Standing on the Water: Stability Mechanisms of Snakes on Free Surface
167
Brischoux and Shine [5] report a statistic correlation between the medium of
locomotion and the aspect ratio of the snake cross-section defined by the ratio of
the height over the width (h/w). Fully aquatic snakes have generally an asymmetric cross-section with h/w = 1.56 ± 0.5 (averaged along the body, without
the tail), that allows for a more efficient thrust against the water. Terrestrial
snakes have a rounder body with h/w 1. For amphibious snakes, the aspect
ratio is intermediate with h/w = 1.2 ± 0.2 [5], corresponding to a slightly elliptically deformed body section. The geometry of the cross section is very important
for static stability since the immersed surface and the center of buoyancy depend
on it (see Sect. 3). The number of vertebra in the vertebral column may vary
from 130 to 300 [8]. The interval between two consecutive vertebra allows lateral
bending (around 10
◦ and 20
◦ ), dorso-ventral bending (few degrees), torsion (very
small) and longitudinal displacement (traction-compression) [8]. The major longitudinal muscles are multi-articular and are attached to the vertebral column
thanks to multiples tendons spanning large intervals varying between 9 and 33
vertebra [7,8]. As we will see in the next section, the large number of degrees of
freedom is a decisive advantage for stabilizing a body on a free surface. Lungs
play also a crucial role in the buoyancy control. The lungs of the snakes are
long and narrow, and the left lung is generally atrophied or lost for non fully
aquatic snakes. For fully aquatic snakes, the lungs can reach 100% of the body
length, while for semi-aquatic snake, like the Nerodia Pictiventris, the lungs end
at 53% of the body length [7]. Note that this snake species experiences a heaving
instability at low speed [7], suggesting a correlation between lungs length and
swimming stability. Graham [6] showed that the Pelamis Pletarus (spending
87% of its time submerged) can vary the volume of its lungs to adjust buoyancy.
During surface resting, the average mass density of this snake is particularly
small with ρ = 555 kg/m
3 , and then goes to 770kg/m
3 when it dives. Thus, the
respiratory system is involved in postural equilibrium and locomotion of aquatic
snakes.
The synergies between the actuation of a large number of degrees of freedom, the ability of controlling its buoyancy and the remarkable perception of its
environment could be at the origin of its spectacular sense of balance on water.
In Situ Observations. In our scientific consortium, we have the exceptional
opportunity to collaborate with a field biologist, Seth LaGrange, who has
observed semi-aquatic snakes exhibiting spectacular poses on water surface.
Some of them are reported in Fig. 1. These snakes are adult Cottonmouths
(Agkistrodon piscivorus) living in close proximity to water and are the most
aquatic species in the genus Agkistrodon. Cottonmouths are large, heavy bodies
pitvipers reaching lengths of 122 cm. They have large, angular heads with facial
pits used for detecting infrared radiation of prey and predators.
The pictures were all taken in southern Illinois on a river called Clear Creek.
The snakes are usually using the water to travel. They also exploit water to
hunt, look for, returning to a basking site, or escaping predators. Their head
is kept inclined and significantly above the water surface. This behaviour can
be motivated by a better used of their infrared sense. We may speculate that
167
Brischoux and Shine [5] report a statistic correlation between the medium of
locomotion and the aspect ratio of the snake cross-section defined by the ratio of
the height over the width (h/w). Fully aquatic snakes have generally an asymmetric cross-section with h/w = 1.56 ± 0.5 (averaged along the body, without
the tail), that allows for a more efficient thrust against the water. Terrestrial
snakes have a rounder body with h/w 1. For amphibious snakes, the aspect
ratio is intermediate with h/w = 1.2 ± 0.2 [5], corresponding to a slightly elliptically deformed body section. The geometry of the cross section is very important
for static stability since the immersed surface and the center of buoyancy depend
on it (see Sect. 3). The number of vertebra in the vertebral column may vary
from 130 to 300 [8]. The interval between two consecutive vertebra allows lateral
bending (around 10
◦ and 20
◦ ), dorso-ventral bending (few degrees), torsion (very
small) and longitudinal displacement (traction-compression) [8]. The major longitudinal muscles are multi-articular and are attached to the vertebral column
thanks to multiples tendons spanning large intervals varying between 9 and 33
vertebra [7,8]. As we will see in the next section, the large number of degrees of
freedom is a decisive advantage for stabilizing a body on a free surface. Lungs
play also a crucial role in the buoyancy control. The lungs of the snakes are
long and narrow, and the left lung is generally atrophied or lost for non fully
aquatic snakes. For fully aquatic snakes, the lungs can reach 100% of the body
length, while for semi-aquatic snake, like the Nerodia Pictiventris, the lungs end
at 53% of the body length [7]. Note that this snake species experiences a heaving
instability at low speed [7], suggesting a correlation between lungs length and
swimming stability. Graham [6] showed that the Pelamis Pletarus (spending
87% of its time submerged) can vary the volume of its lungs to adjust buoyancy.
During surface resting, the average mass density of this snake is particularly
small with ρ = 555 kg/m
3 , and then goes to 770kg/m
3 when it dives. Thus, the
respiratory system is involved in postural equilibrium and locomotion of aquatic
snakes.
The synergies between the actuation of a large number of degrees of freedom, the ability of controlling its buoyancy and the remarkable perception of its
environment could be at the origin of its spectacular sense of balance on water.
In Situ Observations. In our scientific consortium, we have the exceptional
opportunity to collaborate with a field biologist, Seth LaGrange, who has
observed semi-aquatic snakes exhibiting spectacular poses on water surface.
Some of them are reported in Fig. 1. These snakes are adult Cottonmouths
(Agkistrodon piscivorus) living in close proximity to water and are the most
aquatic species in the genus Agkistrodon. Cottonmouths are large, heavy bodies
pitvipers reaching lengths of 122 cm. They have large, angular heads with facial
pits used for detecting infrared radiation of prey and predators.
The pictures were all taken in southern Illinois on a river called Clear Creek.
The snakes are usually using the water to travel. They also exploit water to
hunt, look for, returning to a basking site, or escaping predators. Their head
is kept inclined and significantly above the water surface. This behaviour can
be motivated by a better used of their infrared sense. We may speculate that