The start of the life cycle of the Ophiocordyceps unilateralis is as a spore on the forest floor. 26 species fall under this clade, most of which are native to rainforests in South America, the Indo-Australian region and SE Asia. Each species affects a different ant. Gaps in the canopy of trees make it difficult for some ants to traverse the rainforest, so carpenter ants travel on the ground. If the spore attaches to the exoskeleton of an ant, it grows hyphae (threadlike strands) to breach it through mechanical pressure and enzymes, such as chitinase, lipases and proteases.1
Once inside the body, the fungus starts to grow and steal nutrients from the ant tissue, and fungal tissue grows. This produces enzymes that start to break down the ant’s tissue, and fungal tissue replaces it. Once fungal cells reach the brain of the ant, and the fungus is of sufficient size (which usually takes 16-24 days)2, it begins to secrete chemicals and take over the ant's central nervous system, which enables it to manipulate the ant.
Guanidinobutyric acid (C5H11N3O2) is one of the compounds secreted by the fungus, whilst normally present in the ant’s CNS, the fungus raises the levels significantly. This causes compulsions, possibly due to serotonergic neurons overwork (neurons in charge of serotonin production) and accumulation of free radicals.3 Another compound, hypoxanthine, has deleterious effects on neural tissue, which allows the fungus to alter the motor neurons of the ant.
This causes the ant to behave strangely, leaving the safety of the ant nest or ant line, and becoming unresponsive to stimuli. 4 general behaviours are induced in the ant, and this is triggered by the time of day according to the ant’s body clock.4 The ant becomes hyperactive, causing it to depart from other ants and it will begin to seek out light (phototaxis) as this will generally cause the ant to climb a tree and light is beneficial to the fungus. The fungus will also induce muscle growth in the ant in the leg and mandible areas so that it can grip surfaces better as the fungus causes it to climb a tree through gravitaxis. The ant only needs to climb about 20cm, as this means that the temperature and humidity of the area is ideal for fungal growth. The final act of the ant is to clamp its mandibles on the underside of a leaf, commonly referred to as the ‘death grip’. This prevents the ant from falling after its remaining tissues are broken down, and allows the fungus’ fruiting body to grow correctly. Immediately after locking its mandibles, the fungus releases chemicals that cause the mandibular muscles to atrophy, so the ant cannot remove them. Once the fungus fully grows, it will release spores that fall in a cone shape to the ground which will infect any ants that pass underneath.
References:
1 - Ophiocordyceps Unilateralis, Evans et al. (2011) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3204140/
2 - Zombie-ant fungus, Petruzzello (2023) https://www.britannica.com/science/zombie-ant-fungus
3 - A role for guanidino compounds in the brain , Hiramatsu (2003) https://pubmed.ncbi.nlm.nih.gov/12701810/
4 - Mechanisms behind the Madness: How Do Zombie-Making Fungal Entomopathogens Affect Host Behavior To Increase Transmission?, de Bekker et al. (2021) https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8546595/