Parasites: masters of natural mind control

How, and why, do parasites manipulate the mind? Chris Smith hears how from Cambridge University behavioural ecologist Jenny York...

Chris - First of all, tell us why parasites would need to manipulate someone's mind or an animal's mind? Why?

Jenny - Parasites make their living on the inside or on other organisms, said a host species, and they're at complete conflict with these other organisms because they are stealing from them. So in order to sort of transmit to the next stage of their lifecycle or to reproduce, in some cases, they have these adaptations that allow them to manipulate the behaviour of their host.

Chris - Can you give me some examples?

Jenny - I can start with probably my favourite example (somewhat biased) of the common cuckoo which we study up at Wicken Fen in Cambridgeshire, here

Chris - Can you do an impression of one?

Jenny - I wish I could!

Chris - You hear the sound so so infrequently these days, because they were on their way, aren't they? They're not so common as they once were. What do cuckoos do then? We all know they lay eggs in the wrong bird's nest. But what do they do to make that better?

Jenny - So as you say they lay their eggs at the wrong nest. There are there are brood parasite, that means they need to put their eggs in the nest of host species in order to reproduce and produce offspring. And they have a whole variety of tricks in order to make this happen and to improve their chances because it's not at all that the host birds' interest to raise these cuckoo chicks. And one really interesting adaptation they have manipulation of behaviour: the female cuckoo - despite being very secretive and cryptic, and hiding and waiting for hosts to leave the nest so she has an opportunity to lay, and taking as little as 10 seconds to lay her egg in their nest - she then after doing so gets this really conspicuous call. But if it were anything else you'd think: "Why is she giving the game away?" She's alerting the hosts to what she's just done... So we were really interested in this call and we looked at the acoustic structure of it it's very similar to that of a sparrowhawk which is a predator of what she is parasitising. Exactly!

Chris - So is that she's doing that in order to make them stay away for longer, and so that they're distracted worrying about the predator. They're not worried about what she might have done in their nest.

Jenny - Yeah that seems to be what's happening. So this call makes them equally vigilant to hearing a sparrowhawk call. So it does get them off kilter. There's something about this call that makes them feel wary about their own safety and so instead of paying attention to what's in the nest and thinking: "Is there something parasitising me?", they're paying attention to looking after their own well-being.

Chris - How do you think the cuckoo's evolved to do that in the first place because that's pretty complicated. To mimic another bird having evolved to lay your egg in the wrong nest in the first place. To then evolve to mimic another bird. You gotta know that the other bird you're mimicking is a threat to the first bird you're trying to parasitise. There's lots of links in that chain.

Jenny - Yeah so it's probably unlikely that they're doing this consciously and using vocal mimicry that some species do. Some species can hear a call and then imitate that call perfectly. It's more likely with cuckoos, because they're not vocal learners, that they just inherit this as instinctive, this call that they have. And over evolutionary time it's become more and more similar to that of the sparrowhawk.

Chris - Amazing. What about, since we're talking this week about manipulating the brain, rather than just external manipulation; what about things that manipulate from within? Parasites that can actually change the way your brain physically works?

Jenny - So there are some lovely examples of this. I don't know if lovely is the right word, to be honest! I think that if you think of the sci-fi movies, you know, Alien - we have the real world alien in the zombie fungus that infects ants. They'll come across a spore of this fungus on the forest floor, and then it will take several days to develop inside them, and at a certain point it switches and causes that behaviour to change that they move away from the colony and up high in the tree where they then bite onto a leaf and stay still until this fungus erupts 24 hours later; erupts from the head and throws spores into the forest carry on.

Chris - So they make the ant into their, sort of, fruiting body?

珍妮-没错。是的。是的。很神奇的。所以他们just manipulating the behaviour directly inside.

Chris - Do we know how the fungus is doing that to the animal's brain?

Jenny - This is something that's getting more and more interest at the moment. This exact mechanism of how parasites are manipulating their hosts. So it's a lot clearer in another example: the kamikaze horsehair worm. So there's a worm that lives inside the body of a cricket, and it needs to get to water in order to get to the next stage of its life cycle, so it must burst out of cricket's body. But the cricket doesn't want to go anywhere near water, that's not part of its everyday life. So by changing the neurochemistry of this cricket, it makes it a kamikaze cricket that goes towards the nearest body of water and then is burst open with a giant worm...