A fungus that turns ants into zombies has survived the global shift from tropical to temperate forests by subtly altering its victims’ behaviour.
Zombie ants are actually carpenter ants from the genus Camponotus which have been infected by a species of parasitic fungus, the best known of which is Ophiocordyceps unilateralis.
In tropical areas, the fungus induces the ant to climb high to the top of a tree and grasp firmly on a leaf with its jaws. It then remains motionless while the fungus draws nutrients from its body. Its elevated position facilitates the dispersal of fungal spores onto the forest floor – and other ants – below.
In temperate areas, however, the behaviour is slightly different. After infection, the ant climbs into the treetops and then grasps onto a twig instead of a leaf.
Zombie ants occur on every continent except Europe. But in 2010, fossil evidence of a zombie ant biting a leaf was discovered in Germany. The fossil dated to 47 million years ago – to a time when wet, evergreen forests circled the globe, including what is now Europe.
Intrigued by the discovery of the German fossil, a team led by David Hughes from Pennsylvania State University in the US set out to discover what factors influenced the zombie ants’ choice between biting a leaf of a twig.
The researchers looked at samples of the ants in museums, photographs and previously compiled datasets. They were also ably assisted by an enthusiastic zombie ant citizen scientist.
“We had a great asset here who is Kim Fleming,” says Hughes.
“Kim is a citizen scientist whose property in South Carolina is festooned with zombie ants hanging on trees. As both an excellent photographer and natural historian, Kim was able to collect detailed data for us on the zombie ants over 18 months by taking continual images of samples on her land. This was precious data that would have been very hard to collect.
“Kim is an author of this paper, but perhaps the greatest recognition of her importance is that the fungal species infecting carpenter ants in South Carolina is now named after her, Ophiocordyceps kimflemingiae.”
The study established that zombies in the tropics always bite on leaves, and those in temperate areas always bite twigs. Phylogenetic relationships between the ants were teased out, and they determined that leaf-biting was an ancestral trait, while twig-biting evolved at different times in different continents – showing remarkable convergent evolution on the part of the fungi.
The analysis confirmed that twig-biting developed in response to the earth’s changing climate and forests.
“What is remarkable here is that we have shown that the complex manipulation of an animal by microbe has responded to selection pressure the climate imposes on animals and plants,” adds Hughes.
“That was a cool finding that really excited us.”
The findings were published in the journal Evolution.