Ophiocordyceps pseudolloydii

The Cordyceps fungus has become a fixture in popular media, at least as the go-to comparison/cause for fictional human zombies. The nominal Cordyceps that most people think of is probably Ophiocordyceps unilateralis - the infamous "zombie ant fungus". But what most people don't realise is that there isn't just "the Cordyceps fungus" - that is just a single species out of many ant-infecting fungi in the Ophiocordyceps genus. That's right - there are multiple species of zombie ant fungi and not they are all different. Each of them have evolved their own ways of getting the most out of their ant hosts.

Photo of infected ants from Fig. 1 and Fig. 2 of this paper

The species featured in today's blog post is Ophiocrodyceps pseudolloydii, and it is found in central Taiwan. This fungus specifically targets a tiny ant called Dolichoderus thoracicus. In the forest of central Taiwan are so-called "ant graveyards" - areas with high density of Cordyceps-infected zombie ants. Such sights are familiar to scientists who study these ant-fungi relationships, indeed, such "ant graveyards" have been found in other parts of the world where ants and Cordyceps fungi co-occur.

A group of scientists set out to document the behaviour and position of ants which have been mummified by O. pseudolloydii. One key thing they observed was that no matter where the zombie ants were found in the forest, the head of the dead ant tends to be pointed towards the direction of openings in the forest canopy. This indicates that the fungus might be using sunlight that comes through the canopy as a cue to steer the host ants into position.

Like other ant-infected Cordyceps fungi, O. pseudolloydii places the host ant in a position which is ideal for spreading its spores, without being dried out in open air. This usually means placing the ant underneath a leaf. But the fungus needs some way of anchoring the ant to the leaf before it can mummify the host and start sprouting into a fruiting body. Ophiocordyceps unilateralis induces a "death grip" in the zombified ants, whereby the ant locks its mandible around the vein of a leaf to secure it in place.

But O. pseudolloydii does not do that - instead of using the ant's mandible, O. pseudolloydii simply sprout a dense mass of fungal tissue which binds the ant to the underside of a leaf. So why doesn't it simply do what its more famous cousin does and make the ant bite down on a leaf vein? Possibly because the ant which O. pseudolloydii infects is much smaller than the carpenter ant which O. unilateralis parasitises. Compared with the carpenter ant workers which can grow up to 25 millimetres (about an inch) in length, the workers of D. thoracicus are merely 4 millimetres long. With such a tiny host a dense mat of fungal tissue is enough to anchor the ant in place.

By doing so, this might allow the fungus to save on making the mind-altering chemical to induce the leaf-vein biting behaviour, which can possibly allow it to produce more spores instead. All Ophiocordyceps pseudolloydii needs to do is make sure the ant is intoxicated enough to crawl to the right spot, and once that is done, the fungus will take care of the rest.

Reference:
Chung, T. Y., Sun, P. F., Kuo, J. I., Lee, Y. I., Lin, C. C., & Chou, J. Y. (2017). Zombie ant heads are oriented relative to solar cues. Fungal Ecology 25: 22-28.