By Fred Paillet, OS Education Chair
You read that right – there are a few zombies on the loose in the Ozarks. By zombies we mean functionally dead creatures with their nervous systems taken over by alien life forms and programmed to do the bidding of those new masters. In high school biology you may remember hearing about one of nature’s ecological cruelties in the form of wasps that parasitize caterpillars. The female wasps do this by stinging the caterpillar to paralyze the bug, and then depositing eggs injected into the still living body. The wasp larvae then mature by eating the living flesh of their helpless victim. The process of ecological success through such a parasitic process has reached its ultimate form in the zombie ant fungus of the genus Ophiocordyceps. This fungus disseminates spores that infect ants but not to simply digest them. The fungus first takes over the ant’s nervous system to program the ant zombie to become the ideal fruiting body for the dissemination of spores. The ant is programmed to climb to the highest point it can manage so that the spores eventually produced can be exposed to the wind to ensure that they travel far and wide in search of more victims. The ant’s head is then forced to bite down hard on the leaf or stem where it has been positioned to form a firm attachment. The fungus then proceeds to consume its victim, forming a solid attachment (the white frilly “skirt” shown in my illustration). At maturity, fruiting bodies sprout from the inside of the ant that has become filled with fungal mycelia and begin to shed spores. Sometimes the fruiting bodies can be recognized as miniature mushrooms (as shown in my illustration) but can come in other forms such as horns, fibers and beads on the end of a string. The parasitized ants thus fulfill the role of a zombie – functionally dead creatures stumbling around as they respond to the will of their internal alien masters.
The complexity and devious effectiveness of the ant zombie reproductive strategy is astounding. One wonders how such an involved interaction of life forms could have arisen. An obvious factor is the sheer abundance of the ant population in the environment. Entomologists such as the famous naturalist Edward O Wilson estimate that ants make up nearly three quarters of the total insect biomass in the tropical rain forest. That’s a lot of sustenance for any life form that can take advantage of that abundant resource. In fact, predation is credited with a fundamental impact on the entire course of evolution. Paleontologists have long noted the so-called Cambrian Explosion. This is a vast expansion of fossilized life forms that suddenly appears in the geological record. One often cited cause for this sudden explosion of diversification is the onset of predation. The earliest life forms were unicellular microbes that must have simply absorbed their sustenance from organic debris in the surrounding medium. They had no need to hide from predators. Then, when multicellular animals emerged with the capability of specialized organs, new possibilities arose. When one precocious “worm” developed the ability to capture and metabolize its living neighbors the whole situation changed. Evolution caused those species that were being eaten to find ways to frustrate their predators while the predators developed new equipment with which to detect and digest their prey.
In that sense, zombie ants used to both provide sustenance and serve as reproductive platforms is a logical consequence of the predator-prey competition. The recent development of quick and inexpensive methods for delineating the DNA genome now provides a way to investigate the deep history of the organisms associated with that genome. A recent on-line lecture provided by the Torrey Society associated with the New York Botanical Garden reviewed the genome analysis of the entire line of insect-consuming fungi. This involved systematic genome build-up (molecular time clock) and the spread of various genes between an ever-widening group of developing species. The very first of these insect-eating fungi appeared a hundred or so million years ago as beetle grub predators. If you do any gardening at all you can recognized such grubs as fat, ghostly white little worms that roll out when you turn over the garden soil. They look like they ought to pack a lot of good food for any predator. Since they live in the soil you can readily see how soil fungi could tap into this resource. From there, the analysis shows this family of fungi expanding to include butterflies and many other insects as their prey in the wider, above-ground world. The ant-zombie fungi constitute one of the most recent developments in this ancient story.
One final question is why this family of fungi had to resort to the zombie-ant strategy. Why bother to go to all the complications involved in that strategy? Why not just eat the ants from the inside as they spread the ferocious fungus when they re-enter a teeming nest full of additional victims?
This strategy was necessary because of what biologists call social immunity. Colonial ants and bees have developed the ability to detect when one of their returning workers carries a pathogen. These diseased fellows are immediately set upon and killed, and their carcasses safely disposed of outside of their abode. The zombie-ant strategy apparently developed as a way to avoid the social “antibodies” incorporated into the colonial behavior of ants and bees. In that way the zombie-ant story is just another chapter in the evolutionary dance between predator and victim that we see being played out between the new covid respiratory disease and the chemistry incorporated into the human system of adaptive immunity. Ants and their zombified condition are obviously more common in the rainforest, but we do have a few of our own Ozark zombies, too.