We've Got a Walker

Neha Patel

Imagine a world where a parasite can enter your body and control your movements, only to be killed by the invader exploding from your head. This sounds like something from a gruesome sci-fi thriller, but for the Carpenter ant (Camponotus leonardi) this is a reality when it becomes infected with the parasitic fungus Ophiocordyceps unilateralis. These ants are popularly nicknamed "zombie ants" because of the alteration in behavior when infected. There are many different types of this genus of fungus, which affect different species of insects, but for this blog we will focus on the interaction between these two species.

An ant with a hyphal stalk growing out of its head (source).

The fungus O. unilateralis is found in tropical rainforests around the world, but much of the studies have been done in Thailand. To observe the behavior of the infected ants, Hughes et al. followed ants after they were taken over by the parasite. The parasite was ingested by the ants when they foraged in an area with the fungal spores. After the C. leonardi was infected, it went through convulsions, causing it to fall down from the canopy away from the colony. This can be seen as beneficial to the fungus, since ants will get rid of their dying brethren, and the parasite might not survive to reproduce. The zombie ant then walked alone on low vegetation, and further more only at a specific time during the day (9:30am-12:45pm). They tended to walk on leaves, which is not a normal behavior for uninfected ants. The ants would eventually bite down onto the underside of leaves, always around noon, and would stay attached and die there. After dissection of the mandible, it was shown that the fungus destroyed the muscle, which is what caused the ant to lockjaw and hold fast onto the leaf. The fungus would then grow out and attach more firmly to the leaf. Finally, a large hyphal stalk would grow from the back of the ant’s head and release spores a few weeks later. The leaves the ants attached to would always curiously be about 25cm above ground.

Planet Earth video showing a different species of ant (bullet ant) being body-snatched by a different Cordyceps fungus, complete with creepy music and all.

In 2009, researchers Andersen et al. published an experiment that confirms the location of the final resting place of the ant is very specific, and contributes to the optimal growth and reproduction of the parasite. This change in behavior of the host is called an extended phenotype of the parasite. To start their study, Andersen et al. categorized where in the trees the ant graveyards were showing up, and what part of the leaf the ants attached to. The infected ants were found about 25cm from the ground, on the underside of a leaf, and 98% of the time biting a vein. Then, to see if this place allows for optimal growth of the fungus from the ant, infected dead ants were moved either all the way to the ground, or 15m up the trees. Heights of the hyphal growths were measured in each place as a measurement for reproductive success. Most of the ones displaced on the ground disappeared, and the ones that remained showed stunted growth, inferring that it wasn’t an ideal habitat for the fungus to grow. The ones higher up in the canopy also had reduced height, possibly because the lower humidity and higher temperatures caused desiccation. The authors concluded that it was probably no coincidence that the average height of the final resting place was at 25cm, which is a above the ground, and has high humidity and lower temperatures.

There is hope for these ants yet, though! It has been noticed that these ants occupy canopies high up, which allows them to avoid the ground where the fungus is waiting for them. Also, they spend as limited time on the ground as they have to before climbing up another tree. There seems to be a risk trade-off of braving the ground and the current nutrient supply of the tree that the ants will consider. This is suggested to be an adaptive response to this insidious and deadly fungus.

Zombie kisses (source).

Much research has been done on the behavior of the body-snatched ants, but not the mechanism of how the fungus achieves this. Despite the fact that scientists have no idea how the fungal parasite controls the behavior of its host, they are already trying to research compounds produced by O. unilateralis for human use. The compounds of interest are napthoquinones, which have been shown to have antimalarial, and anti-tumor properties. In addition, in acidic environments, they turn a bright red color. So, in 2005, Unagul et al. published a paper on how to optimize the extraction of a napthoquinone compound called 3,5,8-TMON, which has a potential of being used as a red dye in food and cosmetics.

Even though these compounds can be useful for our lives, I don't know how I feel about getting them from a fungus that turns something into a zombie. I'm probably consuming too much sci-fi and zombie media ("The Walking Dead"), but I'm scared culturing these in a laboratory will lead to the body-snatching of humans. As a scientist, I know there is a very low probability of this happening, but I am okay with being ridiculous and paranoid!

Now, I leave you with a bad photo edit I put together of Rick Grimes of "The Walking Dead" being chased by zombie ants:

Run!!!!!! (source) (source)

References:

Andersen, S.B., S. Gerritsma, K.M. Yusah, D. Mayntz, N.L. Hywel-Jones, J. Billen, J.J. Boomsma, and D.P. Hughes. 2009. The life of a dead ant: the expression of an adaptive extended phenotype. The American Naturalist 174:424-433.

Hughes, D.P., S.B. Andersen, N.L. Hywel-Jones, W. Himaman, J. Billen, and J.J. Boomsma. 2011. Behavioral mechanisms and morphological symptoms of zombie ants dying from fungal infection. BMC Ecology 11:13.

Isaka, M., P. Kittakoop, K. Kirtikara, N.L. Hywel-Jones, and Y. Thebtaranonth. 2005. Bioactive substances from insect pathogenic fungi. Accounts of Chemical Research 38:813-823.

Pontoppidan, M., W. Himaman, N.L. Hywel-Jones, J.J. Boomsma, and D.P. Hughes. 2009. Graveyards on the move: the spatio-temporal distribution of dead Ophiocordyceps-infected ants. PLoS ONE 4:e4835.

Unagul, P., P. Wongsa, P. Kittakoop, S. Intamas, P. Srikitikulchai, and M. Tanticharoen. 2005. Production of red pigments by the insect pathogenic fungus Cordyceps unilateralis BCC 1869. Journal of Industrial Microbiology and Biotechnology 32:135-140.