How do limbless animals move?
LOOK AT the snail or the slug. At breakneck speed, it covers a couple of inches per minute (although at the 1995 World Snail Racing Competition in England, the record was 13 inches in 2 minutes). We humans, who are a thousand-fold faster even while strolling very leisurely, denigrate a fellow who takes a long time to do anything as a slow coach, doing things at a snail's pace.
This comparison is pure anthropo-arrogance, considering that we are far bigger in size and have arms and legs. The poor snail, on the other hand, is a limbless animal — no arms, legs, fins, gills, wings, feathers, nothing. The fact that it moves at all is a marvel. How does it do it? How do limbless animals move?
A whole slew of words are used to describe their motion, each with a specific meaning. To creep is to move with the body on or close to the ground. This is different from crawling, a word that is used typically with limbed animals such as insects, or our own infants who move about on hands and knees.
Worms and snakes slide, slither and side wind. Sliding is moving in continuous contact with a smooth or slippery surface. Slithering is sliding along a surface from side to side. Sidewinding is best exemplified by the rattlesnake, which moves about in loose sand by throwing loops of the body forward. These three types of movement are so typical of snakes that we call them together as `snaking'. The most common type of snake movement is lateral undulation. Here, the snake alternately tightens and relaxes a set of muscles along each side of its body. This produces horizontal waves that travel down the body. Moving thus, a snake can speed up to 10 kmph.
At least many snakes have a rough skin, scales, which aid in friction and thus holding on to surface. Thus it is that when they shed skin during the moulting period, they become pretty much immobile and vulnerable. In effect, they become large worms. Look, in this connection, at the inch worm. As it covers its own body length in a few seconds, it progresses by bringing its rear end of the body forward and then advancing the front end. This mode of laborious movement makes it look like a mobile Greek letter Omega (<108,SYM,87>).
But the real winner is the snail or the slug. In order to locomote, it first releases a drop or two of a sticky mucus on the ground. Next it applies a bit of downward pressure. This allows the mucus to thin and yield, helping the snail advance a bit. This action is repeated, producing a yield-heal cycle, allowing our hero to proceed on its purpose in life at its placid pace and its own rhythm. Dr. Mark Denny of the University of British Columbia, Vancouver, Canada, who studied the role of the mucus in the movement of slugs (Nature, 1980), remarks that the yield-heal cycle allows it to act as a material ratchet, facilitating forward movement, but resisting backward movement; the result is effective adhesive locomotion. The composition and the viscid nature of the mucus, and the shape and spread of the bottom surface of the snail body are vital components of the movement made for each other!
In this connection, Drs. L. Mahadevan (of Harvard University) and S. Daniel and M.K. Chaudhury (both of Lehigh University, Bethlehem, Pennsylvania) have published an interesting paper entitled "Biomimetic ratcheting motion of a soft, slender, sessile gel" in the January 6 issue of Proceedings of the National Academy of Sciences. In their attempt to understand how limbless animals move, they used a chemical worm, a gel made of polyacrylamide, as the model. It is soft, and slender as a worm's body; it is also sessile, touching the surface on which it is placed, just as a leaf touches the stem just at the base. When placed on a glass plate covered with a silicon rubber sheet on top, and vibrated, the polymer rod moves just as a snail moves on ground. The composition of the polymer gel was pre-adjusted so as to have a thin liquid layer wetting the surface. This interfacial layer acts as the mucus and offers the yield-heal cycle, and prevents the polymer rod from adhering strongly to the rubber surface.
They also did a simple mathematical analysis of the process. When vibrations occur as pulses, a large shear stress is generated in the thin mucus layer, so that it yields and flows. As the pulse propagates from one end of the body to the other, slip occurs leading to motion. The rod (and by analogy the snail) moves by pulling or pushing on those parts of the mucus that are not flowing. There is an asymmetry here — movement is perforce in one direction, and not symmetric rocking or oscillating. This asymmetry is brought about in two ways. One is the fact that the pulse moves in one direction, and the other is the material behaviour that switches between flow and no-flow.
The authors conclude that the behaviour of their chemical worm mimics many types of animal motions, namely creeping, crawling, inching and slithering. They also conclude that their model leads to "a unified view wherein all these modes arise naturally as symmetry-breaking bifurcation transitions of slender bodies when the muscular forces exceed the buckling load". Figure out that prose!
A point to note, as they write in their paper, is that unlike biological locomotion, the engine is not overboard, since the gel is driven from outside. Also the interaction between the body and the surface is not `active'; no feedback loop exists that warns the body of external conditions, allowing for change in gait. Animals depend on such feedback from the environment, and adjust their movements accordingly. Touch a snail and it retracts into its shell, lying there motionless, hopefully making predators lose interest. While the chemical worm might not mimic all that the animal does, the model does become a useful first step in devising biologically inspired robots.
That is one of the points behind why study something like snail movement. It is not only plain curiosity, or just `because it is there', but we can learn to make robots and micro-machines, inspired by what we learn from biology. Indeed, there is a book Biologically Inspired Robots (by Shigeo Hirose, Oxford University Press, 1993), and a graduate course on the subject at the Colorado State University, among others.
D. Balasubramanian
dbala@lvpei.org
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