The parasite takes over the ants' brains, causing them to cling to blades of grass against their will. The life cycle strategy of the lancet liver fluke is unique, with snails, ants and grazing animals all unwitting participants. Researchers are currently studying the precise mechanisms behind this intriguing form of mind control.
When the ant is dissected, the encased parasite (white oval-shaped structure) can be seen spilling out of the hind body. Photo credit: Brian Lund Fredensborg
Imagine waking up with your chin gripping the tops of swaying blades of grass and not knowing how you got there. This is the reality faced by ants infected with the lancet liver fluke, a tiny parasitic flatworm. The life cycle of a liver fluke is complex, almost imaginative, and it first hijacks the ant's brain. The unsuspecting ant climbs up and uses its powerful pincers to grab the tip of the grass blade, making it easier for herbivores such as cattle and deer to eat it.
Researchers at the University of Copenhagen's Department of Plant and Environmental Sciences have discovered that the parasite's ability to control ants is more cunning than previously thought. Impressively, the parasite even allows ants to crawl back onto grass blades when it's too hot.
The unsuspecting ants climb onto the blades of grass and use their powerful mandibles to grip the tops of the blades, making them more susceptible to being eaten by herbivores such as cattle and deer. Image source: University of Copenhagen
"It's very clever to have ants climb high up in the grass so cattle or deer can graze in the cool mornings and evenings, and then climb back down to avoid the sun's deadly rays," explains Associate Professor Brian Lund Fredensborg. "Our findings reveal a parasite that is more complex than we first thought."
Research on this parasite has just been published in the scientific journal Behavioral Ecology.
Researchers tagged hundreds of infected ants in the Biedstrup Forest near Roskilde, Denmark.
"It takes some dexterity to stick colors and numbers to the ants' tails, but it allows us to track them for longer periods of time," Brian Lund-Fredensborg said.
Researchers tagged hundreds of infected ants in the Biedstrup Forest near Roskilde, Denmark. Image source: University of Copenhagen
They then looked at the infected ants' behavior in relation to light, humidity, time of day and temperature. It's clear that temperature has an impact on ant behavior. When temperatures are cooler, ants are more likely to cling to the tips of grass blades. When temperatures rise, ants abandon the blades of grass and crawl back underneath.
"We found a clear correlation between temperature and ant behavior, and colleagues joked that we had found the ants' zombie switch."
Once the liver fluke infects an ant, hundreds of parasites invade the ant's body. But only one parasite can enter the brain and affect the ants' behavior. The rest of the liver flukes are hidden in the ant's abdomen.
"Here, there may be hundreds of liver flukes waiting for the ants to carry them to their next host. They are encased in a capsule that protects them from the host's stomach acid, and the liver fluke that controls the ants dies," explains Brian Lund-Fredensborg. "The liver fluke sacrifices itself, so to speak, for the other ants." Animals infected with multiple liver fluke species may suffer liver damage as the parasites move through the host's liver and bile ducts.
Associate Professor Brian Lund Fredensborg. Source: University of Copenhagen
Brian Lund-Fredensborg points out that there are many other examples of parasites changing animal behavior. Therefore, parasites that hijack their host's behavior play a larger role in the food chain than many people imagine. Fredensborg believes the new study reveals a severely underestimated group of organisms.
"Historically, parasites have never received real attention, despite scientific information showing that parasitism is the most common form of life. Partly because parasites are difficult to study. However, the hidden world of parasites forms an important part of biodiversity, and by changing the behavior of their hosts, parasites can help determine who eats what in nature. This is why parasites are so important to us."
This tiny liver fluke is widely distributed in Denmark and other temperate areas of the world. The researcher and his colleagues will continue to study the parasite and exactly how it takes over the ants' brains.
"We now know that temperature determines when the parasite takes over the ant's brain. But we still need to figure out which chemical the parasite uses to turn the ant into a zombie."