The brain-encysting lancet liver fluke Dicrocoelium dendriticum induces Formica polyctena ants to climb and bite to vegetation by the mandibles in a state of temporary tetany, making it more likely to be eaten by grazers such as cattle and deer. According to new research, Dicrocoelium dendriticum can even get the ants to crawl back down the blade of grass when it gets too hot.
The infected Formica polyctena ant climbs up and clamps its powerful jaws onto the top of a blade of grass, making it more likely to be eaten by grazers such as cattle and deer. Image credit: University of Copenhagen.
Once Dicrocoelium dendriticum infects a Formica polyctena ant, several hundred parasites invade the ant’s body.
But only one makes its way to the brain, where it can influence the ant’s behavior. The rest of the liver flukes conceal themselves in the ant’s abdomen.
“There can be hundreds of liver flukes waiting for the ant to get them into their next host,” said Dr. Brian Lund Fredensborg, a researcher at the University of Copenhagen.
“They are wrapped in a capsule which protects them from the consequent host’s stomach acid, while the liver fluke that took control of the ant, dies. You could say that it sacrifices itself for the others.”
“Animals infected with many liver flukes can suffer liver damage as the parasite moves around the host’s liver and bile ducts.”
Over 13 non-consecutive days during one year in the Bidstrup Forests near Roskilde, Denmark, Dr. Fredensborg and his colleague, Wageningen University Ph.D. student Simone Nordstrand Gasque, observed a total of 1,264 individual ants expressing the modified behavior. They then individually marked a subset of 172 infected ants.
“It took some dexterity to glue colors and numbers onto the rear segments of the ants, but it allowed us to keep track of them for longer periods of time,” Dr. Fredensborg explained.
The authors then observed the infected ants’ behavior in relation to light, humidity, time of day and temperature. It was clear that temperature had an effect on ant behavior.
When the temperature was low, the ants were more likely to be attached to the top of a blade of grass. When the temperature rose, the ants relinquished the grass and crawled back down.
“Getting the ants high up in the grass for when cattle or deer graze during the cool morning and evening hours, and then down again to avoid the sun’s deadly rays, is quite smart,” said Dr. Brian Lund Fredensborg, a researcher at the University of Copenhagen.
“Our discovery reveals a parasite that is more sophisticated than we originally believed it to be.”
There are many other examples of parasites that alter animal behavior. As such, parasites that hijack their host’s behavior have a greater hand in the food chain than many might think.
“Our new study sheds light on an extremely underrated group of creatures,” Dr. Fredensborg said.
“Historically, parasites have never really been focused on that much, despite there being scientific sources which say that parasitism is the most widespread life form. This is in part due to the fact that parasites are quite difficult to study.”
“Nevertheless, the hidden world of parasites forms a significant part of biodiversity, and by changing the host’s behavior, they can help determine who eats what in nature. That’s why they’re important for us to understand.”
The findings were published in the journal Behavoral Ecology.
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Simone Nordstrand Gasque & Brian Lund Fredensborg. Expression of trematode-induced zombie-ant behavior is strongly associated with temperature. Behavioral Ecology, published online August 24, 2023; doi: 10.1093/beheco/arad064
