It's that spooky time of year once again when the arrival of Halloween in the How it Works office is marked by a parade of ridiculous costumes. A favourite costume last year seemed to be the zombie and we had a small army of them shuffling around, dripping fake slime over people's desks. It was, er, lovely.
The 'human' version may be fictional but in the animal kingdom, zombies have no mercy, controlling their victims' minds and using them as living puppets.
They're not quite the classic brain-eating, gormless slow-shufflers of horror movies, but in the animal kingdom, the zombie threat is very real. The culprit? Parasites: small organisms with complex life cycles that set up camp inside their animal hosts. These gruesome body snatchers are able to control the animals' minds, using them as living-dead puppets and steering them to positions of optimal benefit.
One classic case is the zombie ant. The parasite is a mind-controlling fungus (Ophiocordyceps camponoti-rufipedis) that manipulates carpenter worker ants into straying far enough from the colony that their social immunity is impaired. The fungus makes the ant bite down underneath a leaf to anchor it before it dies. The fungus then uses the ant's corpse to grow. It also releases spores that rain down and infect more ants, and so the nightmare continues.
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One more insect group responsible for zombifying its victims and turning them into mindless drones are wasps. The jewel wasp (Ampulex compressa) injects cockroaches directly through the brain, depositing venom into two specific locations that render the roach's free will useless. The wasp leads the cockroach to a burrow and lays an egg on the roach's abdomen. The zombie roach only dies once the egg hatches and the larva devours it piece by piece.
By Hughes D, Andersen S, Hywel-Jones N, Himaman W, Billen J, Boomsma J [CC BY 2.0], via Wikimedia Commons
Another wasp species, the green-eyed wasp (Dinocampus coccinellae) makes light work of harnessing the power of the ladybird. The wasp lays her eggs inside the bug, and new evidence suggests that a virus also attacks the ladybird's brain, paralysing and enslaving it as a zombie babysitter. The larva emerges and weaves a cocoon between the ladybird's legs so the paralysed bug acts as a bodyguard until the larva is ready to leave. Amazingly, a quarter of ladybirds recover from their zombification!
Animals are biologically hard-wired to fear and flee from their predators. However, one parasite is capable of reversing a rodent's natural fear of felines, and even encourages the rat to actively seek them out - driving it to its own death by the teeth and claws of a kitty.
Related to the parasite that causes malaria, Toxoplasma gondii is a single-celled pathogen that infects many types of mammal and bird, causing a disease known as toxoplasmosis. Like every life cycle, Toxoplasma has to reproduce, and the only place that this specific microbe can do that is in the gut of a cat. When it infects rodents, Toxoplasma reverses the fear of cats in a rodent's brain, and encourages it to seek out felines by making the rat attracted to the scent of cat urine. Inevitably, this results in the infected rat getting eaten, allowing Toxoplasma to continue its life cycle within the feline hunter.
The oceans are no safer with Killifish in California which have been discovered to play host to a mind-sucking parasite that alters behaviour in order to further its own species. These zombie fish are infected with a fluke, a small, parastitic worm that reproduces in the guts of sea birds. The flukes are able to limit the production of serotonin in the fish's brain, which makes the fish very restless. Ordinarily shy of the surface and its dangers, infected fish actively swim near and even flick the water surface, greatly enhancing the changes of getting plucked out and eaten by a bird.
So this Halloween, be thankful for a mild scare from a passing witch or the icy wind of a nearby ghost because in the natural world things are far, far worse.
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