All amphibians, reptiles, birds, and mammals are pentadactyl tetrapods—vertebrates with four limbs, each with five digits. Snakes, as reptiles, are also pentadactyl tetrapods because their lizard ancestors were fully limbed.
The skull of a Green Anaconda (Eunectes murinus) exhibiting the highly flexible bones of the skull, the three bones of the lower jaw (quadrate, compound, and dentary), and six rows of recurved teeth, which are typical of snakes and located on the maxillae and pterygoid-palatine (inner) bones of the upper jaw, and dentary bones of the lower jaw.
The snake skeleton comprises a skull and a spinal column. Because snakes possess extremely elongate, flexible bodies, they may have up to 500 vertebrae, although 120–240 is more common. Each vertebra is attached to a pair of ribs, which in the absence of a sternum are independent, being interconnected only by powerful intercostal muscles that enable the many modes of snake locomotion. The lack of a sternum allows the rib cage to expand outward so that the body can accommodate large meals, egg clutches, or litters of neonates. The outward expansion and mobility of the ribs is obvious in the dorsoventral flattening of a basking viper, the hooding of a cobra, and the lateral body compression of a swimming seasnake.
All snakes lack front limbs, but the vestiges of the pelvic girdle and hind limbs are present in the boas, pythons, and some other primitive snake groups. Externally, they are represented by a pair of curved horny spurs on either side of the cloaca (genital-excretory opening). Spurs are largest in males, which use them to court the female during copulation.
SKULLS AND TEETH
Unlike the skulls of mammals, turtles, or crocodilians, those of snakes exhibit kinesis, meaning that they are hugely flexible, and the individual bones are capable of the articulation required to manipulate and swallow prey. The large gape of a snake’s mouth is achieved because the lower jaw comprises six separate, flexible bones. The tooth-bearing dentary bone is attached to a toothless compound bone, which in turn is attached to the skull via an elongate quadrate bone. This arrangement permits considerable mobility in all planes, further enhanced by the fact that the left and right dentary bones are not fused at the chin. Many snakes can expand their lower jaws extremely widely to accommodate large meals, and advance each side of the lower jaw independently as the prey is swallowed.
Most snakes have six rows of recurved solid teeth, arranged on the dentary bones of the lower jaw, and both the maxilla (outer) and pterygoid-palatine (inner) bones of the upper jaw. A few snakes lack teeth from some bones—for example, the blindsnakes (Typhlopidae) lack teeth from the dentary bones, the threadsnakes (Leptotyphlopidae) lack teeth from the maxillae, and African egg-eating snakes (Dasypeltis) possess only a few teeth on the rear of the dentary and maxilla. The homologous nature of solid, ungrooved snake teeth makes it easy to distinguish snake fossils from those of lizards, which exhibit greater diversity of tooth type and shape.
Snakes have the same internal organs as other vertebrates, but due to the elongation of their bodies they are arranged less symmetrically. Of two lungs, usually only the right lung is functional and it may run for a third the length of the body, while the left lung is small and vestigial. There are two elongate kidneys, also arranged asymmetrically, a liver, a pancreas, a gall bladder, and a heart that varies in its location depending on the snake’s lifestyle—for example, whether a diving seasnake or climbing treesnake. The heart has three chambers, comprising two auricles and a ventricle, unlike the four chambers of mammalian and crocodilian hearts. The digestive system comprises an esophagus, a stomach, and small and large intestines. The sexual organs of a female consist of a pair of elongate ovaries, while males have a pair of testes and a paired hemipenis. This vulnerable organ is inverted inside the base of the tail until it is required.
VENOM-DELIVERY MECHANISMS & FANGS
Venomous snakes have specialized venom-delivering mechanisms that culminate in their fangs. The most primitive are rear-fanged snakes, with enlarged, grooved teeth on the rear of the maxillae, down which venom trickles into a bite wound. In members of the venomous Elapidae and Viperidae families, the fangs are located on the front of the maxillae. In elapids, they are fixed in position, although the kinesis of the skull allows considerable movement. The vipers have short, toothless maxillae, to which are attached extremely long fangs that are hinged so that they can swing back horizontal to the skull when not in use. When a viper strikes, the flexibility of the skull and maxillae enables the fangs to swing forward like sabers, the highly kinetic skull absorbing the shock of the strike.
Snakes are highly sensory animals whose sense organs differ from those of mammals. They lack an external ear or a tympanum, but they do possess a highly developed inner ear, with which they detect vibrations picked up by the columella bone, attached to the quadrate bone of the jaw. Snakes are not technically deaf; they just hear in a different way to other terrestrial vertebrates.
The snake’s eyesight is also misunderstood. The retina of a vertebrate eye contains visual cells: rods, for night vision; and cones, for color vision and visual acuity. Fossorial blindsnakes may have eyes that are little more than photo-sensitive cells, warning them when they are exposed to daylight, but other snakes possess more elaborate vision. The pupils of diurnal snakes are round, whereas those of nocturnal or crepuscular snakes are vertically elliptical, or “catlike,” providing the eye with more control over how much light reaches the retina. Many diurnal snakes also have dichromatic or trichromatic color vision.
The laterally positioned eyes of a snake provide it with 100–160-degree vision, but probably the best vision of any snake is that of the diurnal Asian treesnakes (Ahaetulla), which have horizontal keyhole-like pupils and a grooved snout, down which they can sight up their prey. This arrangement provides a 45-degree overlap in forward vision from both eyes, effectively providing binocular vision. These treesnakes also have a highly sensitive fovea centralis, a cone-heavy depression in the retina, enabling them to detect the slightest movements of a camouflaged lizard in the vegetation, and accurately judge distance to target.
In those species that spend their time underwater or buried in the sand, the eyes are often located in a more dorsolateral position, permitting vision without exposing the head. The Namib Sidewinding Adder (Bitis peringueyi) is one such sand-dweller with dorsally positioned eyes.
All snakes have a forked tongue. Located in the front of the lower jaw, this is often in continual movement, flicking in and out of the closed mouth through a small opening, the lingual fossa. Environmental molecules are transported on the tongue to a vomeronasal organ (olfactory sense organ) in the roof of the mouth, known as the Jacobson’s organ, allowing snakes to track down either a mate or prey, and find their way around their home range. But they do not need to flick their tongues in order to smell—snake nostrils are also packed with sensitive olfactory tissues.
The Asian vinesnakes (genus Ahaetulla) probably possess the best vision of any snake. They have horizontal pupils and can sight up their lizard prey down grooves on the snout, judging distance to target due to the considerable overlap in the vision from both eyes in front of the snout.
Many snakes that feed primarily on endothermic (warm-blooded) animals have evolved the ability to hunt in total darkness. Pythons, boas, and pitvipers (Crotalinae) all have thermosensory pits that detect the infrared body heat of their prey, enabling an accurate strike. In pythons and boas, there is a series of labial pits in the lip scales, whereas pitvipers have a single loreal pit on either side of the head (located between the nasal and preocular scales—see scalation diagram opposite). More rudimentary structures, known as supranasal sacs, are present on the heads of Old World vipers (Viperinae), and may also function as infrared-sensitive receptors for hunting.
There are also several less-studied sensory receptors in snakes. The tentacles of the Tentacled Snake (Erpeton tentaculum), for example, are thought to detect vibrations in water that indicate the presence of fish. Similarly, the strange spinous, tuberculate scales of filesnakes (Acrochordus) are believed to detect swimming fish in cloudy water.
SEXUAL DIMORPHISM AND DICHROMATISM
Males and females of many snake species are almost indistinguishable, but there are clues to their gender. Males generally have longer tails than females, with a moderately bulbous basal area where the hemipenes are located. Females, meanwhile, may have shorter and more tapering tails, and often longer bodies than males. Females of some species are also much larger than males—for example, female Green Anacondas (Eunectes murinus) and Reticulated Pythons (Malayopython reticulatus) may reach around 20–23 ft (6–7 m) and 20–33 ft (6–10 m), respectively, while males are only around 10–13 ft (3–4 m) and 13–16 ft (4–5 m) in length. Larger females can carry more eggs or neonates, but in some species the sizes are reversed—female King Cobras (Ophiophagus hannah) reach only around 10 ft (3 m), while the largest recorded male was reportedly over 16 ft (5 m) in length.
Some species exhibit sexual dichromatism, whereby males and females have different coloration or patterns—for example, the male Northern Adder (Vipera berus) is silver-gray with black markings, while the female is brown with dark brown markings. Sexual dimorphism (differing body shape or size) is rarer in snakes than dichromatism.
The Malagasy Leafnose Snake (Langaha madagascariensis) exhibits both sexual dimorphism and sexual dichromatism. The brown male bears a conical spike-shaped protuberance on his snout, while in the gray female this resembles a serrated spike
Snake scales are composed of keratin. The dorsal scales of the body are either smooth or keeled (ridged), and are usually arranged in imbricate (overlapping) rows. The ventral scales are usually broader, and also imbricate to permit locomotion on land, although many seasnakes have sacrificed their broad belly scales to enhance lateral body compression for swimming. The scales of the head are either a series of large “scutes,” as in most colubrids or elapids, or are reduced in size to numerous undifferentiated granular scales, as in most vipers and some pythons. The number and arrangement of a snake’s scales provide important clues for species identification.
As snakes grow, they need to shed their skins. A snake approaching a slough will exhibit “milky” eyes as the cells break down to separate the old and new skins, and when the eyes become clear, the snake is ready to shed. It rubs its snout on rough objects to begin the process, then crawls out of the old skin as it becomes snagged on rocks and twigs. Snakes do not possess eyelids, hence their unblinking gaze, but instead they have transparent coverings over the eyes known as “brilles” or “spectacles.” These structures, which resemble contact lenses, are sloughed along with the rest of the skin, as is the skin on the forked tongue.
The dorsal scales of snakes are usually smooth (A) but many aquatic or desert dwelling snakes (especially keelbacks and vipers) have keeled (ridged) scales (B), while the scales of the filesnakes (Acrochordus) are tuberculate (C).