Snakes What is a snake l It is

Snakes

What is a snake? l It is more than just being legless – Amphisbaenians are legless. – Caecilians are legless – Legless lizards l 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.

Snakes are paraphyltic with lizards. l Snakes are a daughter group of the lizards.

Among reptiles, we can use the following to define snakes: 1) Animal has 4 legs – lizard. l Animal has no legs – go to 2. l 2) Scales arranged in rings around body – Amphisbaenian l Scales overlapping and tile-like – go to 3. l 3) Eyelids present – legless lizard. l Eyelids absent – go to 4. l 4) Single row of ventral scales – snake. l Several rows of ventral scales – legless lizard. l

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.

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

Origin and evolution of snakes l The fossil record for snakes is not particularly good, thus, exact relationships are not accepted without argument. l Snakes first appeared between 100 and 150 MYA during the early Cretaceous. l 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.

Origin and evolution of snakes l Next snake in the fossil record is Simoliophis, from N. Africa and Europe. It was a marine snake from about 100 MYA. l 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).

Origin and evolution of snakes l Shortly after the Cretaceous, there were 7 families of snakes. They were in the midst of an adaptive radiation. l 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).

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

Origin and evolution of snakes l 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.

Snakes vs. Legless Lizards l 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.

Snakes vs. Legless Lizards l 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.

Extant Snakes l 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.

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

Extant Snakes l There are derived and underived – Underived snakes include snakes: Anomalepididae l Typhlopidae l Leptotyphlopidae l – 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.

Extant Snakes l All other snakes are derived. l 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.

Extant Snakes l 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.

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

Elapids l 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.

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

Vipers l 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. ’

The Big Snakes l l l Anaconda Eunectes murinus: 18. 9 m reported, 11. 4 m collected. Reticulated Python reticulatus: 10 m/9 m (Nat. Zoo spec was 7. 6 m, 138. 3 kg) Indian Python molurus: 6 m African Python sebae: 7. 6 m/4. 9 m Amethystine or scrub Python Morelia amethistina: 8. 5 m/3. 7 m. Common Boa constrictor: 4. 5 m.

Morphology and Function l 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).

Morphology and Function l Dwarf snakes – Smallest snakes are about 10 cm (Typhlopidae: blind snakes) – Are they primarily fossorial or sub/litter insectivores?

Morphology and Function l Shape l 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.

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

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

Morphology and Function l 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.

Locomotion l 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.

Locomotion l 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.

Locomotion l Concertina Locomotion – Common in burrowing snakes – Head 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.

Locomotion l 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.

Locomotion l 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.

Senses l 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.

Senses l 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.

Senses l 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.

Senses l Vision – 2 species of African snakes have horizontal pupils, w; hich 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.

Senses l Smell – Jacobsen’s 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 Jacobsen’s organs, and the molecules identified. – Important: tongue is forked, providing ‘binocularity. ’

Senses l 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.

Senses l 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.

Senses l 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 -2 mm in diameter. They are rounded elevation or pimple surrounded by a circular depression. There is a nerve ending just below. – Pits are larger, 3 mm in diameter, and may be oval. Also have nerve endings.

Senses l 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.

Senses l 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?