Parasites Make Their Hosts Sociable So They Get Eaten


Many animals come together to find safety in numbers. In teeming herds, flocks, shoals and swarms, it’s harder for a predator to track and isolate any single individual. This is the standard view of animal groups, as espoused in countless nature documentaries. It rests on one big assumption—that the animals are in charge of their own behaviour. And that is not always true.

French scientists Nicolas Rode and Eva Lievens have found that two species of shrimp form swarms because they are being controlled by parasites in their bodies. Some of these need to get into birds to complete their life cycle. Others need to pass from one shrimp to another. They all achieve this by controlling their hosts and making them gather in large conspicuous swarms. The parasites can more easily jump into fresh hosts if the shrimps are swarming, and the swarms are more easily spotted and devoured by flamingos.

It’s a sinister twist on animal gatherings: Rather than finding safety in numbers, the shrimp are being collectively herded towards death’s door by unseen forces.

Rode and Lievens studied two species of brine shrimp—small crustaceans that looks like legs and eyes attached to a splinter. (You probably know them as sea monkeys.) Each one is just a centimetre long, but they gather in temporary swarms that can stretch for up to 2 metres. There are many possible explanations for these swarms, but none of them can fully explain what’s going on. The shrimp aren’t gathering in places rich in food or nutrients.  They’re not avoiding predators, because they’ll still swarm in water that’s too salty to support fish. And while some species gather to mate, others reproduce by cloning themselves. Sex cannot be the only explanation.

Rode and Lievens turned to parasites. If animals are behaving in odd ways, it’s always worth considering that parasites could be influencing them. Many of these body-snatchers are also pharmacists that commandeer the brains of their hosts to create head-banging caterpillarsdocile cockroachessuicidal crickets and fearless rats.

Written By: Ed Yong
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  1. I have to say it: religion qualifies as another perfect example of such a “social” parasite.

  2. I’ve been down the pub three times this week already.

    I think I’m “Hosting”.


  3. “Parasites Make Their Hosts Sociable So They Get Eaten”

    The headline is wrong. Suggests intent.

    should read:

    “Parasites That Make Their Hosts Sociable Get Eaten”

  4. In reply to #4 by aquilacane:

    “Parasites Make Their Hosts Sociable So They Get Eaten”The headline is wrong. Suggests intent.should read:”Parasites That Make Their Hosts Sociable Get Eaten”

    Surely it’s the host that gets eaten, and the parasite is along for the ride. So it should say “Hosts that are made sociable by their parasites get eaten”.

    {/pedant mode} ^_^


  5. Well that would explain Jesus hosting the Last Supper and saying: “Eat my flesh and blood.”

  6. Maybe its some kind of parasite that causes people to be herded into churches by unseen forces.

  7. Parasites never cease to amaze and disturb me, often at the same time. They make all that business with the Ichneumonidae seem kindly by comparison.

    Host-suicide seems to be a stellar example of an extended phenotype, the recruiting of other bodies and other material of the environment to serve the genome’s benefits for generation after generation. The creepy thing about it is that it seems to cover those cases in which a behaviour of an organism can be maladaptive for it but beneficial to another, provided the other takes steps to bring it about. What if many human altruists, far from being the pinnacles of moral behaviour, were actually dupes misled by someone skilled enough in persuasion to delude them into thinking it was a sacrifice for another cause, say a cause for family or for friends? After all, there’s no rule forbidding a species from manipulating its own members.

    This sort of host-destroying adaptation has even been found in fungi that force their ant hosts to climb high vegetation before dying. What’s notable, though, is that the host species are almost always small animal species: snails, ants, spiders, and rats. Is this because larger species have brains too complicated to control for simple parasites, or is it simply because they have yet to evolve the necessary adaptations?

    Or are they among us, right now, lurking unseen?

  8. In reply to #8 by Dublin-atheist:

    Maybe its some kind of parasite that causes people to be herded into churches by unseen forces.

    I think that was Marx’ view, roughly speaking.

  9. In reply to #10 by PERSON:

    In reply to #8 by Dublin-atheist:

    Maybe its some kind of parasite that causes people to be herded into churches by unseen forces.

    I think that was Marx’ view, roughly speaking.

    Marx’s view, given the “opium of the people” quotation in full, was that religion was a source of comfort and escapism for the oppressed proletariat (hence “the sigh of the oppressed creature”), not that it was a manipulative parasite. In any case, I think it would be unhelpful, not to mention a bit patronizing, to speculate about religion being like a real parasite. Religion is a set of beliefs people act upon and apply based on how intuitive they seem and based on the behaviours and explanations of others. It’s not a living organism or a human consciousness, or even a set of phenotype-coding genes with metaphorical desires.

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