Summary: Ants infected with the liver fluke are forced into life-threatening scenarios, all to promote the fluke’s complex life cycle. Scientists have revealed that manipulating flukes is even more complex than previously thought, with temperature serving as a trigger for ant behavior.
In cooler temperatures, infected ants cling to grass blades, making them more likely to be consumed by grazers. As the temperature rises, the ants climb down to protect themselves from the deadly heat.
Key facts:
- The lancelet weevil takes control of the ant’s brain and forces it to attach itself to blades of grass during colder periods, maximizing the chance of it being consumed by grazers.
- Research has revealed a temperature-controlled “zombie switch” that controls when an ant clings to grass or seeks shelter.
- Of the hundreds of flukes inside an infected ant, only one controls the brain, while the rest await transmission in the ant’s abdomen.
Source: University of Copenhagen
Imagine coming to, jaw gripping the top of a swaying blade of grass, not knowing how you got there. Such is the reality for ants infected with the liver fluke, a small parasitic flatworm.
Liver flukes have a complicated, almost insanely conceived life cycle that begins with the hijacking of an ant brain. The unsuspecting ant climbs up and clamps its powerful jaws onto a blade of grass, making it more likely to be eaten by grazers such as cattle and deer.
Scientists from the University of Copenhagen’s Department of Plant and Environmental Sciences have discovered that the parasite’s ability to control ants is even more cunning than previously thought. Impressively, the parasite can even make an ant climb back down a blade of grass when it’s too hot.
“Getting the ants high up in the grass when cattle or deer are grazing in the cool mornings and evenings, and then down to avoid the deadly sun’s rays, is pretty clever. Our discovery reveals a parasite that is more sophisticated than we first thought,” explains Associate Professor Brian Lund Fredensborg, who conducted the study together with former graduate student Simone Nordstrand Gasque, now a PhD student at Wageningen University in the Netherlands.
A study of the parasite has just been published in a scientific journal Behavioral ecology
Zombie “on/off”
The researchers tagged several hundred infected ants in the Bidstrup forests near Roskilde, Denmark.
“It took some skill to stick colors and numbers on the back segments of the ants, but it allowed us to track them over a longer period of time,” says Brian Lund Fredensborg.
They then monitored the behavior of the infected ants in relation to light, humidity, time of day and temperature. It was clear that the temperature had an effect on the behavior of the ants. When the temperature was low, ants were more likely to be attached to the top of the grass blade. When the temperature rose, the ants left the grass and climbed back down.
“We found a clear correlation between temperature and the behavior of ants. We joked about finding the zombie switch in the ants,” says Brian Lund Fredensborg.
Trojan Horse
Once a liver fluke invades an ant, several hundred parasites invade the ant’s body. But only one reaches the brain, where it can influence ant behavior. The rest of the liver flukes hide in the ant’s abdomen.
“There could be hundreds of flukes here, waiting for the ant to get them to its next host.” They are encased in a capsule that protects them from the subsequent stomach acid of the host while the liver fluke that has taken over the ant dies. You could say that he sacrifices himself for others,” explains Brian Lund Fredensborg.
Animals infected with multiple liver flukes can suffer liver damage as the parasite travels around the host’s liver and bile ducts.
Nature’s greatest influencer
Brian Lund Fredensborg notes that there are many other examples of parasites changing animal behavior. Parasites that take over the behavior of their host have a bigger part in the food chain than many would think. According to Fredensborg, this new study sheds light on an extremely underappreciated group of creatures.
“Historically, parasites have never been much of a focus, even though there are scientific sources that say parasitism is the most widespread form of life. This is partly due to the fact that parasites are quite difficult to study. Yet the hidden world of parasites makes up a significant part of biodiversity, and by changing host behavior they can help determine who eats what in nature. That’s why it’s important that we understand them,” he says.
The small liver fluke is widespread in Denmark and other temperate regions around the world. The researcher and his colleagues will continue to investigate the parasite and exactly how it takes over the ant’s brain.
“We now know that temperature determines when the parasite takes over the ant’s brain.” But we still need to find out what cocktail of chemicals the parasite uses to turn ants into zombies,” concludes Fredensborg.
Box: Liver fluke life cycle
- zombie ant: The liver fluke infects the ant’s brain and causes it to attach itself to a blade of grass, so it is eaten by its next host – a cow, sheep, deer or other grazer. Meanwhile, a large swarm of flukes waits in the ant’s abdomen to be transferred.
- Grazer: Once a herder eats an infected ant, it also becomes infected with the liver fluke. Having taken control of the ant’s brain, the liver fluke dies in the host’s stomach acid. The larger swarm in the ant’s abdomen is protected by a capsule that dissolves only once in the host’s gut. Here, liver flukes find their way through the bile ducts to the liver, where they suck blood and develop into adult flukes that begin to lay eggs, which are then excreted in the host animal’s feces.
- snail: Once the fluke eggs are expelled, they lie on the ground waiting for a snail to crawl by and consume the feces. Inside the snail, the eggs develop into larval flukes that reproduce asexually, where they can multiply into several thousand.
- Slime Ball: To leave the snail and move on to their next host, the larval mites cause the snail to cough, expelling them in a clump of mucus. Ants are attracted to the slime balls and consume them, swallowing the fluke larvae in the process.
About this news from neuroscience research
Author: Michael Jensen
Source: University of Copenhagen
Contact: Michael Jensen – University of Copenhagen
Picture: Image is credited to Neuroscience News
Original Research: Closed access.
“Expression of trematode-induced zombie-ant behavior is strongly associated with temperatureBrian Lund Fredensborg et al. Behavioral ecology
Abstract
Expression of trematode-induced zombie-ant behavior is strongly associated with temperature
Parasite-induced modification of host behavior to enhance transmission to the next host is a common phenomenon. However, field studies are rare and the role of environmental factors in inducing host behavior modification is often not taken into account.
We investigated the effects of temperature, relative humidity (RH), time of day, date, and proxy irradiance on the modification of ant behavior Formica polyctena (Förster, 1850) by the brain lancet of the liver fluke Dicrocoelium dendriticum (Rudolph, 1819).
This fluke causes the ants to climb and bite into vegetation with their mandibles in a state of temporary tetany. A total of 1264 individual ants expressing the modified behavior were observed on 13 non-consecutive days over one year in the Bidstrup forests of Denmark.
A subset of these ants (N= 172) was individually tagged to monitor attachment and release of infected ants in relation to changes in temperature. Infected ants primarily attached to vegetation early and late in the day, corresponding to low temperature and high relative humidity, likely coinciding with the foraging activity of potential herbivorous final hosts.
Temperature was the single most important determinant for induced phenotypic change. On warm days, infected ants switched between the manipulated and non-manipulated states several times, while on cool days, many infected ants remained attached to the vegetation throughout the day.
Our results suggest that the temperature sensitivity of infected ants serves a dual purpose—exposing infected ants to the next host at an appropriate time while protecting them from exposure to high temperatures that could increase host (and parasite) mortality.
