SNAKE "DESIGN"

There are more than 2,500 modern species of snakes. The evolutionary, creationism, and intelligent design models make different predictions about the types of variations that can be observed within the groups of snakes. Which is supported by the actual evidence?

water snake

EVOLUTIONARY MODEL

If evolution has occurred, populations can acquire new features as they adapt to their niche, at each level of classification. The same types of modifications which separate one group from another may be observed within a group.

CREATIONISM MODEL

While evolution may occur within a "kind", one "kind" cannot evolve from another. As a result, each "kind" has no relation by descent to any other "kind" and an examination should prove which organisms are completely unrelated to each other. The variations which occur within a "kind" should be minor compared to those which differ between "kinds".

INTELLIGENT DESIGN

If intelligent design has occurred, then some/substantial evolution could have taken place in snake lineages. However, each separate "design" should be impossible to develop over time. The variations on a specific "design" should not significantly alter the "design" or create a complex new "design".

 

VARIATION WITHIN A "KIND"

Are all snakes a single kind?

If creationists conclude that all snakes belong to the same "kind", this is problematic because it doesn't explain why fossil lizards possess features that are shared with modern snakes and fossil snakes can possess ancestral, lizard-like features such as legs. Also, if creationists accept that the variation within all snakes has evolved (that is, that snakes represent a single "kind"), this then means that creationists can reconcile an enormous amount of evolution with their religious faith (especially those that believe the earth is only a few thousand years old). Creationists would have difficulty claiming that a relatively small amount of change in one lineage could not occur, even given millions of years, if they accept a far greater amount of evolutionary change as occuring in a few thousand years.

There is a great deal of variation between the modern families of snakes. Modern snakes range in size from several inches to 33 feet.  Snakes can be thin or stout and their cross section can be circular, triangular, oval, or flattened ovals (Mattison, 1995). The most primitive snakes possess two functioning lungs while all advanced snakes utilize only the right lung. Snakes of the family Tropidophiidae have reduced or lost their left lung but developed another lung which forms from the trachea. The left lung is reduced in Boyleriidae and Boidae. Aquatic snakes of the family Acrochordidae possess a mucus membrane flap which seals off the internal nares. The eyes of the primitive snake group Scolecophidia are small and vestigial.

    The number of vertebrae in a snake’s body varies from under 100 in vipers to over 300 in most colubrids, more than 400 (with almost 300 ribs) in pythons, and almost 600 in a fossil snake (Bauchot, 1994). 

snake skeletonsnake skeleton

  No snake, modern or fossil, is known to have any vestige of arms or pectoral girdles.  Vestigial pelvic girdles exist in the families Typhlopidae, Leptotyphlopidae, Aniliidae, Boidae, and the subfamily Cylindrophinae of family Uropeltidae (Mattison, 1995).  Males may use these for intermale combat (Greene, 1997).  Some primitive snakes have nerves in pelvic region suggesting their ancestors had legs (Mattison, 1995).  Boas and pythons retain a cylinder a bone in their spurs and remnants of limb muscles (Ernst, 1996).  Modern boas and pythons still retain small skeletal remnants of their hind limbs (pythons still possess an ilium, ischium, pubis, and short femur).  These spurs are visible in the upper center of the following photos of the cloacal region of a boa (not to be confused with the hemipenes at the lower left of the photographs).

spur

The heads of snakes may also possess a number of modified scales forming “horns”, “eyelashes”, and “tentacles”.  In the snake family Acrochoridae, the scales are covered in short hairlike filaments and raised scales give a few snakes a hairy appearance.   Hognose snakes have a modified rostral scale.  Snakes lack eyelids and in some primitive snakes, scales cover the eyes.  In most snakes, a specialized scale called the brille covers the eye.  The number of scales on the underside of the body varies from under 100 in slug eaters to more than 500 in seasnakes.  (Greene, 1997; Mattison, 1995). 

snake skulls1snake skulls

snake skulls

DIET AND LIFESTYLE

The earliest snakes probably ingested small prey. All snakes are carnivorous but the size of the food ingested varies from insects and eggs to porcupines and crocodiles.  There are even large snakes which can prey on large mammals including antelope and humans.  Boas, pythons, and vipers ambush their prey.  (Greene, 1997). Primitive snakes of the group Scolecophidia tend to consume large numbers of prey items at a single feeding (such as worms, ants, and termites) and can fill the entire length of their digestive tracts. The most primitive modern snakes (Scolecophidia), the most primitive families of Alethinophidian snakes (Uropeltidae and Anilidae), and the most primitive families of Macrostomatan snakes (Xenopeltidae and Loxocemidae) are all burrowing and many possess adapatations for burrowing. Other variations occur within families (discussed below).

ARE SNAKE FAMILIES SEPARATE "KINDS"?

If creationists equate the "kind" with the taxonomic rank of "family" (as many do), this creates an even greater hole in their nonexistent fossil record since each of these "kinds" should date to the first days of life on earth. If there are 18 or so modern snake families, then the fossil record should provide evidence that these 18 "kinds" of snakes have always existed. The smaller the unit which is defined as a "kind", the more difficult it is to argue that snakes which seem to be very similar are as unrelated to each other as they are to trees and insects and people (given that all "kinds" are equally unrelated). Even if the "kind" is equated with the level of family, an enormous amount of variation would have evolved since the creation of the "kind."

A great deal of variation can occur within a snake family. Different species of boas have adapted to terrestrial, burrowing, arboreal, and aquatic lifestyles. The venomous snakes of the family Elapidae may be terrestrial, burrowing, aquatic or marine. Size can vary. The smallest boas (Exiliboa and Charina) measure about 50 cm while the largest measure more than 11 meters. Pythons range in size from 1 m to about 10 m.

Snake families can vary in their skeletal features. Snakes of the family Anomalepididae are the only squamates in which the prefrontal bone stretches posteriorly over they eyes. In the 3 families of the Scolecophidia, the lower jaw varies in its composition (and may contain a large quadrate bone or a large compound bone which composes much of the lower jaw). The snakes of the family Bolyeriidae are the only snakes to retain the ancestral features of a coronoid bone in the lower jaw and ventral articulations between vertebrae in the trunk (hypapophyses). Snakes of the family Bolyeriidae are the only tetrapods whose maxillary bones are divided into two sections. In the Scolecophidia, families vary in whether of not the premaxilla is toothless (Pough, 1998).

LOCOMOTION

There are a variety of styles of snake locomotion.

garter snake

Rectilinear crawling utilizes the epidermal scales rather than the ribs to crawl.  It requires enlarged ventral scales which is an adaptation of the advanced snakes. Concertina movement occurs when loops of the body allow the snake to push the anterior body forward and pull the posterior body from behind.  It is less efficient than lateral undulation and, since it is known in blindsnakes, may represent the ancestral form of locomotion (Greene, 1997).Sidewinding is an adaptation to movement over hot surfaces because there are only a few points of the body which contact the ground at a given moment. Sidewingding evolved as an adaptation to movement over sand in unrelated snakes such as sidewinder and desert horned viper (Ernst, 1996). 

     A few snakes can jump, some of which can reach 3 feet above their starting point.  Colubrids are the fastest snakes.  Black mambas can reach speeds of 11 km/hr (Greene, 1997).  The genus Chrysopelea of southeast asia can glide up to 100 meters using specialized scales and its ability to flatten its body (Ernst, 1996). 

There are different kinds of scales.  The head can have broad, flat scales as seen in the water snake in the following image.

water snake
Modifications of the skeletal system can exist within snake families. Elongate ribs in the necks of some cobras allow them to produce a hood as a warning mechanism; similar neck-flatting behaviors are performed by other snakes which lack the ribs which form the hood. In the family Tropidophiidae, a vestigial pelvis is present in all species except Tropidophis semicinctus. Facial modifications allow the genus Casarea (family Bolyeriidae) and sea snakes (family Elapidae) allow them to depress their snouts. In spitting cobras, the holes through which venom passes face forward.

DIET

  Some snakes are generalists while others are specialists. For at least half the species of snakes, reptiles are a significant part of their diet and often the major part.  Many eat birds, especially young ones.  In the northern hemisphere, rodents may be a main food source of food.  Some snakes eat bats and some feed on large quantities of frogs’ eggs.  Egg-eating snakes have specialized vertebrae which cut through egg shells. All vertebrate groups have snakes which prey on them.  Some snakes are specialists—one snakes only feed on eels, another only feeds on sleeper goby fish, another only fish eggs.  Most of the snakes that eat fish can also eat amphibians (Mattison, 1995).  There are snakes which have largely limited their diets to each of the following groups of animals: snails & slugs, worms, centipedes, ants, crayfish, fish eggs, eels, frogs and toads (such as the hognose snake), lizards, snakes, birds, rodents (Ernst, 1996).  Only a few colubrid snakes can actually chase their prey (Greene, 1997).  Although some snakes eat by constriction in which the prey is asphyxiated by the squeezing coils of the snake, many snakes which are not constrictors wrap their bodies around their prey to reduce their struggling while they swallow. Some snakes use their tails as lures (Mattison, 1995). Some boas and many pythons possess heat-sensitive pits on their snout to help them track warm-blooded prey.

TEETH AND VENOM

     The types of teeth vary in different groups of poisonous snakes.  Snakes classified as aglyphs lack fangs while opistoglyphs possess fangs in the backs of their mouths which are often grooved.  Many colubrids are opistoglyphs.  Snakes classified as proteroglyphs and solenoglyphs possess fangs in the front of their mouths.  The fangs of solenoglyphs close at rest are designed for a deep injection of poison (Bauchot, 1994).  

     Venom glands evolved from modified salivary glands.  About half of advanced snakes produce venoms.  This venom was useful as ancestral snakes ingested larger and larger prey (Greene, 1997).  All lizards and snakes have glands in their heads.  All snakes more advanced than blindsnakes possess supralabial salivary glands. Duvernoy’s glands open near or through the teeth.  True venom glands open into a fang and are possess by all elapids, vipers, and some other advanced snakes (Greene, 1997).  The venom glands in Crotalinae (rattlesnakes) are derived from labial glands (Bauchot, 1994).  Poisonous snakes vary in the percentage of their poison which they inject per strike from 6-10 % to 55% (Greene, 1997).      

     All venoms contain enzymes which break down proteins.  Other components contained in only some snake venoms include polypeptide toxins, peptide bradykinin potentiatiors, hyaluronidases, thrombin-like enzymes, NGF, ACh, and biogenic amines.  Venoms exist in more than 10 snake subfamilies (Ernst, 1996). 

     Some colubrids are poisonous and have been responsible for human deaths, although no poisonous colubrids occur in the U.S.  Duvernoy’s gland is known in a third of colubrid species in about 100 different genera.  Colubrid venom is not injected; it enters the prey as the snake bites.  The venom of some colubrids is stronger than others (Greene, 1997). 
REPRODUCTION

The shape of the female cloaca corresponds to the hemipenes of males of that species (Mattison, 1995).  Mud snakes can lay between 4 and 104 eggs.  In some species of the family Typhlopidae, the females maintain the eggs inside their body for extended periods so that they are laid closer to hatching. Females provide care of the eggs in Leptotyphlops dulcis (Leptotyphlopidae). In the family Boidae, female pythons coil over their eggs and heat them with muscular contractions. In the family Elapidae, a few species (Ophhiophagus and Naja melanoleuca) build nests and provide parental care for the young. The Mountain viper and mud snakes are the only members of their families (Viperidae and Colubridae respectively) known to coil over their nests.The king cobra is the only snake which builds a nest (Badger, ).
Viviparity (live birth) evolved more than 30 times in snakes and is even evident in one fossil snake.  Closely related snakes can vary in reproduction by oviparity/viviparity.  The percentage of snake species which give live birth increases as more northward.  In the far north, live birth is the only type of reproduction in snakes.  (Greene, 1997).  While some members of the family boidae lay eggs (pythons), others (boas) give birth to live young. The species Trachyboa boulengeri is the only viviparous species in the family Tropidophiidae. Both oviparity and viviparity are known in vipers and females may retain the eggs or young inside their bodies for extended periods of time. Both oviparity and viviparity are known in the family Elapidae (and both lifestyles are also known in the sea snakes of the subfamily Hydrophiinae). Live birth is known in the subfamily Viperinae (common night adder and European viper), Crotalinae (copperhead, jararaca), Boinae (Cuban boa), and Natricinae (brownsnake) (Ernst, 1996).  

In a few species, multiple females can lay eggs together.   A number of female snakes coil around eggs; python females actually contract around the eggs to provide heat (Mattison, 1995).  Many snakes lay eggs which need to develop some time before hatching, some lay eggs which hatch within days after being laid (such as the smooth greensnake), some retain their eggs in the female’s uterus without a shell (boas), some form placentas (garter snakes), some brood over their eggs (pythons).  Egg number in snakes ranges from 1 to 100. 

     Many colubrids mate for a few minutes while some vipers (such as the Western diamondback) can mate for more than a day.  Male combat is known in more than 100 species in diverse groups of snakes.  Cloacal secretions of male gartersnakes form cloacal plugs in females which prevent other males from mating. The hemipenis of the genus Rhamphotyphlopts (Typhlopidae) is distinct from all other squamates in that is solid and in the way it is extended from the body.

    Some snakes are sexually mature before two years of age while some become sexually mature after 4 years.  The Western Diamondback can survive 21 years (Greene, 1997).   While most snakes typically mate in spring through early summer, some snakes (such as vipers) mate in summer and fall and store sperm for subsequent fertilization.  Some females have stored sperm for as long as 6-7 years (Badger, ).

SKIN

Rattlesnake rattles are the remains of thickened skin which are not lost when the skin is shed.   At each shedding, another bead is added to the rattle.  The rattles from young and older rattlesnakes are depicted in the following images.

rattlesnakes rattlesnake

Some snakes enlarge or flatten their bodies and many open their mouths “gaping” to intimidate would-be predators.  Feigning death is a strategy used by a small number of unrelated snakes, including hognose snake and the North American genus Storreria (Mattison, 1995).  In a few snakes, tails may break as a defense.  The Australian death adder flattens its body when threatened and the king cobra extends its hood (Greene, 1997).

There are many people who feel that the shapes and colors of modern snakes can be beautiful. Many provide camouflage. Some provide warning or mimic the warning coloration of venomous snakes. Since coloration varies among closely related snakes, even in the same genus, the evolution of beauty and the coloration which is so important to a specific lifestyle would be explained by evolution, even in the creationist model. The following image depicts the warning coloration of a poisonous coral snake.

Micrurus carralhoi

coral snake

    The following image is of a non-venomous colubrid snake which mimics the coloration pattern of coral snakes.

Oxyrhopus quibeicoral snake

 

IF THESE FEATURES ARE "DESIGNED", THEN WHY HAVE RELATED SPECIES REWORKED THEIR DESIGNS?

It could be argued that there an animal's style of reproduction needs to be designed because of the complex interactions of anatomical structures, physiological mechanisms, and instincts. However, if such a complex "design" is indeed so "intelligent", then why did so many snakes rework their designs? It could be argued that an organism is "designed" to match its environment. However, these designs apparently needed reworking since significant variations occur within close relatives.

Given the extent of the variations among snakes, it is difficult to make an argument for "irreducible complexity". Live birth is not essential to being a snake, nor is venom, nor is a particular type of movement. Each variation could have evolved to supplement an already successful snake "design".