James Cook University Sign (© Magi Nams)
This morning, I felt rather forlorn at having to miss a James Cook University afternoon seminar about ‘zombie’ ants due to an appointment. However, much to my delight, ABC Radio National re-broadcast Saturday’s All in the Mind program early this afternoon, and one of the topics covered in a feature about how parasites change brains was ‘zombie’ ants.1
The interview I heard on ABC radio and the seminar Vilis attended at JCU both featured the research of Dr. David Hughes, a behavioural ecologist traveling the world in search of ‘zombie’ ants, that is, ants infected with a parasitic, behaviour-altering fungus.1 Dr. Hughes has studied this parasitic relationship in Thailand, Brazil, and the southern USA,1 is currently in Townsville after sampling rainforests in the Cairns region for a week (where he found new species involved in this deadly relationship), and plans to also sample rainforests in the Amazon, Honduras, Sri Lanka, and Malaysia.1 As Vilis told me this evening, “He had a map with all kinds of dots showing interesting places he’s been, and more dots showing interesting places he’s going to.”
So, more specifically, what are ‘zombie’ ants? In a nutshell, they are ants which have been invaded by spores of a parasitic fungus and subsequently display radically altered behaviour that is directed by the fungus.1 The ants at the heart of the matter are carpenter ants living in the canopies of tropical rainforests.1 The parasitizing fungus Ophiocordyceps also inhabits such forests and releases spores that waft through the air and settle on prospective hosts.1 When one of the fungal spores settles on a carpenter ant, it bores through the ant’s cuticle and into its body.1 Inside the ant, the fungus grows, spreading through the body and entering the head, but not the brain.1 It exerts its control over the ant by releasing a ‘cocktail’ of chemicals (unknown as of yet) which affect the ant’s brain and cause it to alter its behaviour.1
Carpenter ants are normally found within nests in the relatively dry and airy treetops of tropical rainforests in many parts of the world, including Australia.1 However, when infected by Ophiocordyceps, carpenter ants leave their nests and descend to a particular height in the understory vegetation where environmental conditions are optimal for the growth and reproduction of the controlling fungus (treetops are too dry and the forest floor is to wet and disturbed).1 When the ants have reached a likely spot, they stagger around for a couple of hours and then crawl onto the underside of a leaf having a particular orientation and bite hard and very precisely into a main leaf vein.1 Then they die and remain suspended upside down, their jaws clutching the leaf in a ‘death grip.’1
This is when the fungus really goes to town. After the ant’s death, the parasite uses the ant’s muscles and other tissues to fuel its growth within the ant.1 Then it changes its strategy, growing out through the ant’s body surface to attach the ant more firmly to the leaf.1 Ultimately, the fungus produces a fruiting stalk that shoots up out of the back of the ant’s head and releases spores that will infect more ants.1 I went online to research this topic more thoroughly and came across a photograph of a ‘zombie’ ant that had expired at the will of the fungus. The ant shell still looked like an ant, but it had the equivalent of a big, brown street lamp pushed upward through the back of its head.2
Aside from being a fascinating and grisly ecological story, the ‘zombie’ ant phenomenon has significant implications for medicine. While the parasitic fungus is growing inside a dead carpenter ant, it defends its life-sustaining catch against other consumer organisms that also want to eat the ant by pumping out antibiotics that act as repellents.1 And this isn’t a new story. The fungus/ant interaction has been going on for millions of years, as evidenced by a fossil leaf showing the distinctive and characteristic bite marks of ‘zombie’ carpenter ants directed to their deaths by Ophiocordyceps. 2 I think there’s a new ‘zombie’ movie in the making here.
Ruby (© Vilis Nams)
Note that I’ve published the post for September 29 – Making Camel Tracks in the Desert.
References:
1. ABC Radio National, All in the Mind. We have ways of making you think – Parasites on the brain! Saturday, October 9, 2010. Accessed 11-Oct-2010. http://www.abc.net.au/rn/allinthemind/default.htm
2. Kate Larkin. Attack of the ancient ‘zombie’ ants. Naturenews. 17 August, 2010. © Nature Publishing Group. Accessed 13-Oct-2010. http://www.nature.com/news/2010/100817/full/news.2010.415.html
