I've always liked reptiles. I used to see the universe as a mammoth snake, and I used to see all the people and objects, landscapes, as little pictures

in the facets of their scales. I think peristaltic motion is the basic life

movement. Swallowing.

- Jim Morrison American Poet and Singer, 1943-

1971

SNAKES

Cool Features

• Poison/Toxins

•Digestive capabilities

•Coiling Abilities •Shedding

Anatomy & Movement

Anatomy of a Snake

Anatomy

• Skeleton

• Digestion

• Senses

• Scales/Exterior

• (Musculature)

Skeleton

• Skull• Vertebrae• Ribs

Skull• Many loosely connected bones

• Many elastic ligaments

• Brain completely enclosed by bone

• Two sides of jaws can be moved separately

•!Not dislocating lower jaw!

• Lower jaw – 2 bones connected at chin by an elastic tissue– Loosely attached to

upper jaw

• Teeth bend back in– Fangs like syringe

• Jaws as multibar linkages– “quadrate”

bonedouble hinge

Vertebrae

• Distinct units

• Many more than in other vertebrates

• 150-430 in #

• Strong, flexible joints↑movement (ball)

Ribs

• A set of 2 attached to each vertebra

• Not connected on the under-side– Can be extended out (room for stomach)

Senses• Internal Ears

– “quadrate” bone focuses cochlea

• Forked Tongue/Jacobson’s organ– directionality

• Body Heat- “pit organs”

• Vibrations (along stomach)

•Can See movement•“Near-sighted”

Digestion

• no chewing• Swallow large prey whole

– Move one side of jaw forward, then the other– Curved teeth stick into prey-prevent escape– Alt. draw each side of jaws backpull prey towards

throat

• ↑saliva• Windpipe pushed forward, over tongue, out

mouth• Can digest entire prey except for hair, feathers

Scales/Exterior

• Complete cover (recall insect exoskeleton)

• Transparent, fused eyelids

• 2 layers: (recall human skin)– Inner layer: living cells

that grow, divide– Outer layer: dead cells

what is shed

• Expandable skin• Often camouflage

Molting/Shedding

• Younger>old• More active>less• More temperate regions>less• Grow cont. throughout life• remove parasites

-Rub nose against rough surfacerip in skin-Crawl out; left inside-out in one piece

Movement

• Lateral Undulation (most common)

• Straight-line/rectilinear locomotion

• Concertina movement

• “Sidewinding”

UndulationEdges of curvature push against substratum

Rectilinear

Used to climb up trees (!)

Concertina movement:

• Move front part of body forward, coil it-press against surface to anchor

• Pull back end forward, coil it

• Back end pressed down-provides leverage for repeat

• Esp. narrow channels

Sidewinding:

• Head and tail as supports

• Lift trunk of body off ground and move sideways

• Move head and tail back into position

• Esp. in sand

Evolution of Snakes, Their Movement and Biomechanical

Behaviors

Evolution of Snakes

• Diapsids ("two arches“): a group of tetrapod animals -developed 2 holes in each side of skulls- 300 million years ago

extremely diverse include all snakes as well as other animals. • some lost either one hole (lizards), or both holes (snakes)• There are 2 distinct clades:• Lepidosauria (includes snakes) • Archosauria• These branched off early from diapsid trunk. • These 2 groups characterized by contrasting patterns in locomotion

& posture. • Clade = group of organisms consisting of single common ancestor

and all descendants of that ancestor • Representative species of the 3 groups of the Lepidosauria :

sphenodontids, lizards, & snakes

Order SquamataSuborder Serpentes (Ophidia)

The phylogeny of snakes is poorly known since snake skeletons are typically small & fragile

This makes fossilization difficult and unlikely

General consensus based on morphology: snakes descended from lizard-like ancestors

There is no general consensus on the phylogeny of snakes, but here are some examples:

Lepidosaurs

• Lepidosaurs retained:• sprawling posture • laterally directed movement of limbs found in primitive

tetrapods• Lateral undulation of vertebral column was also

important method of locomotion for most lepidosaurs• “lateral undulation” reached highest degree of

development in snakes• Loosely separated skull bones (allowed prey to be

swallowed whole) = another important lepidosaur feature

Relation to Lizards

• Lizards and snakes are considered to be a single clade: Squamata (scaled reptiles).

• Snakes and lizards share many distinct features in the structure of their skull;

• Both lizards and snakes have legless forms with jaw structure that allows them to swallow prey whole

• Snakes originated much later in the fossil record than lizards

• Based on these similarities theory:• ancient group of monitor-like lizards began to follow

burrowing way of life (tunneling through loose dirt and sand in search of prey) just as some lizards do today.

Where do snakes come from?

• Over period of MILLIONS of years…• these burrowing lizards lost their limbs & their external

ears (helps them burrow more easily)• replaced their eyelids with a clear brille or spectacle to

protect their eyes while digging • About the time dinosaurs reached their apex:• one group of these burrowing lizards gave up

subterranean lifestyle & emerged to the surface • AND developed a new legless mode of locomotion• AND rapidly diversified to invade a large number of

ecological niches• Today we classify the various descendants of these

legless lizards as…

Snake Ancestors

• 4 fossils bear closely on the ancestry of snakes: • Pachyrhachis • Podophis • Lapparentophis• Dinilysia. • The marine squamates Pachyrhachis and Podophis: considered most

primitive snakes (because have a well-developed hind-limb skeleton)• Terrestrial snake Lapparentophis (called "oldest snake“) is

represented by vertebrae only, but clearly snake vertebrae• Dinilysia: has skull that is “mosaic” of lizard & primitive snake

characters, but vertebrae are like those of boa-like snake.

Pythons & Boas

• 1st of modern terrestrial snakes to appear were relatives of the living boids, or boas and pythons

• large heavy-bodied snakes with a rather primitive and heavy skull structure

Colubrids

• About 36 million years ago: a group of smaller, faster snakes appeared which competed with boids for food & living space

• colubrids, or "typical snakes”• enlarged belly scales, enlarged head scales,

reduced left lung, no traces of pelvis or hind limbs

• Includes flying snakes and some water snakes

Opisthoglyphs

• About 15 million years ago, snakes began appearing which had a number of greatly enlarged teeth at the rear of their jaw (referred to as opisthoglyphs or "rear-fanged" snakes)

• Sandsnake is example

Proteoglyphs

• Shortly after, another group of snakes developed more refined venom apparatus. (proteroglyphs)

• Proteroglyphs have short fixed fangs which have migrated (by reducing the size of the maxillary bone) to front of mouth

• Cobra is example

Solenoglyphs

• By about 10 million years ago, most highly specialized of snakes appeared in fossil record

• Vipers-characterized by extremely long fangs

Rattlesnake

• A few million years ago: a group of pit vipers developed structure at end of tail, made up of interlocking pieces of unshed skin, which could be loudly rattled and used as warning device against predators.

• Rattlesnakes: thought to be most specialized of all the living snakes.

WARNING: “the whole picture of early snake evolution has been muddled by jargon-filled, convoluted arguments, the problem, as always, being the basic similarity of snakes and lizards.”

- J. Alan Holman

Swimming Snakes

Swimming Snakes• Largest group of sea snakes (hydrophiids) evolved from Australian terrestrial elapids

that returned to the marine environment around 30 million years ago. • approximately 70 species of sea snakes live in our modern oceans (account for 86%

of marine reptile species alive today)• Sea snakes have specialized flattened tails for swimming and have valves over

their nostrils which are closed underwater • Differ from eels --don't have gill slits & have scales• Need to breathe air, so usually found in shallow water where they swim about the

bottom feeding on fish, fish eggs and eels.• Persistent myths about sea snakes: they can't bite very effectively. • However: their short fangs (2.5-4.5mm) are adequate to penetrate the skin• They can open their small mouths wide enough to bite a table top.

How do they do it?• Snakes elongated bodies = pre-adapted for efficient swimming• most sea snake species developed paddle shaped tail that further

enhances ability to move in water • Sea snakes can spend: 30 minutes - 2 hours diving between

breaths. • They have 2 major adaptations that allows them to do this: • 1: they have 1 elongated cylindrical lung that extends for almost the

entire length of body ( very efficient for gas exchange)• They are able to carry out cutaneous respiration (oxygen diffuses

from sea water across snake’s skin into tiny blood vessels & carbon dioxide diffuses out)

• 2: Sea snakes have nostril valves that prevent air entering lung while underwater.

• Nostril valves open inwards & are held shut from behind by erectile tissue engorged with blood (like a penis)

“Explaining what it takes for a snake to glide sounds a bit like an episode of Sesame Street: today's program is brought to you by the letters J, S, and C.”

- Socha

Flying Snakes• found only in South and Southeast Asia,• Roughly circular in cross-section • aren't actually able to fly BUT can glide or parachute (like flying squirrel)• Socha worked with paradise tree snake, Chrysopelea paradisi (what we will

examine)• When flying snake prepares to jump: dangles like letter J from branch• Then flings itself upward & away from branch fall at steep angle that

could be described as “plummet”• After falling less than 10 FEET, it flattens entire body (but not tail) &

undulates through air in an S shape by moving head from side to side, as if on land (but more lateral movement)

• Snake uses its ribs to change body shape- flattens from head to vent. • S-shaped undulation keeps body parallel to the ground and allows for

stability as snake falls• While gliding, these snakes make turns up to 90 degrees and always

seemed to land without injury • flares its ribs so far outward that its belly becomes concave• With its body molded into a highly flattened C, the area of the snake's

ventral silhouette-silhouette when seen from below-NEARLY DOUBLES

Why do they do it?• One of most important factors in snake's midair shift from free fall to

glide: dramatic increase in width of animal's body. • flattening of snake animal into an airfoil: increase in body width

effectively halves ratio of snake's body weight to area of its underside,

• a measure known as wing loading/a crucial indicator of aerobatic talent

• Experts in aerodynamics suggest: snake's tight S-bends make its entire body act like a highly slotted wing (like in airplane)

• because of the way air flows through gaps, such wings develop more lift at low speeds.

• gaps between the bends of the S-shaped snake in flight could produce more lift than snake would have if it shot, arrow-like, through the air.

• Any extra lift is crucial for maneuvering while gliding.

"Snakes are part body and part tail, and they have ribs up until the tail. They flatten their ribs and make themselves Frisbee-like in form. This gets them aerodynamically fit for gliding. ”

- Socha, LiveScience

ADVANTAGES

• Moving through air from tree to tree bypasses a host of earth-bound predators

• Flying snake threatened by an arboreal animal can just launch itself out of tree

• Saves energy and time

PREDATION

Via Constriction

(not suffocation)

• Former Hypothesis: Constriction leads to death by suffocation and collapse of the lungs

of the snake’s prey (minutes)

• New Hypothesis: Constriction arrests the circulation of prey and thus leads to death

faster than would suffocation alone (seconds)

Brad Moon Experiments

• Small boa constrictors can squeeze with pressures up to 4 PSI (pounds per square inch). This is strong enough to squeeze blood vessels closed in mice and kill them by circulatory arrest…imagine what kind of pressures a 30 foot anaconda might exert!!

• Pressure responses to simulated limb and body movements (a) and ventilatory movements (b) during constriction of a dead mouse by Pituophis melanoleucus (pine snake). Coil was loosened at point (c); at peak (d), the snake pulled the mouse out of the coil to start swallowing it.

PRESSURE

T I M E

INTERMITTENT CONSTRICTION• Constriction appears to involve continuous squeezing, but is

usually intermittent.

• Gopher snakes (Pituophis melanoleucus) & King snakes (Lampropeltis getula) squeeze mice continuously whenever they struggle, but hold the constriction posture without actually squeezing whenever the prey is still.

• Muscles use a lot of energy when they exert force, so by squeezing intermittently only when necessary, the snakes probably save a lot of energy.

• Holding the constriction posture even when not squeezing allows a snake to squeeze again very quickly if the prey starts to move again.

EPAXIAL MUSCLES

• Epaxial: located above or behind an axis, such as the spinal axis or the axis of a limb

• highly active during striking and coil formation• only intermittently active during sustained constriction

suggests that epaxial muscles participate in constriction but are not the only muscles used in constriction.

• These long muscles and tendons are probably not the most important constricting muscles, they do not interfere with constriction.

Viperid snake: Oblique view of trunk muscles showing the complexity of muscle & tendon arrangements

Muscles: 1, interneuralis; 2a, medial head of multifidis; 2b, lateral head of multifidis; 3, interarticularis superior; 4, longissimus dorsi; 5, spinalis; 6, semispinalis; 7, interarticularis inferior; 8, levator costa; 9, iliocostalis; 10, transversus dorsalis; 11, transversus ventralis; 12, obliquus internus dorsalis; 13, obliquus internus ventralis; 14, tuberculocostalis; 15, intercostalis quadrangularis; 16, supracostalis lateralis superior; 17, supracostalis lateralis inferior; 18, intercostalis externus; 19, intercostalis ventralis.

PERISTALSIS

• In the snake’s digestive tract, the circular and longitudinal muscle of the muscularis externa contract in sequence to produce a peristaltic wave which forces the digested material along the alimentary canal. During a peristaltic movement, the circular muscles contract behind the digested material ; then a contraction of the longitudinal muscle follows which pushes the digested food further along the alimentary canal.