{ 2008 03 19 }

Possessed ants and other animals - What has gotten into them?

What do you make of ants that climb to the top of leaves and stay there, or grip onto the tip of leaves with their mandibles and hang there until they die? These are behaviours atypical of them - are they possessed? Have they gone mad? Take a look at the following video for an explanation:

In the video, spores from a parasitic fungus from the genus Cordyceps enter a bullet ant, infecting its body and its mind. The spores enter through the ant's spiracles, which are holes in its exoskeleton that allows it to breathe. The spores then attach on to the ant's soft tissues (though avoiding its vital organs) and start to grow. 

Once it reaches sexual maturity, it is time for the fungus to sporulate. This is when the ant is directed to climb upwards and grip onto the plant with its mandibles.
How is this so? At this point, the fungus starts growing into the ant's brain, and produces chemicals which alters the way in which the ant perceives pheremones. The fungus then manipulates the ant to perform such a behaviour.
Why the specific behaviour of climbing upwards? Well, the parasite directs the ant to climb upwards so that when the fungus disperses its spores, the airborne spores can land on a bigger area and potentially affect more insect hosts. Thus, when the ant is high up off the ground, the fungus then consumes the ant's brain and, like out of a science fiction movie, shoots out of its head and grows for 3 weeks before exploding and spreading its airborne spores over the area. And the cycle repeats.

One interesting snippet of information from the video is how the other bullet ants from the colony were able detect the infected ant and also recognised the danger it posed to them, moving the infected ant as far away from the colony as possible. Some questions that comes to mind: How are they able to detect the infection? Is it a learnt or an instinctive behaviour? After millions of years of co-evolution between the ants and the fungus, ants have learnt that to survive, they had to be able to smell and also chemically detect the parasitic fungi infection. They would then immediately take action to prevent the infestation of their whole colony. To do this, they have in place a sentry system, where soldiers guard the queen and her brood by preventing entry to enemies, included infected ants. When these ants are detected, they are quickly killed and taken far away from the nest.

Another interesting bit was on how there are thousands of different species of Cordyceps fungi, and each targets a specific, unique insect host. The series of images shown in the video on this was, for me, oddly beautiful. It seemed as if different varieties of colourful fungi were growing out of strange yet creative flowerpots that only Mother Nature could have provided. This also goes back to the previous point where co-evolution has also resulted in the need for fungus to recognise its specific insect host through biochemical signals.

"In general, members of Cordyceps show strict host specificity, although the degree of specificity differs from species to species. Some parasitize only a single host species (e.g., Cordyceps sobolifera, strictly on the nymph of Platypleura kaempferi in Japan), while others utilize a range of hosts from a particular taxonomic group (e.g., Cordyceps militaris, on the pupa of various moths) (Shimizu 1994Citation ). The majority of Cordyceps species utilize insect hosts from various orders such as Homoptera, Lepidoptera, Coleoptera, Hymenoptera, and Diptera, suggesting that major host shifts between insect orders have occurred in the genus." (Interkingdom Host Jumping Underground: Phylogenetic Analysis of Entomoparasitic Fungi of the Genus Cordyceps, Molecular Biology and Evolution, 2000)

In the video, the main fungus shown was the Cordyceps unilateralis, which specifically targets ants.

 
(Image taken from http://www.utexas.edu/courses/zoo384l/sirena/species/fungi/)

This is a fly with the parasitic fungus, Cordyceps dipterigena.


(Image taken from http://www.utexas.edu/courses/zoo384l/sirena/species/fungi/)

And of course, if you haven't realised by now, the cordyceps used in Traditional Chinese Medicine (TCM), is also from the Cordeceps genus. The most commonly used species is Cordyceps sinensis, and is also known as terms such as caterpillar fungus and vegetable caterpillar. And, (gasp!) as you might have guessed, this species is not unlike its relative shown in the video except that rather than parasitise ants, it parasitises caterpillars! More specifically, the larvae of particular types of moths such as the bat moth, Hepialus armoricanus. The larvae is said to ingest or inhale the spores of the fungus. During the pupa stage underground, the fungus then dissolves the internal organs of the pupa, and fills up the hollow shell before bursting through the pupa and above ground to disperse its spores. Cordyceps is said to be effective in treating kidney and lung problems, and is a very popular choice of chinese medicine used.


(Image taken from http://taos-telecommunity.org/epow/EPOW-Archive/archive_2003/EPOW-030428.htm)

If you are interested in other parasite-host relationships, you can go to the following links:
Snails infected by parasitic worms, causing tentacles to look like prey for birds [video]
Black wasps lays eggs in aphids [video]
Ants infected by liver fluke worms display abnormal behaviour [video]
Suicidal grasshoppers [article]
Ants neglect own brood to take care of caterpillars [article]
Parasite make ants resemble berries [article]

As can be seen from above, most of the time, parasites manipulate their hosts' behaviour to benefit themselves. In the example of the bullet ant, the cordyceps fungus manipulated the ant's mind and made it climb upwards onto a plant, so that its spores can be dispersed onto a wider area. However, in this article, instead of feeding on more types of plants to provide a supply of live tissue for the parasites as food, the caterpillars that got infected started eating only plants that were toxic to the parasites. Why exactly does this happen is still not known to scientists, but this gives us a fresh insight on the parasite-host relationship where in this case, the host wins the battle for once.

Therefore, possessed ants and other animals - what has gotten into them? Now you can answer that question - parasites.

References:

by Chen Shang Teng Evelyn on 19-Mar-08 12:36 AM
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