Respect For the Fungus Overlords

By Carl Zimmer | July 28, 2009 5:05 pm

When I first learned about the fungus Cordyceps, I refused to believe.

I was working on a book about the glories of parasites, so I was already in the parasitic tank, you could say. But when I read about how Cordyceps infects its insect hosts, I thought, this simply cannot be. The spores penetrate an insect’s exoskeleton and then work their way into its body, where fungus then starts to grow. Meanwhile, the insect wanders up a plant and clamps down, whereupon Cordyceps grows a long stalk that sprouts of the dead host’s body. It can then shower down spores on unfortunate insects below.

I mean, really.

Yet this video from David Attenborough faithfully depicts the actual biology of this flesh-and-blood fungus. I also discovered that Cordyceps is not the only species that drives insect hosts upward. You don’t even have to visit a remote jungle to see one. Here in the United States, houseflies sometimes end up stuck to screen doors thanks to a fungus called Entomophthora muscae. And the lancet fluke Dicrocoelium dendriticum uses the same strategy to get into cows.

Call me naive, but I assumed that creatures as freakish and wonderful as Cordyceps and company would attract enormous amounts of scientific attention. Yet I was frustrated to discover that hardly any research has been carried out on their powers of manipulation. That’s a shame, because you cannot assume that these parasites are indeed manipulating their hosts. It’s possible, but it’s just a hypothesis that requires testing. One alternative explanation might be that when insects get infected with these parasites, they just lose their bearings. What looks like an adaptation to us–getting your host to a good place for spreading your offspring–may just be a coincidence.

So it is with great delight that I see that David Hughes of the University of Exeter and his colleagues are bringing some real science to bear on these X-Files creatures. Based on their research, these creatures have indeed evolved finely tuned adaptations for using their hosts. In fact, they’re even more sophisticated that I had given them credit for.

cordyceps440.jpgThe scientists are studying a fungus in Thailand called Ophiocordyceps unilaterius. It doesn’t infect just any insects, but only ants. And it doesn’t infect just any ant, but almost always infects a single species, called Camponotus leonardi, a species that makes its nests high in trees and wanders around the forest floor for food. If Ophiocordyceps really has evolved to manipulate its host, then the scientists predicted that the manipulation would boost their reproductive success in a number of ways. The alternative explanation would be that,  as has been shown with other parasites, they don’t actually benefit from the change in their hosts.

The scientists dissected infected ants to see how the parasites used up their bodies to grow. They also looked around the jungle to see where the ants ended up. It turns out that the ants don’t end up randomly scattered across the jungle. Instead, Ophiocordyceps-infected hosts are concentrated in “ant graveyards.” The scientists took careful measurements of the conditions in these graveyards, looking for any common factors they might share. They also plucked some infected ants from their leaves and moved them to different spots to see how their position affected their success in infecting other ants.

The death grip of the ant is very precise. All ants bite the underside of a leaf (another species of Ophiocordyceps is known to make its hosts bite bark, and another bites twigs). The scientists found that 98% of the ants bit a leaf vein. Most of the ants ended up on leaves on the north-northwest side of the plant. The ants ended up on leaves low to the ground–at a height of about 25 centimeters. It just so happens that there are consistent differences in the air conditions at different heights in the jungle. The higher you go, the drier and warmer the air becomes. This is telling, because the rate at which fungi grow is very sensitive to humidity and temperature. Studies on fungi similar to Ophiocordyceps show that they need very humid conditions to grow well, and temperatures between 20 and 30 degrees C.

When the ants ended up in this particular kind of spot, the fungus grew in the same consistent pattern. It developed an orange tube that ran the length of the animal, rich in carbohydrates. A big black ball of fungal tissue grew in the ant’s abdomen, and white threads, known as hyphae, spread through the rest of the body. Within 24 hours after death, the hyphae began to emerge out of their host, making the ants fluffy. Some of the hyphae grew out to contact the leaf, lashing the host in place. By the second day, a pinkish stalk began pushing out of the ant’s head. As the entire ant became covered in a mat of hyphae, the stalk grew to be twice as long as the ant itself. At first the fungus produces asexual spores, but after a week or so, the stalk produces sexual spores. Each night, both kinds of spores rain down on an area of about a square meter on the forest floor.

When the scientists moved infected ants higher up into the canopy, the fungi grew abnormally and never fully developed. On the other hand, when the scientists moved the ants to the ground, the ants simply disappeared–devoured most likely by other animals or washed away by rain.

The scientists published their findings recently in the journal American Naturalist. There they conclude that the ant becomes what Richard Dawkins would call an extended phenotype–an extension of the parasite itself, which helps it pass on its genes to the next generation. In a jungle, animals and plants evolve to exploit distinct niches. Ophiocordyceps exploits a niche of its own by directing its host to a place in the canopy where it can find the best conditions for growing quickly. (It should be pointed out that only infected ants end up on the underside of north-northwest leaves 25 centimeters up from the ground–it’s not the sort of place a healthy Camponotus leonardi is found.)

The resting place of the ants benefits the fungus not just because the humidity and temperature are good. It also helps the fungus extend the usefulness of the ant corpse. On the underside of a leaf, the ant’s body is protected from sunlight and heavy rain. The fungus also protects its cadaverous house by making antibiotics to keep other microbes from invading and stealing the valuable food inside. Far from destroying the ant’s body, the fungus uses its hyphae to strengthen the insect’s exoskeleton into what the scientists call “a protective case.” Inside the ant, the scientists propose, the fungus builds its orange tube in order to store up nutrients from the ant’s fluids, so that it can fuel its own growth more reliably over the long term. These adaptations together may allow the fungus to produce a steady shower of spores for days on end in order to infect as many unlucky ants below as possible.

Like their hosts, parasites face many threats to their survival these days. Just as overfishing may wipe out my dear namesake tapeworm, I worry about the rampant deforestation of tropical forests, which may wipe out not just trees, but the insects that depend on them, and the fungi that depend on them in turn. We should not wish extinction on Ophiocordyceps, as gruesome as it may be. It has much to teach us. We might even borrow some of its tricks–Ophiocordyceps antibiotics show promising signs of fighting against malaria and cancer. And it would be a true shame for this particular bit of science fiction to disappear from the world of science fact.

[Image: PLOS One, via Creative Commons Licence]

CATEGORIZED UNDER: The Parasite Files

Comments (31)

  1. Dave C

    Fascinating! Carl, if it weren’t for you, I would probably never have become enamored with parasites (as strange as that sounds). I hope you never stop writing about them!

  2. Absolutely brilliant. Thank you, Carl. I just love this!

  3. I’ve seen that David Attenborough video at least a dozen times, and it never ceases to amaze.

    Thanks for the update! Parasites are truly fascinating.

  4. Fantastic! You are fast becoming one of my favorite science writers. I can’t wait to pick up a copy of Microcosm!

  5. Incredible. Have any parasites been discovered that change mammalian – and dare I say it – human – behavior?

    If those ants had a culture, or a religion, would they come up with a theological reason why they love the underside of a North facing leaf?

    Thanks – love this stuff, gotta get your book.

  6. Christina Viering
  7. Allison Bland

    Great post! I’ve been curious about this parasite ever since seeing it on Planet Earth.

  8. Joe

    This fungus-ant relationship is one of the great exhibits at the Museum of Jurassic Technology in Los Angeles.

  9. JustAsItSounds

    douglass truth #5 :

    Have any parasites been discovered that change mammalian – and dare I say it – human – behavior?

    Toxoplasma gondii is a protozoan that infects cats (it’s primary host) and a variety of other warm-blooded creatures. In rats the larval form, forms cysts on the rats’ brain and modifies their behaviour so that they become ‘fearless’ they no longer avoid light and tend not to stick to walls/skirting boards as they normally do but wander into the open more. More importantly, for toxoplasma gondii, infected rats no longer fear the smell of cat urine and may even be attracted to it.

    All of these modified behaviours mean that the toxoplasma infected rat is more likely to end up inside a cat – where the larval form matures to the adult and pumps out toxoplasma oocysts that end up in the cat’s poop.

    Toxoplasma also infects humans, normally it is not a problem for the human hosts unless they are somehow immuno-compromised (did I just make up that word?). However there has been some research that suggests that t.gondii infection does have an effect on it’s human hosts – from a slightly increased prevalence of depression to ‘… slower reaction times and a 6-fold increased risk of traffic accidents among those infected’

    http://en.wikipedia.org/wiki/Toxoplasma_gondii

    Carl: Here’s some Toxoplasma on the Loom, too.

  10. Martin

    Too creepy, this is lke the Alien chestburster. Wipe it out!

  11. This is such a cool story that it inspired me to make a little comic drawing of it. I hope some of you have a look. Thanks, Carl!

    http://www.markoftedal.com/2009/08/zombie-ant-fungus.html

  12. That is both fascinating and frightening. Thanks!

    (and they told me science can’t be made interesting!)

  13. Wow, that is so X-files! How creepy!!! I love it, I may have to blog about it myself or I’ll have nightmares about it.

  14. “The fungus also protects its cadaverous house..” — a house _like_ a cadaver? Very like a cadaver, I’d say. In fact I’d say, “The fungus also protects its cadaver house…” since it’s not just like a cadaver, it is a cadaver!

    tOM

  15. noir noir

    very interesting post. The rabies virus also affects its host in a similar way. Afflicted dogs salivate profusely (the virus is concentrated in the salivary glands) and has a lower threshold for aggression and biting.

  16. Forest

    Perhaps you know Paul Stamets? In a lecture, he discusses the use of cordyceps for toxic free pest control… and has applied for a patten for his idea(s) and experimentation.
    YouTube video: 6 ways mushrooms can save the world.
    http://www.youtube.com/watch?v=XI5frPV58tY

  17. SouthernFriedSkeptic

    My only thought after seeing this was:

    Science Fiction is Nature’s b*tch.

  18. CassandraT

    This is just like something out of a science fiction story. So very gruesome!

  19. jigo

    Researchers claim to have found the first evidence of ‘zombie’ ants in the fossil record. They have matched peculiar cuts on a 48-million-year-old fossil leaf with the ‘death bites’ made by modern ants infected by a fungal parasite. The research is published in Biology Letters.

    The idea of examining the fossil record for evidence of the distinctive bite marks came to David Hughes, a behavioural ecologist at Harvard University in Cambridge, Massachusetts, as he was sitting in on a palaeobotany undergraduate course. He had just returned from fieldwork in southern Thailand where he had been studying fungal parasites infecting carpenter ants and controlling their behaviour. “Could this parasitic relationship have evolved much earlier in Earth’s history?” he asked himself.

    “The distinctive pattern of the bite marks and their association with the veins of the leaf is actually quite striking and unusual to see in fossil leaves,” says Paul Kenrick, head of research at the Natural History Museum’s Department of Palaeontology in London. “This find is telling us something significant about the evolution of the interactions between organisms and their dependencies.” If the marks are those left by dying ants, the complex ant–parasite relationship must have evolved at least 48 million years ago, he says.

    -NATURE MAGAZINE-

  20. Harry

    After watching the video, I was more amazed on the concept of the circle of life. This is one way of the nature to control biological population. I was actually scared and feel pity for these insects infected. I wonder why there is no mosquito on the video? I would want this to happen to them.
    ———–
    Harry
    expert on Mosquito Bite Allergy

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The Loom

A blog about life, past and future. Written by DISCOVER contributing editor and columnist Carl Zimmer.

About Carl Zimmer

Carl Zimmer writes about science regularly for The New York Times and magazines such as DISCOVER, which also hosts his blog, The LoomHe is the author of 12 books, the most recent of which is Science Ink: Tattoos of the Science Obsessed.

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