The earthworms use touch to communicate and influence each other's behaviour, according to research published in the journal Ethology.
By doing so the worms collectively decide to travel in the same direction as part of a single herd.
The striking behaviour, found in the earthworm Eisenia fetida, is the first time that any type of worm, or annelid, has been shown to form active herds.
"Our results modify the current view that earthworms are animals lacking in social behaviour," says Ms Lara Zirbes, a PhD student at the University of Liege in Gembloux in Belgium.
"We can consider the earthworm behaviour as equivalent the of a herd or swarm."
Ms Zirbes and colleagues were originally interested in how earthworms interact with other microorganisms in the soil.
These interactions are part of the important ecological role of that earthworms play.
However, the researchers began to notice that the earthworms seemed also to interact with each other.
"In experiments, I noticed that earthworms frequently clustered and formed a compact patch when they were out of the soil," Ms Zirbes told the BBC.
Follow the leader
So Ms Zirbes and her colleagues set up a series of experiments to test how earthworms decided where to go, and whether they preferred to travel alone or in groups.
They chose the earthworm Eisenia fetida, which tends to live near or at the soil surface, typically within the litter lining forest floors.
First, they placed 40 earthworms into a central chamber, from which extended two identical arms.
The idea was to leave the animals alone, and then to see how many earthworms moved to either arm over a 24-hour period.
Over 30 identical repeats of the trial, the worms preferred to group within one chamber over the other.
"We noted that earthworms moving out of the central chamber influenced the directional choice of other earthworms.
"So our hypothesis was confirmed: a social cue influences earthworm behaviour," says Ms Zirbes.
Touching moments
A second experiment tested how the worms affected each other's behaviour, investigating whether the worms use either chemical signals or touch to decide which chamber to move to.
The researchers placed one worm at the start of a soil-filled maze, with two routes to a food source at the end.
After the worm chose its route to the food, the researchers added a second worm to see if it followed the same route as the first.
However, after repeated trials, the second worms were no more likely to take the same route as their predecessors. This indicated that the worms did not leave a chemical trail behind them that communicated their direction of travel.
Yet if two worms were placed together at the start of the maze, they were more likely to follow one another, suggesting that they used touch to communicate where they were going.
In two-thirds of these trials, the worms followed each other.
"I have observed contact between two earthworms. Sometimes they just cross their bodies and sometimes they maximise contact. Out of soil, earthworms can form balls," says Ms Zirbes.
A modelling study then showed that, by using touch alone, up to 40 earthworms could follow each other in a similar way, explaining how herds of the animals preferred to move together into one chamber in the initial experiments.
"To our knowledge this is the first example of collective orientation in animals based on contact between followers," the researchers wrote in the journal.
"It is also the first one of collective movements of annelids."
Defensive posture
The researchers suspect that other earthworm species may behave in a similar way. They now hope to investigate why the animals come together to form herds.
One reason may be that clustering helps the worms protect themselves.
Individual Eisenia fetida earthworms secrete proteins and fluids which have antibacterial properties, potentially deterring soil pathogens.
They also secrete a yellow fluid to deter predatory flatworms.
Gathering into groups may increase the amount of fluids covering the earthworms and hence better protect individuals, the researchers say.
By Matt Walker
Editor, Earth News
http://news.bbc.co.uk/earth/hi/earth_news/newsid_8604000/8604584.stm
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