Transcript Slide 1

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, 19431971
SNAKES
Cool Features
• Poison/Toxins
•Digestive capabilities
•Coiling Abilities
•Shedding
Anatomy & Movement
Anatomy of a Snake
Anatomy
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Skeleton
Digestion
Senses
Scales/Exterior
(Musculature)
Skeleton
• Skull
• Vertebrae
• Ribs
Skull
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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”
bonedouble hinge
Vertebrae
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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 backpull 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
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Younger>old
More active>less
More temperate regions>less
Grow cont. throughout life
remove parasites
-Rub nose against rough surfacerip in skin
-Crawl out; left inside-out in one piece
Movement
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Lateral Undulation (most common)
Straight-line/rectilinear locomotion
Concertina movement
“Sidewinding”
Undulation
Edges of curvature push against substratum
Rectilinear
Used to climb up trees (!)
Concertina movement:
• Move front part of body forward, coil itpress 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 Squamata
Suborder 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
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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 "rearfanged" 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 jargonfilled, convoluted arguments, the problem,
as always, being the basic similarity of
snakes and lizards.”
- J. Alan Holman
Swimming Snakes
Swimming Snakes
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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
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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!!
P
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• 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.
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.