Vertebrate Natural History - Lecture 10 - Spring 2001
Vertebrate Locomotion Continued.....
Vertebrate Locomotion Continued
** - Slithering and related forms of motion employed by snakes are the consequence of their evolution — from animals that lost their appendages — historically such animals live a burrowing existence.
** Consequently the entire body (anatomically) evolves for locomotion — the body is elongated with an increased vertebral column (usually between 100 and 400 trunk vertebra in a snake) — versus 16 in a typical mammal. Moreover, each pair of vertebra bears a pair of ribs — that are flexible and supportive.
** - Increased vertebra = increased flexibility, but complicates control problems (although the number of degrees through which a snake can bend is remarkable).
** - Snakes in general are able to propel themselves in 4 distinct ways:
I. Lateral Undulation(s) = is the most common mode of snake locomotion and is probably the preferred form of movement for snakes as well as for limbless lizards.
** - This type of undulation is the most primitive in vertebrates and involves body movement along an S-shaped path.
** - The snake begins to undulate laterally by bending the forward part of its body. This movement establishes a wavelike muscular contraction that travels down the snake's trunk. These contractions do not propel it by acting on the ground under the snake's belly .......
** - Rather, these contractions cause the snake's body to exert force laterally on irregularities in the snake's path (elevations, pebbles, depressions etc....)
** - The snake pushes itself from point to point and creates loops in its body. The outside of each loop forms a contact point. The speed at which a loop progresses down the snake's body toward the tail = the snake's forward speed.
** - Moving loops in this lateral undulation form of movement — appear stationary with respect to the ground (experiments conducted with peg boards demonstrate this type of movement (at least 3 irregularities were required for effective movement). On a smooth, flat surface, lateral undulations are not achievable.
** - Long, slender snakes like the black racer, can use this mode more efficiently than the shorter, heavy bodied snakes like a rattlesnake or cottonmouth.
** - Although a snake propels itself laterally, it does apply other forces on the ground ---- a vertical force (weight of the snake) – and a horizontal force as a result of friction (exerted by muscular activity).
** - Friction is produced by lateral undulations in 2 ways: (a) on the underside of the body sliding on the ground, and (b) on the sides of the body sliding against objects. In both instances, frictional forces act in opposition to the force inducing motion: thus, in lateral undulations, friction has to be overcome by muscular effort.
** - There is a selection pressure to reduce the amount of friction associated with a snake's skin — and that friction is generally unnecessary for movement in a snake. This has been demonstrated in experiments with a snake on a frictionless, lubricated surface with pegs (it can still crawl).
** - Forward lateral undulations (continuous forward progression) by a snake requires a minimum of 3 contact sites at all times. The snake pushes against two to generate force and the third to balance the forces produced at the other two — so the body can move in a particular direction.
** As the number of contact points there is compromise:
(a) snakes are flexible and must expend energy to make their bodies rigid for movement. The more contact points, the less energy exerted to create rigidity.
(b) on the other hand, the greater the number of contact points, the shorter the amplitude of the snake's curves, the wider the angle of force application and the greater the waste of force in the snake's direction of movement.
Lateral Undulations do not work if: (a) a snake is traveling down a tunnel with parallel sides, or (b) on flat surfaces lacking elevations, depressions etc...
II. An alternative to lateral undulations is Rectilinear Movement - which differs from lateral undulations in two respects: (a) it involves the application of force downward vs. laterally, and (b) it is effective only if friction is established between the snake's skin and the ground.
** Rectilinear Movement is a very slow form of movement. It is made possible by the snake's loose skin. The snake fixes several series of scutes – and starts to move the skin between them. As the body of the snake moves forward, the skin is stretched, pulling the forward-most scutes of each series out of contact with the ground, while additional scutes are continuously pulled up to the rear edge of the series.
** This type of movement shows little sign of movement within the snake's body —
** - In rectilinear locomotion, the 2 sides of the snake must move symmetrically, rather than in alternation. This type of motion enables the snake to across flat surfaces and to advance in a straight line when stalking prey.
** However, most snakes do not have the musculature & loose skin necessary for rectilinear movement.
Such snakes can still use static friction (force applied parallel to the contact surface) to move in a relatively straight line by employing:
Concertina Progression
** This type of movement occurs when there are fewer irregularities associated with the substrate or when the curvature of the animal's path is uniform — so that lateral undulations are not possible.
** - In Concertina Progression, the snake draws itself into an s-shaped curve similar to lateral undulations — and sets the curved portion of its body in static contact with the ground.
** Motion begins when the head, the neck and the forward part of the body are extended by forces transmitted to the ground in the zone that remains in stationary contact.
** After the front end of the snake has moved forward a short distance, it stops. This establishes a new zone of stationary contact in which horizontal forces are exerted against the ground — and the rear end of the snake's body is pulled forward (inchworm effect).
** Note – concertina progression is about as common as lateral undulations, and one often sees a long snake combining the two kinds movement. Rat snakes climb trees using both methods (double keeled scales on the belly).
IV. Sidewinding -
** - This is mechanically the most efficient mode of snake locomotion.
(a) Large S-shaped loops are formed in the snake's body – the head is on the ground – and the neck is bent and also on the ground,
(b) The posterior portion of the body is progressively laid down parallel to the head. Several body segments may be in contact with the ground,
(c) very rapid style of locomotion which reduces body contact to the ground & allows for the avoidance of predators.
Mostly found in desert snakes such as rattlesnake species.
