Even mind-controlling, spore-erupting, zombifying parasitic fungi can't escape the effects of climate change. Researchers studying the habits of zombie ants observed that different species tend to hang onto different parts of a plant. While some show a predilection for leaves, others chomp on twigs – and this preference is determined by climate. The paper has been published in the journal Evolution.
Zombie ants sound like something straight out of a B-horror movie but the phenomenon is actually fairly common. It is found on most continents and affects roughly half of all carpenter ant species, each with its own personalized breed of fungi.
Individual ants are infected by falling spores and when the fungus takes hold, it eats its unfortunate victim from the inside. The fungus then gains control of the ant's movements and forces it to climb high into the branches of a tree where it latches onto a leaf or twig and doesn't let go. When the ant dies, spores spew from the body, infecting any unlucky ant that happens to be crawling below.
"In tropical areas, zombie ants bite onto leaves, but in temperate areas, they bite twigs or bark," David P. Hughes, associate professor of entomology and biology at the Pennsylvania State University, said in a statement.
A preference for twigs and bark in more temperate regions is likely because trees here are deciduous and there would be a serious shortage of leaves in the fall and winter months. In contrast, in the tropics trees are evergreen, meaning that leaves are abundant all year round.
Though now extinct in Europe, there is fossilized evidence to show zombie ants did once exist there. A fossil found in Germany that dates back to 47 million years ago shows an ant frozen in time biting into a leaf. All those millions of years ago, the vast majority of the planet's forests were evergreen and it is likely, the researchers say, that all zombie ants clung onto leaves.
But as temperatures cooled, forests closer to the poles became deciduous. Leaf-biting zombie ants wouldn’t have lasted very long as they would have dropped to the floor in the fall months, so the fungus wouldn't have had adequate chance to reproduce. Therefore, climate change triggered a process of natural selection. The result: zombie ants with a preference for twigs or bark.
During the study, researchers monitored ant behavior in three areas and found that their hypothesis held up. Not only that but in temperate zones, 90 percent of dead zombie ants wrapped their legs around the twigs to make sure their position was as secure as it could be.
This twig-wrapping and twig-biting behavior would likely have evolved at different times depending on the location.
"We can estimate that these changes occurred between 40 and 20 million years ago," explained Hughes. "However, because of the scarcity of zombie ant fossils, we can't be any more specific than that at the moment."