It is the monsoon. The sudden increase in humidity has something stirring that is neither an animal nor a plant. In the moist corners of the woods, little mushrooms are sprouting; they are spore-bearing “fruiting bodies” of fungi that remain hidden as colourless masses (mycelia). Fungi are generally considered to be members of a “third kingdom”, next to animals and plants.
Most fungi are primary decomposers and degraders. Some, called mycorrhizal fungi, lead a symbiotic life with plants, enabling the latter to absorb minerals in return for sugars. Some are pathogenic (disease-causing), infecting living plants and animals. Most of the fungi release spores to disperse and colonise different geographic areas.
On one of her foraging trips, an ant steps onto a fungal spore (called conidium) or breathes one in through her respiratory opening (spiracle). All animals, including us, inhale fungal spores all the time, but the fungal spore picked up by this ant intended to find her. This spore travels through her intercellular spaces and slowly feeds off her tissues. One monsoon day, she clambers upon a leaf as if in a trance. This isn’t her usual behaviour. She drops her head, and her legs barely move. Sometimes, an infected ant like her may bite into the base of a branch or wrap her legs around a branch due to contraction of the muscles in what is called a “death grip”. Through the night, a stalk of pale-white mass erupts from her head-joint, her exoskeleton separates segment-for-segment as the fungal filaments glue her in place.
Pathogenic fungi are common. Most are known to infect plants; there are about 7,000 species that cause plant rust disease alone. Fungi that cause diseases in animals are also diverse; over 1,000 species are known. Some cause diseases in humans as well. All it takes is a spore entering the body. For instance, the fungus that entered the ant’s body and turned her into a walking zombie is a type of fungus called entomopathogenic fungi.
There are at least two groups of distantly related fungi that infect invertebrates. Biologists are just beginning to understand how fungi manipulate host behaviour. Indeed, the dramatic change we observed in the ant we followed was caused by this fungus. Once the time for spore-formation comes, it uses a chemical signal that makes the host seek out open areas where the chances of the spores spreading through the air are promising.