A fungus hijacks ants, steering them to the perfect spot to die.
In tropical forests, carpenter ants infected by the fungus Ophiocordyceps unilateralis leave their colony, climb a plant to about 25 centimetres and clamp their jaws onto a leaf vein. Then they die, and a stalk sprouts from the ant's head to shower spores onto the forest floor below.
The fungus was discovered by Alfred Russel Wallace in 1859 and targets ants of the tribe Camponotini, which includes carpenter ants. Its main host, Camponotus leonardi, nests high in the rainforest canopy and travels along aerial trails. Where gaps in the canopy are too wide to cross, the trails dip briefly to the forest floor, and there spores stick to the ants' exoskeletons and break through using pressure and enzymes.
Inside, the fungus spreads as yeast-like cells, massing beside the brain and in the muscles and secreting chemicals that alter behaviour. The infected ant suffers convulsions that knock it out of the canopy. It then climbs a plant and bites a leaf vein with abnormal force, typically about 25 centimetres above the ground, on the north side of the plant, where humidity is 94 to 95 per cent and temperatures 20 to 30°C: ideal conditions for the fungus. One study in Thailand found the biting synchronised around solar noon. The jaw muscles then waste away, locking the ant in place in what is called the death grip.
The fungus kills its host, reinforces the body from inside, anchors it to the leaf and secretes antimicrobials to keep competitors out. Within 4 to 10 days a stalk grows from the head and releases spores. Dead ants pile up in 'graveyards', sometimes at more than 25 per square metre. A 48-million-year-old fossil leaf from Germany's Messel Pit bears the same dumbbell-shaped bite marks, suggesting the manipulation is ancient.
It is one of the best-known examples of an 'extended phenotype', in which a parasite's genes show up in its host's behaviour. The ants fight back: they groom spores off each other, carry infected nestmates away from the colony and keep their trails off the ground where they can. The fungus has its own enemy too, a parasitic fungus that leaves only around 6 to 7 per cent of its spore-producing structures viable. What was thought to be one species now looks like many, each tuned to a particular ant. And despite its fame from The Last of Us, some scientists call the label 'zombie ant' catchy but misleading.
Source: Wikipedia — Ophiocordyceps unilateralis · Text summarised from Wikipedia (CC BY-SA 4.0)
