T
here are approximately 3,400 extant species of
snakes, having a cosmopolitan distribution and
occupying a huge range of habitats. The evolution-
ary success of this group is due in part to their incredible
locomotory abilities. An obvious diagnostic feature of
snakes is their state of ‘leglessness’: the earliest snakes
possessed tiny hind limbs, however through the course of
evolution these were lost. Early-diverging snakes (e.g.
boas and pythons) still retain vestiges of the pelvic girdle
and hind limbs, however these are functionless. Natural
selection was responsible for the loss of limbs, so snakes’
body plan must confer adaptive benefits. Nevertheless,
being legless poses significant challenges in terms of
locomotion.
two additional sets of zygapophyses - paired bony
processes that interlock each vertebra with the vertebrae
above and below. These help limit torsion, without
dramatically restricting the lateral bending of the vertebral
column. But moving on land using lateral undulations is
much harder than in water. Tetrapods use their limbs to
generate thrust. The solid framework of bones and muscles
functions as a system of levers, transmitting force to the
substrate and powering the animals along. Without limbs,
snakes lack these propulsive forces. Instead, their anatomi-
cal basis for movement involves their long backbones
(comprising several hundred vertebrae) and complex,
multi-segmented muscle chains and tendons. On surfaces
with some texture, snakes’ scales create passive friction,
with less directed towards the front than the back,
Stem snakes (the earliest snakes identified from the fossil enabling forwards movement. The amount of friction can
record) evolved approximately 125 million years ago, and be actively adjusted by modifying the angle between the
crown snakes (modern snakes) evolved about 105
scales and the substrate – too much would impede move-
million years ago. Both originated on land (rather than in ment.
aquatic settings), and snakes’ distinctive long, limbless
body plan appears to have evolved as an adaptation for
modes of
burrowing. This hypothesis is supported by the analysis
of fossils (Dinilysia patagonica) linked to ancestral
snakes; these possess a unique inner ear structure shared
are recognised.
by extant burrowing snakes and lizards, but which is
absent in snakes living in water or above ground.
Furthermore, all extant snakes have an elongate body with The mechanics of movement.
a relatively short tail - a trait they share with burrowing
lizards.
Six modes of snake locomotion are recognised: lateral
undulation, sidewinding, concertina, rectilinear, ‘slide-
Like ancestral fish and then the first land-based tetrapods, pushing’ and saltation. Several different modes may be
ancestral reptiles inherited a form of locomotion based on employed simultaneously at different points along the
alternating lateral undulations of the body. Most snakes
snake’s body. Species differ in their tendency to use a par-
retain this pattern, however the lack of legs and highly
ticular mode, and this is associated with differences in
undulatory nature of movement places very high twisting body plan and adaptations to the substrate they frequently
forces on their vertebral columns. To cope, snakes have
encounter. Lateral undulation – characteristically called
serpentine movement – is the most widespread mode of
locomotion. Concertina locomotion and sidewinding are
also common, and are used when there are insufficient
substrate projections necessary for lateral undulations. All
generate propulsive forces through laterally flexing the
vertebral column by contracting axial muscles.
‘SIX
SNAKE
LOCOMOTION
During lateral undulations, horizontal waves travel down
alternate sides of the body. Although limbed reptiles also
often move with lateral undulations, snakes differ
because lacking limbs to provide fixed points for
generating propulsive force, they instead rely on moving
their body continuously to push past fixed irregularities
(e.g. stones, grass tussocks, bumps) in the environment.
Despite each point generating a sideways force, the
lateral forces on opposite sides of the body cancel out,
leaving a net rearward force which propels the snake
forwards. Studies have revealed that laterally-undulating
snakes move by continuous posterior propagation of
alternating unilateral muscle activity, with limited
contribution from the tail. Whilst this form of locomotion
works well on rugged substrates, it gets a snake nowhere
on a smooth surface!
Sidewinding is effective on low-friction, shifting sub-
strates like sand or mud. Watching a snake sidewinding is
’