Monday, August 27, 2007

Zombie Ants from Outer Space

I promised myself that I wouldn’t do another post with a biological example of the viral model so soon, but Mike the Boyfriend (my sister’s boyfriend, not mine) just told me about this and it’s too cool to delay.

Ok, I admit it, the heading’s a tad misleading: the ant isn’t actually undead, nor is it from outer space. But “Dicrocoelium dendriticum” just wasn’t a very compelling title.

Anyway, there’s a parasite out there that can sneak inside the still-living body of an ant; take control of it; and force it to behave in a suicidal manner. The parasite is called a Lancet Fluke, and it sure jumps through some hoops in order to achieve its purpose; a wonderful example of very specialized adaptation.

The Fluke’s ultimate target is the belly of a herbivore such as a cow or sheep.

This is how it gets there:

Thousands of fluke larvae snuggle into a nice big pile of herbivore poop*
A snail comes along and eats the poop (accidentally ingesting a bunch of fluke larvae).
The snail wanders around, leaving a trail of ooze riddled with now-juvenile flukes.
An ant comes along and eats the snail-ooze. No, really!**
When it eats the snail-ooze, the ant unwittingly eats hundreds of juvenile flukes.
One lucky fluke navigates inside the ant to its brain, nestling in to the area that controls movement and mandibles:

  • While the temperature is warm – daytime – the fluke does nothing. This allows the ant to act normally during the day when all the other ants are active.
  • When the temperature drops below 10C – nightfall – the fluke takes over. It makes the ant climb a blade of grass and latch onto it with its mandibles. The ant remains suspended from the grass all night.

  • When the temperature rises above 10C again – daytime – the fluke relaxes control and the ant resumes its normal activity.

This cycle continues until a herbivore who is out for a stroll one evening happens to eat the blade of grass – ant, flukes and all.
The herbivore digests the hapless ant, releasing the flukes who spread out into its stomach where they can meet like-minded individuals.
Pairs of the now-adult flukes perform a special hug which results in the creation of a fluke egg.
The unlucky host of the party stops eating; it weakens and eventually dies. But before passing on, the herbivore – you guessed it – poops out the fluke eggs, taking us all the way back to step 1.

As I mentioned up top, this is another example of the viral model:

The fluke spreads to ever more sites by hitching a ride on snails then herbivores then snails etc, occasionally getting carried by one of them to a new place it's never been before.

I know this whole process sounds like it comes from a bad ’60’s movie, but it’s real and actually pretty neat. Makes you wonder, how on earth did the fluke evolve in such a specialized manner..?

...And are there any other mind-controlling parasites out there? (Hint: yes.)

(I learned most of this from and some of it from Behavioral and Morphological Changes in Carpenter Ants Harboring Dicrocoeliid Metacercariae, Carney 1969.)

* I’m sorry that 2 out of 3 posts involve the wonders of poop as a transport mechanism. I’m not obsessed with the idea; apparently excrement is just far more useful to the parasites of the world then I ever knew***!

** If you think eating snail-ooze is gross, then why didn’t you complain about the part where the snail ate the herbivore crap?? Maybe you’re just more open-minded than I…although I think that this would show rather conclusively that open-minded people contain fluke worms…

*** By the way, this is why you’re supposed to wash your hands after going to the bathroom! Who knows, if too many people ignore this rule, maybe a similar parasite will evolve that causes people to leave the norms of human society and stay up all night consuming grass…Wait a second: are we too late?? Is it
already among us…?

(picture found randomly at

Tuesday, August 7, 2007

The Viral model giveth, and it taketh away

So the last post showed one way in which a species improved its odds of success in the world by harnessing the power of the viral model. Here's a look at another organism which uses the model to deadly effect, simultaenously strengthening its species while devastating another.

Behold the Mountain Yellow-Legged Frog!

Doesn't look very threatening, does it?
...And actually, it isn't. But this is:

That is a picture of Batrachochytrium dendrobatidis (Bad End, if you will) - a fungus which is lethal to various amphibian species such as frogs, toads, and salamanders. In this picture, the fungus has taken up living under the skin of some hapless mountain frog. The frog won't survive this encounter - it's headed for a bad end.

The fungus reproduces by firing a spore (a fungus seed) out into the world. The Bad End spore lives in water, mud, and dirt; it is very tough and has been shown to survive up to 10 years in a site without a host.

Nobody knows exactly how Bad End kills its host, but we do know that it has spread - fast. The spores are skilled at catching a ride on the feet of passing animals - including the people who are actually out there researching the Bad End! This allows the spore to hitchhike out of the immediate area and spread the species to more and more sites.

Does that ring any bells? How about this?

Why do we care? Well, to quote somebody smarter than me*:
"Over the past 30 years, [the frog] has disappeared from up to 95 percent of its historic range, and its absence is impacting other organisms. Garter snakes that used to prey on these frogs are now declining. The frog's decline is leading to an unraveling of a high-elevation ecosystem."

Unraveling ecosystems are A Bad Thing. Even if you have zero ounces of pity, or (correctly) consider this just another case of natural selection determining which organisms deserve to survive, you may well enjoy the predictability of your daily life as a human being in an affluent society. The destruction of entire ecosystems can have unpredictable effects: the world is a complex system, and the obliteration of a big chunk of it can really come back to hurt us and our way of life in ways we can't predict.

So you don't even have to be nice to want to help the frogs - you just have to be selfish!

*I learned about chytridiomycosis here and here.
(frog image from this site, where it is credited to David Liittschwager.)
(fungus image lifted from this site, where it is credited to A. Pessier, University of Illinois.)