Carpenter ant infected with parasite clings to a twig

A carpenter ant left biting and clinging to a twig after its death from a fungal infection. Credit: Kim Fleming

Evolution

Zombie ants’ final resting place sealed by the trees

Climate influences an infected insect’s last act.

A fungus that turns infected ants into powerless ‘zombies’ has adapted to climate conditions in different locales by modifying its victims’ behaviour.

After the fungus (Ophiocordyceps unilateralis) infects an ant, it commandeers the host’s nervous system, forcing the insect to march up a tree to grab a twig or leaf in its jaws. The ant dies quickly but remains locked in place. Eventually, the fungus bursts through its head, spreading fungal spores.

David Hughes at Pennsylvania State University in University Park and his colleagues examined zombie ant cadavers and images from collections around the world, and recorded what the ants were biting when they died. In tropical climates, where trees do not shed their leaves, zombie ants were more likely to bite leaves than twigs. But the reverse was true for ants in temperate climates, meaning insects were likely to stay put as leaves dropped.

Analysis of O. unilateralis DNA showed that the fungal variety that encourages twig-biting evolved independently several times.