Zombie Creatures: What Happens When Animals Are Possessed by a Parasitic Puppet Master? [Slide Show]
From fungi to flies, some parasitic species have figured out how to control their host's behavior to get what they need. See what happens when bugs go really bad
Credits: STEVE YANOVIAK
CAT AND MOUSE A parasite that lives to change mouse behavior may also be altering the way humans act. The parasitic protozoa
Toxoplasma gondii thrives by cycling through feline and rodent hosts. When it infects mice, the brain-dwelling parasite makes them more daring and, in particular, less afraid of the scent of cats (so it can get passed back to the feline hosts when they eat the infected, emboldened rodents).
The chemical changes brought on by the parasite appear to have some of the same effects on humans, who can be infected by ingesting parasite eggs from cat feces. Research by Kevin Lafferty, a research ecologist with the U.S. Geological Survey, has found that the parasite can cause women to act more moralistically, and men less so. And "when looking at human societies, those traits correlated pretty well with the prevalence of T. gondii infections," Levri says. Other research has shown a higher incidence of risky behavior in people who are infected. ISTOCKPHOTO/SHIFFTI
A BERRY SCARY ANT Ants do their best to avoid being snatched up as snacks. Their dark coloring can help them blend into their surroundings and prevent would-be predators from spotting individual targets in a nest. For a parasite that seeks to find its way back into a bird host, however, this arrangement needed some alteration.
The nematode parasite ( Myrmeconema neotropicum) manages to turn Cephalotes atratus ants' gasters (enlarged part of the abdomen) the color of local red berries—the kind that birds eat—and also impel the false berry-baring hosts to venture away from the colony, making them easier prey. When the exposed victim is snatched up by a bird, the latter is infected with the parasite. After the bird digests its tainted treat, it passes along the parasite passenger in its droppings, which stand waiting to infect other unsuspecting ants. STEVE YANOVIAK
A FUNGUS AMONG US Ants infected with a spooky breed of fungus (
Ophiocordyceps unilateralis) have been shown to march to that parasite's orders. Usually tropical treetop-dwellers, these infected Camponotus leonardi ants will climb down to low leaves that are just in the right spot for the fungus to thrive (at very specific orientation, temperature and height), then chomp onto a leaf vein with a so-called "death grip".
The ant then dies as its innards are metabolized by the fungus inside. Eventually, the invader sprouts from the ant's body and releases spores that drift to the ground, creating a silent circle of certain death for any other ant that passes through it. DAVID HUGHES
CRICKET-ICIDE Crickets can't swim, but the harrowing hairworm (
Spinochordodes tellinii) doesn't care about that detail. After growing inside of a cricket's body and feasting on its insides, the hairworm will inexplicably compel the cricket to throw itself into a body of water, where the ruthless body snatcher can emerge and enter the aquatic phase of its life cycle.
"It was amazing to see hundreds of crickets at night totally under the control of the parasite inside and jumping into the water," says Frederic Thomas, a scientist at the Genetics and Evolution of Infectious Diseases research group in Montpellier, France, who described the phenomenon in the Journal of Evolutionary Biology. WIKIMEDIA COMMONS/BILDSPENDE VON D. ANDREAS SCHMIDT-RHAESA Advertisement
A COCKROACH PAWN In a well-documented example of external parasite control, an emerald cockroach wasp (
Ampulex compressa) enslaves a much larger cockroach ( Periplaneta americana). The wasp injects a neurotoxin into the cockroach's brain. This toxin kills off the roach's ability to control its own movement but doesn't paralyze it entirely. The wasp is then able to grasp the roach's antenna and lead it into a nest before laying an egg in the live cockroach's body.
Permanently incapacitated, the cockroach is unable to escape and is eaten from the inside by the growing wasp larva. FREDERIC LIBERSAT
A CATERPILLAR BODYGUARD Why would a caterpillar protect wasp cocoons from invaders? The wasps (of the genus
Glyptapanteles) are controlling the caterpillar. "Just before the wasp [larva] emerges from the caterpillar, it produces a toxin that causes the caterpillar to be a bodyguard" for the wasp larvae, explains David Hughes, a research fellow at the University of Exeter. "It doesn't allow the caterpillar to do all the other normal caterpillar stuff, like walk away" to escape, he adds.
Researchers are still not sure how the caterpillar is engaged in this final duty. "It's a behavior that's induced by a parasite that is not even in the host anymore," says Edward Levri, an associate professor of biology at The Pennsylvania State University. Other caterpillars have been forced to create cocoons for parasitic wasp larvae that emerge from them, he adds. "The caterpillar is almost dead, and then its last goal in life is to create a cocoonlike house" for the vulnerable wasp larvae. As he points out, "if [the larvae] aren't covered up by the web, another parasitoid will inject eggs into them," initiating another meta-level of parasitism. WIKIMEDIA COMMONS/JOSE LINO-NETO Advertisement