the anterior region to be extended forwards. The process
is then repeated. Concertina locomotion is used on
surfaces that are unsuitable for lateral undulations, but
where enough static friction exists to prevent backwards
slippage. This form of locomotion is most commonly
used when crawling through burrows or tubes - lateral
undulations are restricted, yet snakes can brace their
bodies against the walls. Concertina locomotion is not for
animals in a hurry. Banded Water Snakes (Nerodia
fasciata) travel only 0.05 total lengths per second using
this form of locomotion, but can achieve 1.88 total
lengths per second using lateral undulations. It also
requires a lot of energy; Eastern Racers (Coluber
constrictor) use seven times more energy when
employing concertina locomotion, compared with lateral
undulation.
Rectilinear locomotion differs from other forms of snake
locomotion in that it does not rely on alternating
contractions of the lateral muscles along the trunk.
Rather, both of the lateral muscle masses act in
synchrony, sequentially contracting and relaxing, which
draws the body forward in a fairly straight line.
Rectilinear locomotion mainly involves two series of
muscles which run from the ribs to the skin of the ventral
surface. The costocutaneous superior muscles pull the
skin forwards relative to the ribs; the ventral scales then
anchor the body to the substrate. Next, the costocutaneous
inferior muscles pull the ribs - along with the vertebral
column, axial muscles, and viscera - forward relative to
the stationary ventral skin. Several waves of these
symmetrical contractions pass down the body at any one
time, so that a number of points of stationary contact are
established. This creates a bizarre effect in which it
appears that the ventrolateral skin is crawling on its own,
whilst the dorsal skin moves at a nearly even rate!
Rectilinear locomotion is most common in large snakes
like boids and vipers, however all snakes are likely
capable of using this mode of locomotion.
Snakes sometimes use ‘slide-pushing’ when travelling on
low-friction substrates. Although similar to lateral
undulation in that it also involves alternating waves of
body motion, in slide-pushing there are no fixed points in
the physical environment to generate forces pushing the
body forwards. Instead, the snake moves its body
incredibly quickly, propagating waves so rapidly that
enough sliding friction is generated to propel it forwards.
This form of locomotion isn’t very efficient – despite all
that wriggling, slide-pushing snakes appear to be simply
flailing about, and only progress gradually.
Finally, saltation is a rare, pretty extreme form of
locomotion employed by the Horned Adder, a short,
heavy-bodied viper from southern Africa. The snake
rapidly straightens its body from anterior to posterior,
which actually causes it to be lifted entirely off the
substrate. Only very small individuals move in this
manner.
Young Peringuey’s Adder (Bitis peringueyi) with trail
in the Namib Desert. Image by MyImages - Micha.