Lateral undulation

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Snakes
General
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Five general topics

• Definition of snakes
• Anatomy
• Ecology
• Adaptations addressing 4 snake-lifeway issues
• Moving
• Acquiring prey
• Swallowing prey
• Digesting prey
• Reproduction
• Evolution and introduction to the basic clades
• Biogeography
• Next Class the Variety of Snakes

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Definition of Snakes, IAnatomy
(and see next slide on general anatomy)

• Snakes lack
• External limbs (mostly)
• Sternum
• External ear openings
• Moveable eyelids
• Bony mandibular symphasis
• Organs neural pathways associated w/ detection
  analysis of chemical info are greatly advanced.

• Snakes focus the eye by moving the lens.
• Viscera are elongated
• Kidneys and gonads are displaced, right forward.
• In advanced snakes, left lung is reduced or
  absent.
• Liver is large and greatly elongated.
• Skull has greatest degree of kinesis among
  tetrapods.

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General Anatomy, advanced snake (female)
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Definition of Snakes, II Ecology

• In general, snakes live in a chemical world.
• Recall the general context of tongue-sensing
  lizards.
• Receiving information
• Conventional-vertebrate smell and taste are
  reasonably acute.
• Use of tongue vomero-nasal organ is intense
  acquisition of pheromones is particularly
  important.
• Sending information Snakes have special cloacal
  glands that provide socio-reproductive
  information about conspecifics.
• In general, modern snakes are adapted to take
  relatively large prey swallow it whole.
  Consider
• Foraging (and moving for other reasons)
• Capturing killing prey
• Swallowing prey
• Digesting prey
• (These general feeding adaptations will be
  covered in more detail later.)

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Basically, snake adaptations must address 4
issues

• How do you move when you dont have any limbs?
• How do you kill prey when you dont have any
  limbs and when your mouth teeth are fragile?
• How do you feed a big body through a relatively
  small hole when
• movement foraging strategies (plus the anatomy
  that supports them) preclude eating lots of
  little stuff
• you cant bite your big food into pieces
• and you have no limbs to tear your big food
  apart?
• How do you digest items with small surface/volume
  ratios and perhaps with protective coverings?

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Locomotion in snakes

• Lateral undulation (most common, next slide).
• Undulatory aquatic propulsion (not illustrated
  sort of an integrated form of lateral
  undulation).
• Wiggle friction (slick surfaces energetically
  inefficient sustained by glycolysis not
  illustrated).
• Rectilinear crawling (next slide 1 big
  snakes).
• Concertina locomotion (another slide
  establishing a stable position with one segment
  of body while another is moved forward).
• Sidewinding (not illustrated).

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Lateral undulation
Overall resultant force

• This is the most common form of serpentine
  locomotion.
• Muscular waves travel along alternate sides of
  the snakes body and generate posterolateral
  forces at fixed points on substrate. Lateral
  components cancel each other out, and the overall
  resultant force moves the snake in a forward
  direction.
• ____________________
• points dappuis

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Rectilinear locomotion

• The Costocutaneus superior muscle pulls ventral
  scale forward relative to rib vertebra.
  Ventral scale then anchors to substrate. The
  costocutaneus inferior muscle then pulls rib
  vertebra ( rest of snake) forward relative to
  anchored ventral scale.
• Waves of such contractions pass simultaneously
  down the snakes body. (This looks as if ventral
  skin inches forward while dorsal skin appears to
  move at a more or less steady rate.)

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Concertina locomotion

• establish purchase with the front part of the
  body
• pull the back part of the body forward
• establish purchase with the back part of the
  body
• push the front part of the body forward
• establish purchase with the front part of the
  body

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Three methods of acquiring prey
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Just plain eating

• Most snakes simply grab their prey, bite it a few
  times to quiet its struggles, and flat swallow it
  down.
• This is a very efficient way of processing prey
  that is not dangerous.
• There is no distinct line between just plain
  eating and envenomation
• The evolution of venom amongst squamate reptiles
  is a complex issue receiving increasing
  attention.
• Many swallowers have (mildly) toxic saliva
  rudimentary delivery apparatus.
• See opistoglyphic fangs on later slide dealing
  with venom-delivery systems.

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Constriction

• Origins of constriction
• Probably evolved for dealing with elongate prey.
• Readily transferable to bulky, rounded prey.
• Killing mechanisms
• Relatively small prey cardiac arrest.
• Relatively large prey tighten w/preys
  exhalations.
• Anatomical constraints on constrictors
• To wind tightly, vertebrae must be short.
• This precludes rapid travel.

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Envenomation

• Venom ingredients
• Consist mostly of digestive-type enzymes and a
  spreading agent.
• The next slide gives a few more specifics.
• Venom types are correlated to a degree with
  Family
• Viperidae are mostly hemotoxic.
• Elapidae are mostly neurotoxic.
• However, venom can also vary by species,
  population, individual, and even life-stage.
• Type of venom-delivery apparatus is more closely
  related to taxonomy (next slide 3).

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Venom a few common components ( taxa effects)
(from Pough et al., 2004)
Component Taxa w/component Physiological Effects
Proteolytic enzymes All, especially high in most vipers Digest tissue proteins peptides.
Hyaluronidase All Reduces viscosity of connective tissue, increases permeability, hastens spread of other venom components.
L-amino acid oxidase All Attacks many substrates causes general tissue destruction.
Basic polypeptides Elapids Block neuromuscular transmission.
Cholinesterase High in elapids low in vipers Unknown (formerly thought to block nerve-muscle connections in elapids).
Phospholipase (esp. Phospholipase A2) All Attacks cell membranes ( therefore works synergistically w/other stuff)
Phosphatases All Attacks high-energy phosphate compounds such as ATP.
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More venom (over-)generalizations

• Some primitive components resemble pancreatic
  enzymes (and are associated with similar
  circulation-inhibitors).
• Viper venoms
• Hemotoxic (hystolytic)
• Usually include slow-spreading digestive enzymes
  of high molecular weight
• Often include agents that increase or decrease
  clotting
• Elapid venoms
• (versus vipers) Fewer components, lower
  molecular wts.
• Neurotoxins multiple, long- and short-chain)
• Pre-synaptic block release of acetylcholine
• Post-synaptic (more common) block combination of
  acetylcholine at receptor sites on muscles
• Atractaspis
• Many viper components plus special cardiotoxins.

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Additional, ecological notes on snake venom

• The vast majority of snakes are non-venomous in
  any human-practical sense of the term.
• In the technical sense, distinction between
  venomous non-venomous is less clear than
  experts once thought.
• Inefficient-looking delivery systems (see next
  slide) may work quite well.
• Evolution of venom
• Primary function securing prey!!!!!
• Secondary functions
• Defense against you? (But many snakes are
  reluctant to use.)
• Venom digestion. If snakes have evolved to
  swallow large prey whole, and if retention of
  undigested prey is unhealthy, then how can snakes
  live (for example) in high mountains (see later
  slide)? Also, contrast typical food venoms of
  elapids versus viperids.

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Four types of venom-delivery apparatus

• A Numerous very small teeth not primarily
  adapted for venom delivery.
• B Fixed, grooved fangs toward rear of jaw
• Sometimes called opistoglyphic fangs
• Colubrids such as boomslangs
• C Fixed, hollow fangs toward front of jaw
• Sometimes called proteroglyphic teeth
• Elapids (cobras, coral snakes, etc.)
• D Folding, hollow fangs toward front of jaw (
  see next slide)
• Sometimes called solenoglyphic teeth
• Viperids (rattlesnakes, vipers, etc.)
• (Note The snake world isnt as clear-cut as this
  typology!)

Do not think of this as a linear, evolutionary
series!
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Fang erection in vipers and pit vipers

• W/ mouth closed (A), quadrate is rotated
  backwards maxilla (black) fang (red) lie along
  roof of mouth.
• W/ mouth open (B), quadrate rotates down, pushing
  pterygoid (etc.) forward, rotating maxilla fang
  into upright position.
• Blue indicates articulations.

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Schematic of viperid ( mostly like elapid)
venom-delivery apparatus

• Squamate head glands are ripe for exaptation
• All snakes except scolecophidians have many (and
  other squamates too?)
• Modified into salt glands, multi-ducted mucous
  glands....
• Duvernoys gland
• Many colubrids
• More consolidated
• Mucous or serous or both
• Advanced venom glands
• Viperids elapids
• Connective-tissue capsule
• Dedicated compressor musculature (operates on
  large lumen in viperids)
• Duct connecting gland to hypodermic fang

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After a snake has control of its food

• two problems remain
• Opening the mouth wide enough to get the
  food-item in the problem of gape
• and transporting the food-item through the mouth
  and into the throat the problem of swallowing.
• (Thereafter, contractions of the axial
  musculature can move the food-item on down the
  alimentary canal.)

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• Adaptations increasing the gape of snakes
• Mandibles are not fused by bony symphasis.
• Mandibles include hinge.
• Elongated quadrates articulate w/ braincase.
• Snout is hinged to braincase.
• (Note Upper jaw elements are not shown. And
  dont worry about the skin it can stretch
  plenty.)

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• Swallowing, I
• Notes
• Lower jaws are not shown.
• On each diagram, left side is shown above right
  jaw.
• Sides work in alternation.
• Top Left palatine maxilla forward right
  palatine maxilla back.
• Bottom Right palatine maxilla forward, left
  palatine maxilla back.
• Note Palatines do most of the work (next slide),
  but dont forget maxillas and lower jaws.

Ventral view
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• Swallowing, II
• (Remember, sides work alternatively only one
  side is shown here.)
• Palatines on one side of head are lifted (1? 2)
  and protracted (2?3) to gain more forward
  purchase in prey (4).
• Muscles from braincase to palatines contract
  (4?5) to pull prey further into snake (as shown,
  snake is moved forward over prey).

Side view
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But even if you can swallow it, you still have to
digest it

• Prey is often large.
• Prey items are often covered with protective
  chitin, feathers, scales, or hair.
• Snake digestive system is extremely powerful.
• Many snakes thermoregulate to increase digestive
  efficiency.
• Some prey body-parts pass through undigested.

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Extreme digestion problems really big stuff

• Big constrictors (e.g., boas and pythons) can eat
  enormous meals.
• They also control their digestive systems
• Turn off during fasts (c.5.5 wks) turn on after
  feeding.
• On increases metabolic rate by 1500-4500.
• Metabolic start-up burns an average of 32 of
  prey-items calories.
• Off reduces metabolism to 50 of snake-typical.
• Increase is about 25-50 in mammals

• Montane vipers eat relatively big stuff cool
  down at night.
• Snake metabolism would be slower than bacterial
  population increase, but
• Deep injection of venom speeds digestion of prey
  from within!
• (Want a few million dollars? I got an idea that
  beats the lottery.)

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Reproduction

• Pre-repro. behavior varies!
• About 70 of snakes lay eggs, while the other 30
  bear young alive.
• Mode of reproduction follows taxonomic lines but
  only approximately.
• Young typically emerge as miniature copies of
  adults.
• Live-bearing modalities run a continuum from
  merely hatching eggs inside (most common) to true
  viviparity.

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Typical squamate male repro. anatomy

• Paired hemipenes are carried in tail everted
  (one at a time) for copulation.
• Hemipenes structure is a critical taxonomic
  feature. Why?

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Preliminary notes on classification of snakes

• Classification is difficult for at least two
  reasons
• Fossil record is not great (fragile skulls).
• Snakes, like frogs, have restrictive body plan
  therefore convergence is a problem.
• There is a primary dichotomy between the most
  primitive snakes (no good common name) others.
• There is a definite clade of modern snakes (see
  next slide), includinggt90 of all snake species
  (common harmless snakes, vipers, pit vipers,
  cobras).
• The intermediate snakes (including boas, pythons,
  and others) are difficult to organize, but I have
  ideas.

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Family relationships among snakes (circa 2004)
 
This branch includes about 90 of living snakes
its organization is not well understood see next
slide!
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Molecular classification (circa 2007)
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Snake Evolution perhaps the biggest mystery in
modern vertebrate paleontology

• Fossils shown at left are Haasiophis
  terrasanctus, an aquatic snake (??) with legs
  from Cretaceous (c. 95MYBP) beds near Jerusalem.
• The rarity of cranial material for fossil snakes
  allows disagreement about origins to persist
• Scenario 1 snakes are derived from small,
  burrowing lizards (majority positionand mine
  see slide-after-next).
• Scenario 2 snakes are derived from large,
  aquatic lizards (old heresy, occasionally
  reconsidered).
• Any evolutionary scenario must account for body
  elongation, leglessness, weird eyes,
  adaptations for feeding on large prey (all
  discussed above).

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Evolution of Snakes Basics to Remember

• Snakes are tongue-sensing lizards and may be
  considered the most successful of the many lizard
  experiments with leglessness.
• Snakes most probable ancestry lies within the
  anguimorph (sensu lato) line.
• Some experts think varonoids are closest living
  relatives.
• Amphisbaenians and dibamids also have their
  advocates.
• Snakes originated at or before the
  mid-Cretaceous.
• The locale of origin was Gondwanaland.
• Most (all?) modern Families of snakes probably
  originated in Southeast Asia.

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My 4-step burrowing-origin theory (This is
current orthodoxy it is also scenario 1 from 2
slides ago.)

• 1 Tongue-sensing ancestors try another
  experiment in leglessness and give rise to
  critters somewhat like the living
  scolecophidians. (Call these Original Snakes.)
• 2 The Original Snakes begin to exploit larger
  but elongated food, sometimes pursuing it above
  ground. (Call these Transition-One Snakes.)
• 3 Surface-hunting Transition-One Snakes evolve
  larger gape in response to abundance of more
  rounded terrestrial prey. (Call these
  Transition-Two Snakes.)
• 4 Transition-Two Snakes give rise to varieties
  better adapted for surface life. (The result is
  early Advanced Snakes such as boas and pythons.)

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Living animals somehwat like Original Snakes

• Consider a lizard capable of efficient burrowing
  and possessing the chemical-sense acuity typical
  of many lizards.
• This animal could follow underground pheromone
  trails left by ants and termites. (If the
  reptiles highly developed cloacal glands could
  produce the right scent-passwords, it could enter
  ant or termite nests feed without danger???)
• Such an animal would be ideally suited to exploit
  the vastly increased abundance of colonial
  insects that typified the mid-Cretaceous.

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Transition-One Snakes(e.g., Anilius scytale,
top photo)

• Fossorial Original Snakes, capable of following
  underground scent-trails, would have opportunity
  to exploit prey that came in larger packages.
• Such potential prey would include earthworms,
  other snakes, and caecilians (lower picture).
• After heavy rains many subterranean organisms
  emerge to the surface.
• These would include Transition One snakes and
  their prey.

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Transition-Two Snakes(top Xenopeltis
unicolor, bottom Loxocemus bicolor)

• Occasional surface-foraging by fossorial snakes
  would provide contact with a vast variety of
  prey-items having inconvenient shapes.
• Strong selective pressures would therefore exist
  for expanding gape.
• Two extant Families represent what I have called
  Transition-Two Snakes.
• Both have rather rigid skulls, but they manage
  wider gape than animals like Anilius (the
  caecilian eater on the previous slide).

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Advanced Snakes

• I consider boas, pythons, their allies to be
  the first advanced snakes.
• They exhibit adaptations such as broad ventral
  scales, flexible skulls, and immense potential
  gape.
• Their evolution and radiations will be considered
  further under biogeography.

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The way I view snake biogeography

• Non-advanced snakes All scolecophidians, all
  Transition-1, and all Transition-2 snakes are of
  Gondwanian origin and remain so by distribution
  except for minor dispersions and raft-transport
  via India to Southeast Asia.
• Advanced Snakes
• Boas and pythons
• These exhibit complex dispersions within, from,
  and back to Gondwanaland.
• A Boas and Pythons slide gives (some, but
  probably not enough) relevant data for addressing
  those mysteries.
• Colubroidea (modern advanced snakes)
• Southeast Asia is origin for these snakes and for
  their several radiations into the rest of the
  world. (See Southeast Asia slide.)
• The African colubroids are particularly
  interesting and probably include both an ancient
  clade and a more recent clade from SEA. (See
  Africa slide.)

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Boas Pythons
1. Note that the big boas (Boinae) the
pythons (Pythoninae) occupy almost all the
tropics but barely overlap. a. Big boas
occur in S. America, Madagascar, and Pacific
islands. b. The single genus, Python,
occurs almost throughout the range of the
Pythoninae. 2. Note that the erycine boas
(Erycinae) are all small, often relictual,
sometimes fossorial, and frequently adapted to
xeric habitats. 3. Guess the biogeography!
Erycinae
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Southeast Asia Key to Modern-Snake Biogeography?

• SEA shares some non-modern snakes with Af.
  S.Am.
• Scolecophidians probably came to Asia by raft
  transport (India).
• Transition-1 and Transition-2 snakes inhabit SEA
  Neotropics they may represent Gondwanian relict
  distributions.
• Python may have dispersed from Australia across
  SEA into Africa.
• Much later, SEA elapids colonized Australia (by
  island-hopping) and radiated extensively there.
• Still more recently, 3 modern Families expanded
  from SEA to northern Asia, to Europe, and to the
  Americas.
• Note temperate- cold-Eurasian snake-faunas are
  similar and are depauperate. Is this because of
  Pleistocene glaciations?
• Obvious fairly recent links exist between the
  snake faunas of the New World and of SEA. How
  many radiations occurred?
• Dispersals into Africa are considered on the next
  slide.

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(biogeography) African mysteries

• Two major African patterns
• Africas non-modern snake lineages show
  affiliation w/snakes of other regions (SEA, SAm,
  Australia). This is explainable by
  Gondwanaland radiations.
• But the most modern African snakes appear closely
  related only to each other.
• I hypothesize 2 time-separate invasions (probably
  of SEA origins) of colubroid founder stocks into
  Africa. The invaders underwent rapid evolution
  and speciation, diverging phenotypically from
  non-African ancestors.
• Molecular biologists still have not teased out
  the relationships in part because geographical
  intermediates have been lost due to climatic
  change (Sahara Desert).