A parasite that takes control of ants’ brains and turns them into zombies is even more dastardly than previously thought.
The common liver fluke is renowned for its ability to hijack the mind and body of ants. It forces them to crawl up and hang out on tall blades of grass, waiting to be eaten by grazing animals such as cattle and deer – the fluke’s next unsuspecting host.
However, researchers from the University of Copenhagen have discovered that the parasite’s ability to control the ant is even more cunning than previously thought after discovering the parasite can ‘switch off’ zombie mode when it gets too hot, allowing the ants to crawl back down.
Essentially, the ants ‘wake up’ high above the ground to find themselves hanging on by their jaw.
‘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 lead author Associate Professor Brian Lund Fredensborg. ‘Our discovery reveals a parasite that is more sophisticated than we originally believed it to be.’
The discovery itself required quite a bit of cunning too, with the researchers tagging several hundred infected ants in the Bidstrup Forests near Roskilde, Denmark.
‘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,’ said Associate Professor Fredensborg, who conducted the research longside co-author Simone Nordstrand Gasque.
Together they observed the infected ants’ behavior in relation to light, humidity, time of day and temperature.
It became clear that temperature had an effect on how the parasites directed the ants. 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.
‘We found a clear correlation between temperature and ant behavior. We joked about having found the ants’ zombie switch,’ said Associate Professor Fredensborg.
The lifecycle of the liver fluke is a rollercoaster. To infect the ant, several hundred parasites invade the ant’s body – but only one makes its way to the brain. From here it controls the ant, while the rest of the liver flukes hide themselves in the ant’s abdomen.
Once eaten by a grazer however, the parasite that took control of the brain is killed by the animal’s stomach acid. The larger flock in the ant’s abdomen is protected by a capsule that only dissolves once in the host’s intestine, enabling them to migrate towards the liver and lay eggs.
These eggs are excreted by the grazer, where they wait patiently for a snail to come along and eat the faeces. The eggs develop into larval flukes that reproduce asexually, multiplying into several thousand parasites.
Now they must escape the snail. The obvious way to do that, apparently, is by causing the snail to cough, which expels them in a ball of mucus – a ball of mucus that is surprisingly tasty to ants, which come along and eat them.
‘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,’ said Associate Professor Fredensborg. ‘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 liver fluke is widespread in temperate regions worldwide, including in the UK. Associate Professor Fredensborg and his colleagues are continuing to investigate the parasite to determine exactly how it takes over an ant’s brain.
‘We now know that temperature determines when the parasite will take over an ant’s brain,’ he said. ‘But we still need to figure out which cocktail of chemical substances the parasite uses to turn ants into zombies.’
The study is published in the journal Behavioral Ecology.