Climate change and zombie evolution

It’s no secret to fans of biology and of excellent video games that zombies are real. There are a few different versions of zombie infection that have different effects depending on the zombifying organism and its host. In this case, we’re talking about zombie ants and the Cordyceps fungi that prey on them.

The image is a silhouette of a

For those who need a refresher, Cordyceps spreads through spores, which attach to the species they specialize in, and grow into its body. Mycelium – the root-like structures that make up most of a mushroom’s body – grows into the host and feeds on it, growing to replace the host tissue with more fungus. As this happens, the host is compelled to change its behavior. It is forced to seek out a good location to die. This means gaining elevation, and clamping onto a twig or a leaf. The ant then dies, and the Cordyceps puts out a “fruit body” like the various toadstools we eat. Spores are released, and drift around until they land on a new host and the cycle can begin again.

So where do climate and evolution come in?

First a brief note about evolution, for any who might need it. It’s impossible for evolution to not happen in all forms of life we know of. Evolution is as much a part of our daily lives as time is – it’s something that we don’t directly perceive, but we experience its passage in a myriad of ways. Do you believe in the supernatural? In gods or in magic? Then life evolves in response to those as well. Regardless of what you believe, the climate is changing, and all life on earth is changing with it.

Recent work out of Penn State has shown that whether the host ants are forced to cling to a twig or a leaf depends on climate:

The German fossil, which dates to around 47 million years ago, shows the ant biting into a leaf. During that era, wet evergreen forests ranged from the equator nearly to the North and South poles. Because all the forests were evergreen, all the zombie ants would have chosen to bite into leaves.

As the climate cooled, temperate forests grew in the northernmost and southernmost areas worldwide. Zombie ants in those areas that bit on leaves would have quickly ended up on the ground when the leaves fell. The fungal species that manipulated its host onto leaves would not successfully reproduce. This implies there was a strong force of natural selection acting on where the ant host was manipulated. Over time, the fungi evolved so that in temperate areas, zombie ants are manipulated to bite onto twigs or bark.

This image is a collage of two pictures, one showing a living or recently-dead brown ant clinging to the underside of a twig with its legs, and biting into its bark. The second image is the same ant with what looks like brown mold all over its body, and a long, narrow fungus fruit sticking down from the back of the ant's head. This image shows a dead ant biting the central vein of a leaf. There is a long, branching mushroom structure growing out the back of its head. There are a couple fuzzy lumps on the mushroom that look like they might be some form of spore-releasing tissue.

I don’t know of any way that this directly affects us, but it will be interesting to see the process of ecosystems collapsing and reshaping themselves under the assault of rapid warming. Many of the changes will be far too subtle to notice if we’re not actively looking for them – like a change in what part of a plant zombie ants bite onto when they die.

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