Crabs adapt to out of body experiences
Using tools Hermit crabs see their shells as an extension of the bodies and adapt their movements accordingly, new research by Japanese scientists has found.
The finding, published today in the journal Biology Letters, is also providing an insight into how we use tools and drive vehicles subconsciously.
The researchers, led by Dr Kohei Sonoda of Kobe University, studied terrestrial hermit crabs (Coenbita rugosus) from Okinawa. The coastal crabs use empty gastropod (snail) shells for protection and frequently change them in search of a better fit.
"There are not many gastropod shells available so they have to make do with whatever they can find, even if it is awkward or uncomfortable," says Professor Yukio Gunji, another of the study's authors.
Some marine species are even known to attach sea anemones in strategic positions to help balance their shells and make them more comfortable to carry, he says.
"Hermit crabs are always on the lookout for a more comfortable fit and will thoroughly investigate any new shell they find before deciding whether it is right for them," says Gunji.
"When they move into the new shell they have to quickly adapt to its new shape and to its weight and balance to avoid collisions and other potential risks."
This involves a fundamental reassessment of their body image and how it fits into a structurally complex environment.
To find out how they adapt, the researchers glued plastic plates asymmetrically to the shells' surface then observed the crabs' progress along a corridor with alternating left and right hand turns.
Initially the crabs seemed unable to balance the shell, however, they rapidly adapted to their awkward new shape. Within 10 to 20 seconds they modified their walk and increased their turning angle when going around corners to accommodate the load.
"This suggests that the shell, when extended by the plate becomes assimilated to the hermit crab's own body," says Gunji.
An extension of our body
He says these findings offer new insights into body consciousness, which is a combination of our body schema - the parts we can control and manipulate - and body image - our understanding of the body as a whole entity.
"A person who plays tennis well uses the racket as an extension of their arm," he explains. "The racket is integrated into the body schema, but the person feels no pain if the racket is damaged because the racket isn't integrated into their body image.
"We assume that body image is dynamically negotiated with body schema to create body consciousness, but there haven't been many studies."
Hermit crabs demonstrate this interaction because they quickly adapt when their body schema is changed (i.e. a plastic plate is attached to their shell), and their body image also dynamically changes to accommodate the plate.
A similar thing happens when we drive a car.
While this research has implications for cognitive science, it is also connected to the study of tool use, says Gunji.
"Previously tool use was strictly defined with many people believing it was dependent on intelligence. Recently though, some researchers have begun to think that the notion of 'tools' is much harder to define because of the way they become integrated into body schema, and how that may change depending on context," he says.
"The hermit crabs' assimilation of their shells as a virtual body and dynamic use of them as tools for shelter may help use redefine the way we think about tool use."