Vol. 16 Issue 3, Fall 2011
By Cliff Fairweather
A few years ago at the Rust Nature Sanctuary near Leesburg, I came across a number of flies perched on blades of Japanese stiltgrass. None of them were moving, even when I moved in close to them. On closer examination, it was apparent that many were in various states of decay, although others were quite fresh looking.
I collected a few specimens for further study and photography and began to ponder what was going on. I was aware that many animal parasites manipulate the behavior of their hosts, and I expected some sort of parasitic wasp to emerge from my specimens. Instead, their bodies simply turned to lumps of mold; I was perplexed.
The parasitic life style might seem pretty gruesome to us, but it is an ecologically important one in terms of its large influence on plant and animal populations and communities. Parasitic relationships are common and have arisen among a wide variety of organisms throughout evolutionary history.
Parasitism is a form of symbiosis, which is an intimate, usually long-term relationship between organisms of two different species. In a parasitic relationship, one species, the parasite, benefits and the other, the host, is harmed. In cases where the host is killed by the parasite, the parasite is termed a parasitoid.
In general, the host provides a habitat, including a food supply, during some part of the parasite’s life cycle. A major survival challenge for a parasite is getting into the right host species at the right point in their life cycle, and an important way that many parasites do this is to manipulate the behavior of their host.
Zombies are the stuff of horror movie classics such as the Invasion of the Body Snatchers or the various incarnations of The Night of the Living Dead, but real-life zombies dwell among us in the form of parasitized animals. An outbreak of pod people is pure science fiction, but many animals, including humans, are susceptible to behavioral manipulation by parasites. Insects are frequent parasite hosts, and parasitized insects aren’t too hard to find.
Even if they weren’t mind-controlling parasites, Gordian worms (aka horsehair worms) are pretty bizarre beasts. The adults occur in freshwater and I have occasionally encountered them while collecting other aquatic organisms. Their long, thin, rather stiff bodies form into complicated knots as they cluster for mating. Each mated female lays millions of eggs on the bottom and dies soon afterwards.
The larvae hatch out and make their way through their life cycle, if they’re lucky, in a complicated and seemingly improbable chain of events. First, an aquatic insect larva ingests an egg, matures to become a land-dwelling adult and dies. A cricket then feeds on the aquatic insect carcass, ingesting the now encysted Gordian worm larva. The worm larva transforms from an encysted form into an active form and draws nourishment from the cricket’s body fluids.
As bad as this already sounds for the cricket, its nightmare has just begun. The worm grows, coiled inside the cricket’s body cavity, to a length three to four times that of its host. Once it matures, the worm manipulates the cricket’s behavior, causing it to wander in a random pattern. If the cricket encounters water, it obligingly jumps in, and the adult Gordian worm, now in its breeding habitat, departs its drowning host.
The lancet liver fluke, a trematode or primitive worm, has a similarly complex life cycle that ends badly for one of its hosts. At various points in its life cycle, the fluke is hosted by a snail, an ant and a cow or other herbivorous mammal. In this sequence, the ant is the victim of behavioral manipulation by the fluke. This happens after an ant ingests the fluke’s larval stage by feeding on a ball of mucous coughed up by an infected snail.
Once inside the ant, the fluke larvae cause the ant to ascend a grass blade or other plant each night and remain there until morning, clamped in place with its jaws. During the rest of the day, the ant behaves normally. While mounted on vegetation, however, the ant is susceptible to being eaten by a cow, sheep, or other grazing animal, thus delivering the fluke to a suitable reproductive habitat.
As for my moldering flies, it turns out that parasitic mind control isn’t exercised by animals alone. In an especially creepy twist, fungi get into the act. Many insects and other arthropods are affected by entomopathogenic (insect-infecting) fungi. Many of these fungi cause their hosts to take up positions that increase the dispersion of fungal spores, such as at the end of a leaf.
Grasshoppers infected by entomopathogenic fungi are manipulated to climb tall meadow plants to release spores into the wind. The flies I observed at the Rust Nature Sanctuary were probably also shedding spores into the air, but spores can also be transmitted by direct contact with other flies. Male flies are not especially finicky about mating partners and will attempt to mate with a dead, fungus-infected female. Indeed, infected females often die with their wings apart, exposing their abdomens in a manner especially enticing to the males.
The exact mechanisms by which parasites exercise mind control are the subject of current research, but alteration of the host’s neural chemistry appears to play an important role. For example, some parasites cause their host to produce neuromodulators, substances such as serotonin, that alter the transmission of nerve impulses. Some parasites also appear to enlist the host’s immune system in altering host behavior.
Host species are not entirely defenseless against parasitic mind control. Cockroaches and other insects use “behavioral fever” to kill parasites, meaning that they find warm places or bask to raise their body temperature above what their parasites can endure. Other hosts simply eat right. Some tiger moth caterpillars kill parasitic tachinid fly larvae in their bodies by eating plants high in alkaloids or glycosides.
When you’re contemplating ghosts, ghouls and demons this Halloween, consider the real-life zombies about in the world and the parasites that control their minds … and hope that you’re safe from their reach. Sleep tight!