PSA: Babies are not parasites.
|This is a parasite.|
(Head louse, by Gilles San Martin)
Of course, parasites have babies, and some free-living organisms have a parasitic early life stage. But the notion I seek to discredit here is that all babies are parasites of their parents and, particularly, that the human fetus is a parasite of its mother. This misconception has become distressingly common among my peers.
It distresses me because I love babies and I love parasites, so I think it's important to understand the distinction between them. In a nutshell: a parasite reduces the fitness of its host; a baby increases the fitness of its parents.
Seems pretty straightforward, right? Yet I will concede that numerous superficial similarities between babies and parasites can lead to confusion. Parasites often live inside the body of another creature, extract their nutrition from its blood, and struggle to escape attack by its immune system. That's starting to sound an awful lot like a fetus . . .
But the host-parasite relationship is one of conflict, while the mother-baby relationship is intrinsically cooperative. Consider the immunology of the two. Host and parasite are locked in an arms race: the parasite evolves ever more complex techniques of avoidance, while the host evolves ever more complex techniques of detection and attack.
Meanwhile, mother and baby cooperate to prevent immunological conflict. The site of this cooperation is the placenta--the big blob of tissue that's genetically part of the baby and physically connects baby to mom. For a long time, scientists thought of the placenta (and by extension, the fetus) as a kind of natural organ transplant. Just as in medical organ transplants, the mother's immune system would have to be suppressed to prevent it from rejecting the foreign body.
But a fascinating review paper in 2010 suggests this is the wrong way to think about pregnancy--that, in fact, the cooperative choreography between mother and child is far more sophisticated:
The trophoblast [placenta] and the maternal immune system have evolved and established a cooperative status, helping each other for the success of the pregnancy. This cooperative work involves many tasks, some of which we are just starting to unveil.True, the placenta uses at least one trick from the world of parasites--a molecule that makes it partially invisible to mom's immune system--but it also oversees an active exchange of molecules and even cells between mother and baby. The full implications of this exchange aren't yet understood, though the mother's contributions undoubtedly protect the baby from infection, and the baby's cells may also offer health benefits to the mother.
All this isn't to deny the fact that a pregnant woman makes certain sacrifices. Notably, she gives up nutrition that could otherwise have gone to her own body. But in sharing nutrients with her offspring through the placenta and, later, milk production, a human mother has it relatively easy. Some species transfer nutrients more, um, directly.
Babies of one rather unusual sea urchin simply graze on their mother's skin to get the early nutrition they need*. And the young of certain spiders consume their mother's entire body--parental sacrifice at its most extreme!**
Then there are the jellyfish children. In some species of narcomedusae, baby jellies hang out inside their parents, slurping food out of the adults' digestive tracts. That's not so weird--I mean, think of regurgitation in birds--but then sometimes they'll go and slurp from an unrelated adult, or even from adults of another species.
The ones that stay with their parents are certainly not parasites. But the ones that feed off other adults are in murkier territory. They're certainly acting a lot more like parasites than if they'd stayed at home.
But what if it's like a "village" scenario, in which all the adults pitch in to raise all the children? Parasitism need not enter the picture; this is simply cooperative parental care. Of course, jellies do not have complex societies, so it's a rather fanciful idea. It becomes even more fanciful if you consider the baby jellies who feed from adults of a different species. It's hard to argue that those little tykes are anything but parasites.
I like the narcomedusae because they illustrate when a baby is just a baby, and when a baby becomes a parasite.
It all boils down to the fact that parent and child have a common goal: the child's survival. Host and parasite, on the other hand, have a fundamental disagreement about the desirability of the parasite's survival.
(Of course, the baby in my belly could have taken over my brain and caused me to write this manifesto.)
|This is not a parasite.|
(Pregnant author, by Anton Staaf)
* I know I learned about these sea urchins from an invertebrate zoologist who ought to know, but I can't for the life of me find the reference now. Frustrating!
** It's worth noting that parents sometimes eat their young, as well, if things don't seem to be working out--reclaiming the nutrients they invested in order to give reproduction another shot later.