A 2011 field study in Thailand found fungus-infected ants bite a leaf vein about 25 cm above the ground, around solar noon, and stay locked on after death.
The ants did not wander off to die just anywhere. Every infected ant the team found before its final bite was walking alone on low plants, and only between 9:30 a.m. and 12:45 p.m. Each one bit into a leaf less than 3 metres from where it was first seen.
That is from a field study of the fungus Ophiocordyceps unilateralis and the carpenter ant Camponotus leonardi in a Thai rainforest, published in BMC Ecology in 2011 and led by David Hughes, then at Penn State University and the University of Copenhagen.
Down from the canopy
These ants normally nest about 20 metres up, in the forest canopy, the paper notes. Infected ants end up far below, in the humid understory. The team saw them walk at random rather than in a straight line, and fall again and again in convulsions, which kept them from climbing back up.
They die on sapling leaves about 25 cm above the soil, a narrow band where conditions suit the fungus. Dead ants pile up in patches the researchers call graveyards.
The bite at noon
The final bite was abrupt. In the 16 bites the team timed, it came around solar noon. The sun was at about 80 degrees at the moment of biting, close to its highest point during the study.
The ants always bite into a vein on the underside of the leaf, the paper says, never the edge or the top surface.
Locked on
Inside the head, the jaw muscles waste away, and fungal cells crowd the head. The paper calls the result lock-jaw: the ant stays attached to the leaf after it dies. Ants seemed to stay alive for as long as six hours after biting.
What the study can't tell you
This is one ant species and one fungus, with the ants watched over eight days in September 2007, with small numbers (21 ants followed, 16 bites timed). The noon timing might be a direct cue from the sun, or from heat or humidity that rise with it. The paper does not say which.
