Snakes

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Snakes
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What is a snake?

• It is more than just being legless
• Amphisbaenians are legless.
• Caecilians are legless
• Legless lizards

• Legg loss seems to be an adaptation to mode of
  habitat use.
• May reduce cost of transport.
• Associated with fossoriality
• May be associated with movement through complex
  habitats.

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Snakes are paraphyltic with lizards.

• Snakes are a daughter group of the lizards.

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Among reptiles, we can use the following to
define snakes

• 1) Animal has 4 legs lizard.
• Animal has no legs go to 2.
• 2) Scales arranged in rings around body
  Amphisbaenian
• Scales overlapping and tile-like go to 3.

• 3) Eyelids present legless lizard.
• Eyelids absent go to 4.
• 4) Single row of ventral scales snake.
• Several rows of ventral scales legless lizard.

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Thus, we know the following

• Snakes have a single row of ventral scales. This
  presumably helsp in locomotion, especially for
  arboreal species.
• They eyes of snakes are overlaid with a single
  clear scale.
• This scale is called the brille (german for
  glasses). They have no eyelids.

• Legless lizards retain vestiges of pelvic and
  pectoral girdles whereas snakes at best retain
  some vestige of the pelvic girdle.
• Legless lizards are not a supple as snakes, they
  are much more rigid.

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Origin and evolution of snakes

• Snakes presumably evolved from the lizards.
• Within 7 families of lizards, there are members
  with limb reduction
• Pygopodidae (Australian snake lizards).
• Scincidae
• Anguidae
• Most likely nearest neighbor are Varanids (w/
  Helodermatidae).
• Note we know of no legless varanids.

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Origin and evolution of snakes

• The fossil record for snakes is not particularly
  good, thus, exact relationships are not accepted
  without argument.
• Snakes first appeared between 100 and 150 MYA
  during the early Cretaceous.
• Earliest snake is Lapparentophis defrennei from
  N. Africa. It was terrestrial. This fossil is a
  good snake, with no obvious connection to any
  earlier herps.

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Origin and evolution of snakes

• Next snake in the fossil record is Simoliophis,
  from N. Africa and Europe. It was a marine snake
  from about 100 MYA.
• The families for Lapparentophis and Simoliophis
  were extinct by the end of the Cretaceous, along
  with 2 other snake families. However, 2 families
  survived to the present (Aniliidae pipe snakes,
  and Boidae boas).

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Origin and evolution of snakes

• Shortly after the Cretaceous, there were 7
  families of snakes. They were in the midst of an
  adaptive radiation.
• Colubrid snakes, which are the most speciose
  today, did not show up in the fossil record until
  the end of the Eocene or start of the Oligocene
  (about 36 MYA).

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Origin and evolution of snakes

• Diversification of the colubrids coincides with
  the reduced diversity of other snakes, including
  boids. Why is this?
• 3 families appeared in the Miocene
• Viperidae (vipers)
• Elapidae (cobras)
• Acrochordidae (file snakes).

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Origin and evolution of snakes

• Just as in mammals, the distribution of snake
  families is dependent on continental drift. Old
  families tend to have worldwide distribution
  while more recent families are restricted
  geographically.

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Snakes vs. Legless Lizards

• Elongation of the body is characteristis of
  snakes.
• There is a reduction in body diameter.
• Left lung is reduced or entirely absent.
• Gall bladder lies posterior to the liver.
• Right kidney is anterior to the left.
• Gonads are similarly displaced.

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Snakes vs. Legless Lizards

• In lizards, elongation also results in reduced
  body diameter. However, here the head is also
  reduced, and there is a consequent reduction in
  the size of prey that can be taken. Snakes have
  solved this problem, lizards have not.

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Extant Snakes

• There are 14 or 15 extant families w/ c. 2400
  species.
• 2 families contain only 1 species.
• 1 family contains only 2 species.
• 1 family contains only 3 species.
• The Colubridae (probably not a natural group)
  contains over 1500 species.

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Family Genus Species
Anomalepidae 4 19
Typhlopidae 3 200
Leptotyphlopididae 2 80
Tropidopheidae 4 21
Bolyeriidae 2 2
Boidae 17 61
Acrochordidae 1 3
Loxocemidae 1 1
Xenopeltidae 1 2
Aniliidae 1 1
Uropeltidae 10 51
Colubridae 287 1500
Atractaspididae 8 55
Elapidae 65 290
Viperidae 25 214
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Extant Snakes

• There are derived and underived snakes
• Underived snakes include
• Anomalepididae
• Typhlopidae
• Leptotyphlopidae
• These are small burrowing snakes w/ rudimentary
  eyes, and smooth shiny scales.
• They are grouped into the infraorder
  Scolecophidia.
• They feed primarily on termites and ants, their
  jaws are incapable of the same level of extension
  or other snakes, and they have few teeth.

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Extant Snakes

• All other snakes are derived.
• They are in the infraorder Alethinophidia.
• West Indian Boas are in the family
  Tropidopheidae.
• Pythons (oviparous) are in Africa, SE Asia, and
  Australia.
• Boas (viviparous) are in the Americas and on
  Madagascar. (Some boas and pythons have heat
  sensitive organs around the upper and lower jaws
  which enable them to find prey in total darkness.

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Extant Snakes

• All other snakes are more recently evolved.
• Unlike previous families, they do not contain
  hind-limb girdles.
• They do not have a coronoid bone.
• Most successful of these are the Colubrids.

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Colubrids

• Have invaded a broad variety of ecological niches
  (no exclusive marine species).
• Dominant in all snake assemblages except
  Australia.
• More or less slender elongated bodies w/ large
  eyes, large scales covering their heads.
• A number of species are venomous, but method of
  delivering venom is not as specialized as vipers
  and cobras.
• Some are viviparous and some are oviparous.

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Elapids

• Elapids (including cobras, kraits, sea snakes,
  mambas, and coral snakes).
• Essentially replace colubrids and vipers in
  Australia.
• Australia is inhabited by more venemous snakes
  than harmless ones.
• Cobras are characterized by hollow venom fangs
  fixed at the front of their upper jaws, and
  specialized sucts that carry the venom from the
  venom glands to the tip of the fangs.

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Elapids

• Cobras may be aquatic, terrestrial, burrowing, or
  climbing.
• They may be oviparous or viviparous.

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Vipers

• Vipers are considered the most derived snakes.
• Evolved after Australia broke off from Pangea
• Southernmost and northernmost snakes.
• Also occur at highest altitude in Himilayas.
• Fangs are long and hinged, so they can be folded
  out of the way.
• Have heat sensitive pits between eyes and
  nostrils hence pit vipers.
• Pits have evolved independently from those in
  Boas and Pythons.
• Some American pit vipers have rattles.

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The Big Snakes

• Anaconda Eunectes murinus 18.9m reported, 11.4m
  collected.
• Reticulated Python Python reticulatus 10m/9m
  (Nat. Zoo spec was 7.6m, 138.3kg)
• Indian Python Python molurus 6m
• African Python Python sebae7.6m/4.9m
• Amethystine or scrub Python Morelia amethistina
  8.5m/3.7m.
• Common Boa Boa constrictor 4.5m.

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Morphology and Function

• Large size
• Large size enables animal to eat a greater
  diversity of food items.
• Large size means animal must eat more food items.
• Large snakes are thus restricted to tropical, or
  diverse areas (Boa constrictor does range into
  semi-deserts of Mexico).

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Morphology and Function

• Dwarf snakes
• Smallest snakes are about 10cm (Typhlopidae
  blind snakes)
• Are they primarily fossorial or sub/litter
  insectivores?

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Morphology and Function

• Shape
• Arboreality
• Arboreal species tend to be slender w/ long
  tails, which facilitates weight distribution over
  potentially unstable branches.
• Some robust bodied arboreal boids fit this
  pattern too. When compared to other boids, their
  bodies are more slender w/ longer tails than
  other boids.

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Morphology and Function

• Shape
• Terrestrial
• Some terrestrial species tend to be long and
  slender too.
• Fast moving
• Active foraging
• Diurnal
• Whipsnakes like Coluber and Masticophis.
• Sand snakes like Psamnophis.

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Morphology and Function

• Shape
• Terrestrial
• Robust bodied species
• Sit and wait predators like Gaboon viper Bitis
  gabonica
• Puff adder Bitis arietans.
• Australian elapids Acanthophis
• Tree pythons Python regius, P. anchietae, and
  blood or short tailed python P. curtus.

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Morphology and Function

• Cross section
• Burrowing snakes tend to be cylindrical
• Terrestrial species tend to be flattened on the
  bottom (icreases friction with ground surface).
• Arboreal species may be flattened from side to
  side.
• Aquatic species tend to be laterally flattened.
• Triangular shape (function unknown) African file
  snakes Mehelya, kraits Bungarus, lesser
  extent, American Indigo snake Drymarchon corais.

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Locomotion

• There is some controversy concerning cost of
  transport in snakes.
• Theoretical work supports idea that limblesness
  is cheaper than quadrupedal locomotion.
• Some empirical work suggests otherwise.
• Problem is comparing long slender legless form
  with limbed short form.

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Locomotion

• Serpentine crawling
• Consists of side to side wrigling/lateral
  undulation.
• Use side of body to push against surface
  irregularities.
• At any moment, multiple points along the body are
  pushing simultaneously against fixed points.
• As animal moves forward, new parts of body come
  into contact w/ fixed points.
• All parts of body follow same path.

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Locomotion

• Concertina Locomotion
• Common in burrowing snakes
• Head and anterior portion of body extend forward,
  back portion of animal is used as anchor.
• Ant. Portion of body used as anchor while
  posterior portion brought forward.
• Elaphe obsoleta also uses concertina locomotion
  when it goes up a tree. Or the outside corner of
  a house.
• In burrowing snakes (Uropeltidae shield tails)
  the sides of the body are kept parallel, while
  the vertebral column is folded laterally. This
  enables the snake to use concertina locomotion in
  extremely restricted spac of burrow.

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Locomotion

• Rectilinear Crawling
• Used by some heavy bodied snakes like boids and
  vipers.
• Use edges of ventral scales to anchor portions of
  body, then pull themselves forward.
• Body extends a bit, anchors, pulls forward. This
  occurs in waves along the length of the body.

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Locomotion

• Side Winding
• Used by vipers in areas of windblown sand.
• Occurs in N. America (Crotalus cerastes), S.
  America, Africa (Cerastes cerastes), and central
  Asia.
• Thus, evolved independently on several
  occaisions.
• Like concertina locomotion in that one part of
  body is used as an anchor.
• Head and neck are raised off ground and thrown
  sideways.
• Head and neck are anchored, rest of body catches
  up.

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Senses

• Vision
• Vision is not particularly good, probably because
  fossorial lizard origins.
• Typhlopids and Leptotyphlopids have only
  rudimentary/vestigal eyes.
• Surface dwelling snakes have re-invented the eye.
  However, many derived features of vertebrate eye
  have been lost. These include ability to focus
  eye via shape changes in eye. Lost in all except
  (or re-invented) Ahaetulla. Other snakes focus
  by changing the position of the lens, as in a
  camera. Result is limited depth of field.

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Senses

• Vision
• Rods and cones are not well organized in snakes.
• Many snakes lack either rods or cones altogether.
• Poor ability to see detail.
• Inability to see stationary objects.
• Enhanced sensitivity to movement.
• Wide field of view.

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Senses

• Vision
• Only a few snakes have movement of eyes (vine
  snakes, Oxybelis).
• Dirunal hunters like garter snakes and whipsnakes
  have the best vision. Usually have large
  circular pupils.
• Nocturnal hunters like lyre snakes (Trimorphodon)
  and cat snakes (Telescopus) have large eyes, but
  vertical elliptical pupils that close donw to
  tiny slits during the day.

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Senses

• Vision
• 2 species of African snakes have horizontal
  pupils, which permit binocular vision. These
  snakes hunt lizards, use vision entirely, and are
  active during the day.
• As noted before, snakes lack eyelids, and have a
  brill instead. Some primitive snakes lack a
  brille, and have several scales covering the eyes
  instead.

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Senses

• Smell
• Jacobsens organ found only in snakes and some
  lizards.
• It is a pair of sacs lined with sensory cells, on
  the anterior portion of palate.
• Sacs open to roof of mouth via narrow ducts, and
  inner ends are connected to the olfactory nerve.
• Tongue is flicked through lingual fossa to sample
  the environment, and then withdrawn to mouth.
  Tips are inserted into Jacobsens organs, and the
  molecules identified.
• Important tongue is forked, providing
  binocularity.

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Senses

• Heat Sensitive Pits
• Found in boas, pythons, and pit-vipers.
• Evolved independently in the 3 groups.
• Pits are lined with epithelial cells with
  thermoreceptors.
• Nerves transmit signals to brain.
• Pit vipers have the most sophisticated pits, with
  2 compartments divided by membrane.

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Senses

• Heat Sensitive Pits
• The inner pit is connected to the outside by a
  narrow pore in front of the eye. This equalizes
  air pressure on either side of the membrane, and
  also enables the animal to evaluate air
  temperature.
• Heat generated by prey is detected by the outer
  surface of the membrane. Comparison of the 2
  enables the animal to differentiate between
  convective heat and heat of the prey. The viper
  can detect .001 C differences, thus enabling
  strikes even in absolute darkness.

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Senses

• Scale Turbercles and Pits
• Found in both snaks and lizards.
• Associated with thinning of the epidermis.
• Exact function unknown, assumed to be sensory.
• Tubercles are 1-2mm in diameter. They are
  rounded elevation or pimple surrounded by a
  circular depression. There is a nerve ending
  just below.
• Pits are larger, 3mm in diameter, and may be
  oval. Also have nerve endings.

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Senses

• Scale Tubercles
• Found in all snakes.
• In primitive forms (Typhlopidae,
  Leptotyphlopidae, Anomalepidae) they are found
  only at anterior portion of animal).
• Distribution is extremely variable in other
  snakes.
• Assumed to be in parts of the body that come into
  contact with environment as the animal moves.
  Organs of touch?
• In rough earth snakes (Virginia striatula) they
  are found only in males, and are thought to help
  males locate females vent during courtship.

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Senses

• Scale Pits
• Found only in higher snakes.
• Not found in all species of higher snakes.
• Absent in Elapids.
• Most numerous on head, especially around the
  snout.
• Lost in burrowing forms. Are they sensitive to
  light?
• Function to secrete oils? Chemical communication?

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