Parasitic worms that zombify ants to do their bidding might be even more clever than we knew. Recent research has found that these parasites can not only compel ants to climb up blades of grass but also to climb back down when the weather gets too hot—all as part of their devious strategy to get eaten by larger animals to continue their complex life cycle.
The parasites are a type of flatworm known as the lancet liver fluke (Dicrocoelium dendriticum). These worms primarily live out their lives as adults inside cows or other grazing ruminants, but they take a long journey to get there.
Worm eggs pooped out by cows will end up in the grass, where they get eaten by snails. The worms reach their next larval stage inside the snail and reproduce asexually into thousands more. The snails respond to the infestation by forming hard cysts around the invaders, which are coughed out as slimy balls of mucus. These slime balls are then unwittingly eaten by ants, along with the worm larvae.
Once inside an ant, thousands of larvae grow into their next stage of life. Most will migrate to the ant’s stomach, safely cocooned, but one will make its way to the brain and hijack it. The infested ant is coerced to climb to the top of a nearby grass blade and clamp down on it, providing an easy opportunity for roaming ruminants to inadvertently eat the ant and its wormy parasites. These thrice-consumed worms finally reach adulthood inside their last host, move down to the liver, feed, mate, and lay the eggs that will restart this gruesome cycle again (the actual brain worm sacrifices itself for its brethren and doesn’t survive this last leg of the journey).
While the basic details of the lancet liver fluke’s life cycle are known, there’s much that scientists don’t understand about this complicated process. So a team of researchers at the University of Copenhagen’s Department of Plant and Environmental Sciences decided to look more closely. They studied more than 1,000 infected ants in the Bidstrup Forests near Roskilde, Denmark for 13 non-consecutive days over the course of a year, painstakingly tagging a subset of 172 ants for better observation.
The team theorised that several factors might affect the infected ants’ behaviour, such as humidity and the time of day. But it was temperature that appeared to have the greatest influence. On relatively cool days, the ants stayed glued to the grass pretty much the entire time. But when the weather got warm, the ants crawled back down and seemingly went about their normal business. This meant that the ants were most often puppeteered by the worms at night and in the morning.
“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. Our discovery reveals a parasite that is more sophisticated than we originally believed it to be,” said study author Brian Lund Fredensborg, an associate professor at the University of Copenhagen, in a statement. “We joked about having found the ants’ zombie switch.”
The findings, published last month in the journal Behavioral Ecology, highlight how little we still know about parasites in general, the authors say. And it will take more research to uncover the specific mechanisms that these flukes use to zombify an ant’s brain.
Thankfully, there’s no need to worry about being turned into zombies by these flukes. Humans do occasionally become infested by them, which can sometimes cause serious harm to the liver and bile ducts. But these infections are rare and humans are only an accidental primary host for the worm.