Earthworms take group decisions, travel in herds: Scientists

Last Updated: Sunday, April 11, 2010 - 17:56

London: 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.

PTI



First Published: Sunday, April 11, 2010 - 17:56

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