Ophiocordyceps Unilateralis and its Zombie Ants
Not the catchiest title I’ve ever written I appreciate!
Many of you will remember my long held appreciation for the life and work of Alfred Russel Wallace and this post is actually going to be dedicated to a discovery of his – the fungus Ophiocordyceps Unilateralis. Cordyceps are a group of parasitic fungi containing over 400 described species. Their hosts are usually insects and each variant often has a particular insect species that it targets. The host of choice for this variant is the Camponotus leonardi ant, otherwise known as the Carpenter Ant, which is found in many forested areas around the world and the two coincide in the tropical forests of Thailand and the Brazilian rainforests.
Spores of the fungus attach to the outside of the ant and, using both mechanical pressure and enzymes, break through the exoskeleton, gaining access to its juicy insides. The fungus then spreads through the ant’s body causing it to convulse uncontrollably and dislodge the ant from its nest and ground it. The fungus then manipulates the ant into climbing the tree again until it is in the perfect position. This means the fungus drives it’s ant until it finds a leaf that meets it’s very exacting criteria – a mean height of 25.20 (± 2.46 cm) above the forest floor, on the northern side of the plant, in an environment with 94–95% humidity and temperatures between 20 and 30°C (68 and 86°F). Once there the ant will clamp it’s incredibly strong jaw onto the main vein on the underside of the leaf and hang.
This is common referred to as the death grip and is vital to the fungi’s lifecycle. The fungi’s hyphae then invade more soft tissues and structurally fortify the ant’s exoskeleton while more mycelia sprout out of the ant, securely anchoring it to the underside of the leave while secreting antimicrobials to ward off competition. When the fungus is ready to reproduce, its fruiting bodies grow from the ant’s head and rupture, releasing the spores. The process from infection to the sporulation can take anywhere between 4-10 days. Infections may lead to 20 to 30 dead ants/square meter and O. unilateralis has therefore been known to destroy entire ant colonies. In response, ants have evolved a clever ability to sense that a member of the colony is infected and healthy ants will then carry the dying one far away from the colony in order to avoid fungal spore exposure and therefore the potential eradication of their population.
Many of these entomopathogen, or insect-pathogenising fungus, exist in nature and each causes a different behaviour that is to the benefit of the fungus but it’s important to note that this isn’t just a characteristic of fungi and is a talent utilised by a variety for parasites. For more examples, check out
- Dicrocoelium dendriticum (Liver fluke)
- Toxoplasma gondii
- Hymenoepimecis argyraphaga
- Myrmeconema neotropicum
- Spinochordodes tellinii
- Glyptapanteles sp.
- Leucochloridium paradoxum
- Sacculina carcini
- Schistocephalus solidus
- Euhaplorchis californiensis
- Heterorhabditis bacteriophora