An ant scuttles through the rainforest. Seemingly brainwashed, the ant is on a mission to ascend the nearest plant and clamp down hard on a leaf. Here the ant will remain for a few days until it dies, in what biologists call a “death grip”. You might think this seems like a weird thing for an ant to do, and you’d be right.
Like something from a horror movie, the ant has been taken over by a parasitic fungus, a member of the genus Ophiocordyceps (AKA Cordyceps). This fungus forces the ant to find a leaf in a prime location for fungal growth, all the while digesting it from the inside. Researchers have discovered that Cordyceps fungi can coordinate an ant’s behaviour with astonishing precision. In one species, almost all infected ants are found on leaves roughly 25 centimetres from the ground, where the temperature, humidity, and leaf orientation are just right. This is a remarkable feat of manipulation, which involves a highly complex interaction between the fungus and the ant, which is still not fully understood. Recent developments have shown that despite how it might appear, the fungus never invades the ant’s brain, but could still be controlling it from afar using hormones.
This begs the question, why would a fungus go to all this effort? Why not just grow from the ground like a normal mushroom? Whilst the strategy seems brutal, in the dog eat dog world of biology, anything goes. If an organism can carve out a niche for itself, then it can be extremely successful. Because the Cordyceps fungus can manipulate an ant to carry it to the perfect growing conditions, it means it does not have to compete with other fungi or worry about ground dwelling predators. Furthermore, by hijacking an ant’s energy resources, the fungus has more energy to invest in reproduction. This is the final piece of the puzzle: once the ant is in place and the fungus has amassed enough energy, it bursts a fruiting body through the ant’s head and rains down spores on further victims.
Ants are not the only ones to fall prey to these coercive Cordyceps. Around 400 species from this genus parisitise different arthropods, the group containing insects, spiders, millipedes and centipedes. Parasitic fungi actually play a vital role in maintaining sustainable insect numbers in the wild, which has led to some interesting attempts to replace harmful pesticides with species of fungi, to limited success. The species used are not harmful to mammals, and the spores can be formulated as a spray, allowing the use of existing pesticide application equipment. However, fungal spores require just the right temperature and humidity, and high exposure to sunlight can actually inactivate them. Nevertheless, as part of an integrated system with other pest control methods, or for specific crops that grow in the right conditions, parasitic fungi could be useful.
The cruel strategy of the Cordyceps fungus, rendering its victims mindless zombies, has provided the inspiration for some recent popular fiction. The hugely successful The Last of Us (2013) is a video game set in a dystopian zombie-ridden future, where a species of Cordyceps has evolved to take control of humans, utilising them to infect other humans. The first stage is violent and aggressive, with the fungus transmitted through biting. But true to nature, the later-stage infected groan with fruiting bodies, ready to disperse spores onto those unfortunate enough to disturb them. Beyond the jump scares, this game seems to strike at a deep fear we may all have of being taken over by Mother Nature.
While a fungus taking over a human is extremely unlikely, you would be surprised at the ways parasites have been found to manipulate our behaviour. One example is the Guinea worm, which resides in water fleas found in stagnant water. Upon ingestion, stomach acid dissolves the water flea, but the Guinea worm persists. The female grows and moves through the body until it reaches an extremity, like a hand or foot, where it resides undetected for up to a year. When it is ready to lay its eggs, the worm releases an acid, causing the skin to blister. This blistering causes the infected individual to put the burning limb into water to relieve the painful sensation. The Guinea worm lays its eggs, which are ingested by the water fleas. And so the cycle continues.
Although nature is often portrayed as beautiful, the reality can be harsh, a consequence of organisms vying to outcompete each other by any means. This directionless drive to propagate is the key to understanding the whole of biology. If an organism can find a way to pass on its genes slightly more successfully than a competitor, then it is treated favourably by evolution. Unfortunately for our ant, it has become an unwitting pawn for Cordyceps in the game of life.
Image credit: Chandan Kumar via flickr