Monday, October 29, 2012

Why Babies Aren't Actually Parasites

[Edited 10/30/12 due to existence of intraspecific parasitism; see comments.]

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 amphibian* 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)

* This used to be "sea urchin" because I was convinced that's what I'd heard, but I recently asked an echinoderm expert, who didn't know what I was talking about. Fortunately, I also recently picked up The Sixth Extinction, which mentioned the curious habits of caecilians.

** 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.


  1. Nice explanation! However, it makes me wonder why biologists choose to stipulate that "by definition", a parasite must be of a different species than its host. It seems a little arbitrary. And species boundaries aren't always so clear-cut, are they? What about a baby mule?

    1. At its heart, parasitism is a relationship by which organisms have their reproductive success reduced by other organisms; and at its heart, reproduction fundamentally increases the reproductive success of an organism. In biological terms, classifying a reproductive relationship as a parasitism is fundamentally at odds by definition. The "species" in the definition is intended to help you make that distinction and is not at all arbitrary; the word parasitism was invented to describe a relationship between different species on an evolutionary time scale. There other intraspecific relationships on an ecological time scale that are combative, such as competition, and there are interspecfic combative relationships as well. So to use parasitism to describe any combative relationship on any time scale would dilute its explanatory power in biology significantly.

      It's true that production of a mule probably hurts the mother's reproductive success. Not all reproduction increases reproductive success! The point is that reproduction evolved to, well, increase reproductive success. That some of these attempts at reproduction result in mules or kids with genetic disorders or stillborn children doesn't mean the process didn't evolve to contribute to reproductive success- just that it doesn't work well 100% of the time.

      Meanwhile, parasitism exist as the result of one species evolving to take advantage of another species. Implicit in the concept of the parasite is that it evolved to take advantage of another species to increases its own reproductive success.

  2. Nicely written Danna!

    You are right, while a developing fetus does some of things that a parasite does, as you pointed out, unlike a parasite the baby also directly contribute to the reproductive fitness of the mother and is ultimately cooperative due to shared genetic interests. While a fetus may use some methods which are superficially similar to those employed by parasites to establish and survive in the womb, they share fundamentally different goals. However, I must point out parent-offspring conflict can arise and is well-known from many species (for brevity, I won't go into them here).

    As an aside, it is know that many mutualists and parasites also use similar methods for establishing in their host. For example, mutualistic nitrogen-fixing bacteria that form nodules in the roots of legume gain entry using molecules which are similar to those used by pathogenic bacteria.

    The narcomedusa is an interesting case - a "context-dependent baby/parasite". There is one point in your post that I might dissent though in regards to the relationship baby narcomedusa may have with adult jellies of the same species that are not their birth parents:

    "If the adults are of the same species, the babies can't technically be parasites"

    I'd argue that they *can* actually be parasite. Intraspecific brood/social parasitism is well-known in birds and bees, and the case of the precocious nacromedusa babies can be broadly comparable to those systems.

  3. Very interesting...putting aside the part of the definition that the parasite must be of a different species (which I also think is a bit arbitrary), would a surrogate mother pregnant with an unrelated fetus then be drifting more into parasite territory? She's still cooperating to keep the baby alive, but so is a crazy anorexic who swallows a parasitic worm in order to lose weight...

    Oh, and I love the picture of you!

  4. I'm so glad that Tommy, who is a totally legit parasitologist, let us know intraspecific parasitism is a thing! I was unsure on that point, and actually only added the bit about parasites and hosts having to be different species at the last minute (for a lazy reason: I remembered PZ Myers saying it in an interview on a similar topic, and then I found confirmation in Wikipedia). I'll edit the text of the article accordingly!

    Oh yeah, and I had some stuff on parent-offspring conflict in an early version of this post, but I ended up cutting it so that I could finish ever. A very interesting field. It does seem that, in some cases, whether you see conflict or cooperation depends on lot on your assumptions and perspective. Just like there are some organisms labeled parasites (like dicyemids!) whose negative effects on their hosts have never been documented, and may well be commensals or even mutualists.

    And yes, Mike, you're absolutely right that species boundaries are often amorphous.

    As for being a surrogate, that's an interesting question! Made incredibly difficult to answer by our complex human society. If the pregnancy doesn't have any serious complications that affect the mother's long-term health and she's well compensated, I'd say it's more of a commensalism or mutualism.

  5. Regarding being a surrogate - thre's Alloparenting ( In fact, it has been suggested that alloparenting might have been one of the key factor in the evolution of human social behaviour.

    Sarah Hrdy wrote a bit about that in "Mother Nature: A History of Mothers, Infants and Natural Selection." and I am sure others have written on that topic as well.

    As for narcomedusa and surrogate jelly babies - well, as you pointed out, they don't have complex societies, so it wouldn't really be considered as alloparenting...

  6. I should have known there'd be a proper biological term for that. The alloparenting article mentions allolactation, so I expect the parallel term for surrogate mothering is allogestation.

  7. I love my children to life, but I'd be blind not to acknowledge that a parasitic relationship exists. I give; they take. And they don't necessarily give in return. Perhaps, when your cuddly creature becomes a teenager, you'll start to see just how parasitic this whole cycle is. If I am taken care of in my old age, then I will consider it a worthy cause; otherwise (and I'm speaking strictly in terms of ROI here), I will have devoted a huge portion of my body, time, and resources to another being without any gain beyond emotional props. Then again, maybe I'm just jaded by the teenage years right now. I can't even give my DNA any worthwhile credibility right now. Well...she is cute...albeit awkward looking in this phase. ;)

    1. Thanks for commenting--you raise a good point! People often use the term "parasite" metaphorically, as you have done, but my purpose here was to explain why the literal definition of "parasite" is incorrect to apply to children. Even if they were actually consuming your flesh, they would not be literal parasites, because parasites reduce fitness and children increase fitness (by carrying your genes forward in time). That may not seem like much of a gain, but it's the only gain that matters in an evolutionary context.

      Anyway, as for the metaphorical usage, I'm really sorry you're feeling that way right now. That's tough. I hope it gets better!

    2. I disagree. In most of nature, the parents will kill their young if ideal conditions are not met to continue rearing. They abort their young. The benefit of passing on one's genes may be outweighed by the detriment to one's individual health and the individual that is playing host, should always have the choice, at anytime to continue such a relationship. Whether we use this argument to hold the label parasite from human offspring, we all agree they do possess very parasitic traits... all but the benefit of proliferation of species. That being said, I think this only strengthens the argument for individual choice.

    3. Certainly, see second footnote above! Filial cannibalism--and other less drastic means by which parents withdraw resources from certain offspring--can enhance fitness by allowing the parent to devote those resources to other offspring (either now or in the future) that are more likely to survive/reproduce, or that are simply more numerous. From an evolutionary standpoint, re-allocating those resources to other offspring is all that matters.

      You seem to be approaching the matter mostly from a philosophical standpoint, musing on the the reproductive choices of individual humans, which is quite a rich topic for discussion but not one I'm interested in hosting here, I'm afraid. =)

  8. "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."

    Does this, then, mean that toxoplasma is not a rodent parasite since under its influence, both rats and the toxoplasma also have the singular goal of survival of the parasite?

    1. Sly, very sly! Indeed, many parasites cause their hosts to engage in behaviors that promote the parasite's survival while being downright antithetical to the host's. I was certainly taking liberties by using anthropomorphic language like "goal," "disagree," and "desire," but if I wanted to continue on in an even more fanciful vein, I could say that the rat's true identity, its essential "ratness" if you will, is being forced into such actions against its will, that its desire to resist the parasite is being tragically overwritten by the parasite but that the goal of the parasite's survival still belongs solely to the parasite. If, on the other hand, I wanted to backtrack into dry academic mode, I could say that all this talk of goals is hogwash; it's purely a question of evolutionary fitness: the child's survival increases the fitness of both child and parent, while the parasite's survival increases its own fitness and decreases the fitness of the host.

    2. Like women getting certain cravings during pregnancy etc. :)

  9. Thanks for your comment! That's an interesting point about evolutionary vs. ecological time scales. (Though as you probably know they blend into each other; biology is so charmingly messy that way.) But intraspecific parasitism really does exist, as in the case of ducks I linked in a comment below:

    And since I can never resist investigating a good etymological question, I now know that the original use of the word "parasite" did in fact refer to an intraspecific interaction within Homo sapiens, and only later took on the biological definition we're discussing here:

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  11. I am a professor of Biomedical Enhineering, and your arguments are weak at best. Now before I get flamed, I am not from the pro-choice camp trying to argue against life or anything like that, but I believe in sound arguments not ones that are simply trying to debunk for the sake of debunking. I teach my students to back their arguments up with citable facts. Additionally, I am currently happily pregnant with my first child (and going through all of the crappy symptoms as a result of this "parasitism"). The baby I am carrying inside me is, by all definitions, a parasite. I don't see why that has to be argued against. It's not necessarily a horrible thing. I have chosen to carry this parasite in me, however, it IS stealing nutrients from me for survival, and it is not conferring any advantages to me. The info you cite on peripartum cardiomyopathy doesn't hold because peripartum cardiomyopathy, by definition, is caused by a pregnancy. The excess exertion required by the heart to support the baby causes heart damage in this case. If the baby confers some sort of advantageous immune suppression in response, it does not negate the fact that the disease itself was caused by the presence of the baby. The strongest case you offer is the lack of inter species difference in a human pregnancy, however, as others have stated this is a pretty arbitrary condition placed on the definition of parasitism. In fact, this stipulation is absent from Mosby's definition of parasite which states that a parasite is "an organism living in or on and obtaining nourishment from another organism." This is a highly revered medical dictionary. A human pregnancy falls under this medical definition.

    1. " is not conferring any advantages to me."

      Actually, from an evolutionary perspective, it's conferring the only advantage that matters: fitness.

      I think what I'm learning from yours and many other reactions to this essay is that the medical and/or personal perspective can be quite different from the evolutionary perspective.

      Congratulations on your happy pregnancy!

    2. I think in this case we could argue a stipulation in the definition of parasitism between proximate and ultimate relationships. In the proximate sense, offspring would fall under the parasite category (especially in the cases of young causing the death of the mother), but ultimately they do increase host fitness, so it could be a case of mutualism in the long run.

      I'm a few months late to the chat, but I thought I might add that.

    3. That is a very fine point. A related duality is the contrast between parasitism in the physiological sense--is one body sapping nutrients from another?--contrasted with the evolutionary sense--is fitness decreased or increased?

  12. What about the sexual parasitism in Angler fish? The male of some species of angler fish are considered parasitic yet, according to your definition, they cannot possibly be considered parasites because they are of the same species as their host. And they do not reduce the evolutionary fitness of their female host, quite the opposite, they provide sperm so that the female can reproduce. Also, a fetus or baby can certainly reduce the fitness of their host. An unhealthy fetus would mean that the reproductive success of it's mother was reduced, especially if miscarried or if it dies during or after birth. It would mean that the mother lost a significant portion of her life, resources and health for nothing. Also, after the baby is born, when the mother is lactating she is significantly less likely to reproduce and can have diminished fertility. Being pregnant and giving birth also significantly increases the mother's likelihood of dying which obviously diminishes her evolutionary fitness to 0. I also know of cases where a complicated birth rendered the mother infertile. Not all parasites harm their hosts(unlike the fetus) since they would have an interest(not unlike the fetus) of keeping their host alive and kicking.

    1. Ooh! I'm delighted that you thought of anglerfish males, which had completely slipped my mind. (Note that this adds to an existing discussion about intraspecific parasitism--see Tommy Leung's comment above about brood parasitism in birds and bees.)

      After some literature browsing, I think I am going to have to be super pedantic and boring and argue that the male anglerfish are not really true parasites. They're called parasites, I think, because they were historically first perceived as parasites of a different species, and only later identified as males of the same species who are actually providing genetic material to the female, thereby contributing to the fitness of both male and female. Also, it's fun to call them parasites, and how could an animal with such bizarre and fascinating behavior *not* be a parasite? The thing is, there *are* cases of what one might call "real" sexual parasitism, in which males or females "mate with a ‘host’ whose genetic contribution is discarded either immediately or after a delay of one generation." (That's a quote from this paper:

      Your other comments about specific cases in which illness or death of offspring (potentially combined with illness or death of mother) would reduce the mother's fitness are, I think, somewhat peripheral the question of whether babies are parasites. There are plenty of ways that any kind of reproduction can go wrong--chickens can die of egg-binding and squid can get caught by fishing boats as they aggregate to spawn. "Gone wrong" scenarios do happen (and can be very, very sad) but they don't change the fact that reproduction gone right increases fitness. (Also, the mother's death doesn't necessarily reduce her fitness to 0 unless all of her offspring are also dead).