Earthworms take group decisions, travel in herds

Contrary to the long-held belief that earthworms lack social behaviour, a new research has found the creatures, which play an important ecological role, use touch to communicate and take 'group decisions' to travel in the same direction as part of a single herd.

Researchers at University of Liege in Gembloux in Belgium, who have discovered this striking behaviour in the earthworm 'Eisenia fetida' for the first time, said a social cue influences earthworm behaviour "Our results modify the current view that earthworms are animals lacking in social behaviour," said researcher Lara Zirbes.

"We can consider the earthworm behaviour as equivalent that of a herd or swarm."

Zirbes and colleagues were originally interested in knowing how earthworms interact with other microorganisms in the soil.

However, they noticed that the earthworms formed herds 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," Zirbes told the BBC.

The surprising behaviour fascinated the scientists to do more research as to how earthworms decided where to go, and whether they preferred to travel alone or in groups.

For their study, the researchers chose the earthworm Eisenia fetida, which tends to live near or at the soil surface, and carried out a series of experiments.

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," said Zirbes.

In another test, 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," said 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 Ethology.