Why hugging the Wall?
A mouse, a newt, a beetle… when placed into the unfamiliar surroundings of an empty, featureless container (not all at once, of course; that wouldn’t be a good idea!) all show a characteristic behaviour that is common to a large number of animal species: they hug the wall. Once they have found the edge of the container, their restlessness reduces, they calm down and may even stop to move altogether.
If you choose a container with a square outline, the animal you put in it will in all probability come to rest in a corner. And this habit of seeking the edge or better still a corner is not restricted to the terrestrial environment, for if you observe what happens when you put a fish into an empty aquarium (well, water has to be in there of course), you will observe the same behaviour: the fish seeks the edge and feels most comfortable in the corner. This phenomenon of looking for maximum tactile (= touch) stimulation, of bringing as much of the body into contact with as much of the surrounding objects as possible is termed “thigmotaxis” and represents a behaviour which is quite independent of seeking darkness. You can, after all, spotlight exclusively the corners of your experimental chamber with no difference in the result, even in the case of the nocturnal and darkness-loving mouse.
It does, of course, make sense to seek protective cover in this way. Firstly, you are less easily spotted this way than out and in the open and secondly, positioned in narrow passages and crevices it is far easier to defend yourself than if you were not surrounded by objects and therefore attackable from all sides. Moreover, at least in Nature, the areas giving you physical body contact are often less well illuminated, adding to their appeal as perfect hiding places.
Thigmotaxis has an inhibitory influence on movement and when the comfort-spending contact is removed from the animal, it will usually be fast to react with an increase in locomotor activity: it will more or less rapidly run away to find a new place that allows it to squeeze in and have its body in contact with. Thigmotaxis is also involved when you suspend a locust in mid-air by glueing its thorax to a thin wire or a stick. Does it fly and flap its wings? It depends: if the locust holds something in its feet, for instance, you allow the insect to hold a piece of wood or some paper, it will not fly. But if you suddenly pull the support away from its feet, it will immediately start to fly, flapping its wings. If you want it to stop, you give it to hold something again and the touch receptors on the feet will inform the locust that it is in contact with an object (a form of thigmotaxis) and that then there is no point to fly.
Although we humans are rarely subject to being suspended in mid-air like our experimental locust, we, too, exhibit a kind of thigmotaxis, a preference for the wall: have you ever observed which tables in an empty restaurant are first occupied by customers when the restaurant opens its doors? Doesn’t that remind you of our mouse, newt and beetle that I had started this essay with?
© Dr V.B. Meyer-Rochow and http://www.bioforthebiobuff.wordpress.com, 2018.
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