The Last of Ants

Kelly’s Field Notes

Common Name: Parasitic Fungi aka Zombie Fungus

Kingdom: Fungi

Order: Hypocreales

Family: Cordycipitaceae, Ophiocordycipitaceae

Genus: Cordyceps, Ophiocordyceps, Beauveria

Species of Note (There are around 400 species of Cordyceps fungi, there are 26 species of Ophiocordyceps currently described):

Cordyceps sinensis - commonly called the caterpillar fungus. Caterpillars infected with this species of Cordyceps are sold as food and as folk remedies in East Asia.
Ophiocordyceps unilateralis - commonly called the zombie ant fungus. Discovered in 1859, this fungus causes the ant to leave its nest and head to the more humid forest floor (suitable for fungal growth). The ant then attaches to the underside of a leaf by hanging on with its mandibles, then dies. The fungus bursts forth from the ant’s head and eventually sends spores out. Infection to death takes between 4 and 10 days. This fungus is susceptible to other fungi which sometimes limits its destruction of ant colonies.
Beauveria bassiana - commonly called white muscardine disease. It infects a variety of insects, forcing them to remain in temperature zones fitting for the fungus which also prevents the ladybugs from behavioral fevering (moving to warmer temperature where the fungus cannot survive).
Not a Cordyceps but interesting - Septobasidium sp. - fungi of this genus parasitize scale insects. Scale insects puncture tree bark to feed on sap and will remain in place, in large colonies, for the rest of their lives. The fungus then moves in and penetrates the scale insects, feeding off of them without killing them and in the process making them sterile. The fungus does not infect every colony member, allowing uninfected members to reproduce and continue to provide a food source for the fungus. The fungus also acts as a deterrent to scale insect predators.

Special note: there are tons of websites spouting the use of Cordyceps as supplements and remedies. Please consult a doctor before trying any new supplements as at best they may do nothing and at worst you may have an allergic reaction or other dangerous adverse side effects.

What Are Fungi:

Fungi are eukaryotic organisms (membrane bound organelles, can be unicellular or multicellular) that are mostly terrestrial but are found in aquatic ecosystems as well. Fungi are usually categorized into 4 groups: Chytridiomycota (chytrids), Zygomycota (bread molds), Ascomycota (yeasts and sac fungi), and the Basidiomycota (club fungi). Cordyceps are in the Ascomycota group and are called sac fungi. Ascomycota are named for their ascus which is a sexual structure containing non motile spores (though some do reproduce asexually). Some familiar Ascomycota are brewer’s yeast, bread yeast, and morels.

Life Cycle:

Special note: Cordyceps fungi cannot currently infect humans due to our high body temperatures. We run so hot we’d denature their proteins, so now worries as you read through this!

Ophiocordyceps unilateralis and O. pseudolloydii as examples-

Infection - an ant becomes infected with the fungus.
Leaf Gripping - the ant climbs the stem of a plant and the underside of the leaf with its mandibles. This gives the fungus the best chance at spore dispersal.
Ant Death - the ant succumbs to the fungus and dies after 4 to 10 days, remaining attached to the leaf.
Fungal Stroma - seven days after death, a fungal stroma (compact mass of mycelium, in this case a long thread-like protrusion) protrudes from the ant’s thorax or base of the head
Perithecia Pad - the perithecia pad forms from the stroma, this is the fungal fruiting body which will spread spores. O. unilateralis reproduce via sexual reproduction and O. pseudollodii reproduces aesexually.

Several species of ants are susceptible to this fungal infection, one paper noted 10 species in their study. Within that study ants seemed to climb plants randomly. The stroma to perithecia pad can happen multiple times on one ant corpse. We don’t really know what compels the ants to seek the underside of a leaf - speculation is either manipulation of muscle fibers and/or chemical manipulation of the brain. The fungus does not enter the ant’s brain. The fungus forms a scaffolding around the ant muscle bundles and interfere with the ant’s nervous system, controlling it at the muscles. Unlike the television show and video game, hosts are not infected through the mouth. Likely what happens is floating spores attach to the ant’s carapace and slowly penetrate their body. Additionally, Ophiocordyceps does not communicate through a wide fungal network, each infected individual is contained.

Cordyceps sinensis -

Infection - In late Autumn the fungus infects ghost moth caterpillars (several species) living in the soil. 4th and 5th instar caterpillars are more likely to become infected while they are shedding their cuticles. They have more resistance to the fungus in the pupal stage.
Caterpillar Placement/Death - Infected caterpillars move to 2-5 cm below the soil’s surface, facing head up, then die.
Fungal Stroma - In Winter, a stroma grows out of the caterpillar’s head, remaining below the soil’s surface.
Fungal Stroma Emerges From Soil - In Spring, the stroma emerges from the soil and produces fruiting bodies, releasing spores, and beginning the cycle anew.

In the presence of other helpful fungal species the mummification time of the caterpillar can speed up to 3 - 5 days from 50 days. It also increases the infection rate from 3% to 60%.

Beauveria bassiana -

Infection - B. bassinaba is found in soils where it is picked up by host insects.
Death - In about 3 days the insect is killed by the fungus.
Mycelium Formation - Mycelium forms about 1 day after death, then a day and a half for spore formation.
Spore Discharge - Five days after death spores are discharged from the body and the cycle starts anew.

This fungus has been used as biological control of many agricultural pest species and mosquitoes. While considered safe for humans to come into contact with, there has been at least one case of an immunocompromised human becoming infected. It has also been known to infect captive alligators and tortoises.

The Last of Us:

In the opener for The Last of Us we see a variety of beautiful fungi growing in a sort of Game of Thrones style opening credits. It’s important to note that the slime mold shown is not a fungus. Slime molds used to be a subkingdom of fungi but are not classified as such any longer. Slime molds now fall under Phylum Amoebozoa (that’s right, with amoebas!).

There is a scene in the show where hyphae (branching filaments of mycelium) are shown growing into “zombie” bodies and connected to each other through the ground. Cordyceps do not work like this, they are pretty localized to their hosts.We do have species of fungi that are connected via a form of “fungal network.” They don’t really do this on their own! Fungi will use their mycelium to tap into tree roots and form vast networks where trees and fungi exchange chemical signals, water, and nutrients. Globally, it is expected that there are trillions of miles of fungal networks but locally they can extend for many miles.

The characters in the show discuss whether or not the fungus has intelligence and/or a consciousness. Intelligence and consciousness are slippery words in biology, but for the purposes of this podcast let’s talk a little about what that might mean in fungi in a general way. We know fungi respond to external stimuli. In lab studies, when hyphae are punctured or sliced the fungus will push resources to the injury to heal it. Hyphae growth rate and branching patterns shift in response to confinement. Fungi that infect humans also alter their growth forms when entering a host which protects the fungus before the human’s system can create an immunological defense. This is a type of forethought. There is a theory catching on called “cellular consciousness” that basically describes cells as “self-referential “knowing” problem-solving entities.” An important quote on this “The fungus is not thinking in the sense that a brained animal thinks, but some of the underlying cellular processes of signal transduction are bound to be homologous.” By homologous Dr. Money (2021) means thinking and the way signaling works through cellular processes are fulfilling the same purpose. Fungi will also move resources to nutrient deficient parts of their hyphae.

If we want to discuss learning, we see that as well. Laboratory studies have shown if shocked some fungi will immediately increase their growth rate after being shocked a second time. The fungi can remember being shocked for up to 12 hours. Another fascinating study shows memory and spatial navigation in fungi. The fungal colony was given a bait wooden block next to its main colony to grow towards, but when the blocks were separated the original block still grew in the direction of where the bait block used to be. And again, while not fungi, there have been many experiments done on slime molds that have similar outcomes, including finding the shortest path to food sources.

Fungi and Humans:

Psilocybin - “magic mushrooms” have been used by many cultures for recreational use and more recently research is being done on their medicinal uses.

Antibiotics - without fungi we would not have penicillin and other antibiotics.

Yeast - from bread to beer, we use this very handy fungi all the time in cooking and brewing. The “mother” in your favorite kombucha is a symbiotic growth of yeast and bacteria.

Fungal Infections in Humans - while none of them create zombies, humans are infected with fungi every day. Sometimes it’s fairly benign, as with Athlete’s foot or a yeast infection, or it can be incredibly dangerous as with Histoplasmosis, caused by a type of fungus that infects the lungs. Claviceps purpurea, or St Anthony’s Fire, causes ergotism through consumption, which causes symptoms of hallucinations and convulsions among other issues.