baby to have some siblings in other words (speaking hypothetically you understand!). But Orson shares more of his genes with himself than he does with his siblings – 50% more! So there is a tug of war, in which baby wants all of Mama’s investment for himself, but Mama wants to save something in reserve, for little brother or sister.  The expectation is that Mama will want to wean baby off the breast earlier than baby wants to give it up. The extent of the conflict varies with things like how monogamous the species is (half-siblings are even worse rivals then full siblings), whether its under k selection or r selection, how uncertain the future is, how old the mother is, her nutritional state. But in general, Trivers’ theory predicts that a mother will seek to invest in breastfeeding just far enough to enhance the fitness of her child but not beyond the point where it might interfere with her chance of having other children. The quality of the milk is also expected to taper off, but this is the mother being stingy, rather than about the infant’s changing nutritional needs.

David Haig recently published a study arguing that infants have been selected for strategies which decrease the chances of a competitor sibling appearing – especially breastfeeding, beyond the point where they are nutritionally dependent on milk, because it dampens the mother’s fertility, making her less likely to conceive siblings (Haig 2013). Even more difficult for a new parent to read, Haig suggests that night waking is a strategy. He claims babies are actually designed to cry in the night a lot, to make sure that their parents are sleep deprived and well, not feeling horny. This puts a different light on the ethics of preventative calpol ; )

Another candidate for the title of biological altruism is the phenomenon of 'non-offspring nursing', allonursing,  observations of which have been claimed in many non-human mammals (see here). Feral cats and birds have been observed taking milk from the teats of elephant seals, for example. Controversy surrounds these claims, however, with some arguing that allonursing is properly viewed as a sort of infant parasitism, especially across species, or in species where the mother births many offspring at once and is likely to be poor at keeping track of which are hers. Mothers feed interlopers because they cannot count above three, for example.

The reason for this scepticism about allonursing being genuinely altruistic is, again, kin selection theory. It just doesn't make evolutionary sense for a mother to suckle babies that aren't hers, we think.  The exception is in species that live in close knit family groups. Here we expect to see much higher rates of allonursing, because mothers are usually going to be nursing the offspring of relatives - her nieces, nephews, grandchildren, in other words.  Since the babies will share genes with the lactating female, her kind donation of milk can be recognised as making evolutionary sense after all, because boosting the fitness of those young boosts her own indirect fitness.

The general view is that when non-infant nursing occurs in species that typically have only one child at a time, it is best understood as milk stealing (Packer 1992).  In species that have large litters (polytocous species), on the other hand, breastfeeding of non-offspring occurs most frequently when social groups are small, and constitutes a form of communal care, underpinned by either kin selection or reciprocity.

In humans allonursing used to be called wet nursing and was commonplace. There is some controversy amongst medical professionals about whether the practice should be encouraged or not, with critics fearing the transmission of diseases this way. Interestingly, Craigslist carries adverts of breastmilk for sale in the states, for $2 an ounce. Here in the UK there are milk banks at hospitals, where new mothers are encouraged to donate spare milk*. At the donation centres the milk is screened and pasteurised before being made available to mothers with health crises or other problems breastfeeding. I think they tend to be well stocked, perhaps providing another example of Richard Titmuss’s famous argument that people are happier to provide biological functions to others for free than if they’re getting paid (Titmuss 1971). Breastmilk donation then, is surely altruistic in every sense of the word (as long as you don't feel too good about it).

This post is dedicated to my friend Sarah who just asked me if ducks have nipples.

*Some mothers really do have too much. I wonder if there is any truth to the claim I heard midwives repeat over and over that new mothers standardly produce enough for twins and then later adapt to just one? This is at odds with what Jared Diamond writes about traditional societies standardly eliminating multiple births)

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