The Book of Snakes: A Life-Size Guide to Six Hundred Species from around the World
656
The Book of Snakes: A Life-Size Guide to Six Hundred Species from around the World
656Hardcover(First Edition)
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Product Details
| ISBN-13: | 9780226459394 |
|---|---|
| Publisher: | University of Chicago Press |
| Publication date: | 10/15/2018 |
| Edition description: | First Edition |
| Pages: | 656 |
| Sales rank: | 392,833 |
| Product dimensions: | 7.50(w) x 10.90(h) x 1.80(d) |
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CHAPTER 1
EVOLUTION & DIVERSITY OF SNAKES
Snakes are elongate animals with fragile skulls and skeletons, which may become disarticulated and separated post-mortem. It is therefore no surprise that relatively few complete snake fossils are available, most comprising a few vertebrae and skull fragments.
THE EVOLUTION OF SNAKES
There are two contrasting schools of thought regarding the evolutionary origin of snakes. One theory proposed that they evolved from a now extinct group of large marine reptiles known as mosasaurs, which dominated the Late Cretaceous oceans. The other theory holds that snakes have a terrestrial origin and evolved from within the Anguimorpha, a suborder of lizards that today contains the slow worms, alligator lizards, monitor lizards, and the venomous Gila monster, and beaded lizards. This latter theory is the more widely accepted, but there is still support for an aquatic mosasaur origin.
The earliest snakes are now thought to date from the Middle Jurassic or Early Cretaceous period, 167–140 MYA (million years ago), with fossil examples discovered in England, Portugal, and Colorado, USA. These fossil examples comprise a few vertebrae and fragments of jawbones, but they can be readily identified as snakes by their strongly recurved teeth, a common characteristic of modern and ancestral snakes. Their discovery suggests a much earlier origin for snakes than the previously accepted Late Cretaceous, 95 MYA.
Early snakes are thought to have inhabited warm, wet, well-vegetated habitats, where they existed as terrestrial, nocturnal, wide-foraging, non-constricting stealth hunters, preying on soft-bodied invertebrates and vertebrates of lesser width than their own heads. A modern comparison might be the Asian pipesnakes (Cylindrophis). The greatest explosion in snake diversity appears to have occurred following the Cretaceous–Paleogene extinction event, 66 MYA. This led to the extinction of the dinosaurs, mosasaurs, and 75 percent of all life on Earth, but it also resulted in the rise of the mammals, a potential prey source of early snakes.
Some fossil snakes display hind limbs, including Najash rionegrina, from Late Cretaceous Patagonia, which has a well-developed pelvic girdle and what are believed to have been functional hind limbs. Three Middle Cretaceous marine species — Pachyrhachis problematicus and Haasiophis terrasanctus from Palestine, and Eupodophis descouensi from Lebanon — also had hind limbs. These species are grouped in the extinct family Simoliophiidae, but body elongation and loss of limbs does not necessarily separate snakes from lizards (see "Skeleton and Limbs").
As recently as 2016, an Early Cretaceous fossil from Brazil was described as Tetrapodophis amplectus. It had an extremely elongate body and four short pentadactyl limbs, and was reported worldwide as the first four-legged snake. But this discovery proved extremely controversial, and paleontologists now believe that the fossil is a dolichosaur, an extinct marine lizard-like reptile.
THE DIVERSITY OF MODERN SNAKES
The snakes (suborder Serpentes), along with the lizards (suborder Lacertilia) and the worm-lizards (suborder Amphisbaenia), comprise the order Squamata, the scaled reptiles. The sister clade (group) of the Squamata is the Rhynchocephalia, the beaked reptiles, a once diverse and widely distributed group of lizard-like reptiles that is now confined to New Zealand, where it is represented by a sole extant species, the Tuatara (Sphenodon punctatus). The Squamata and Rhynchocephalia together form the superorder Lepidosauria, the sister clade of the Archosauria, which contains crocodilians, birds, and extinct dinosaurs and pterosaurs.
Modern snakes are divided into two infraorders, the Scolecophidia (worm snakes) and the Alethinophidia (true snakes). The Scolecophidia comprises five families of small fossorial (burrowing) snakes. Although appearing primitive among living snakes, these are actually highly derived, having specialized considerably for their subterranean existence.
The Alethinophidia is divided into the Amerophidia, a small group that has not spread beyond Latin America, and the Afrophidia, which contains the majority of the true snakes. The Afrophidia are the "Out of Africa" clade, because the continent appears to be the group's evolutionary cradle, from where it radiated worldwide. The Afrophidia is further divided into the Henophidia ("old snakes"), which contains the boas, pythons, pipesnakes, shieldtails, and several smaller families of small-mouthed snakes, and the Caenophidia ("recent snakes").
The Caenophidia is divided into two superfamilies. The Acrochordoidea today contains just three species of aquatic filesnakes (Acrochordus), but once included the now extinct Nigerophiidae and Palaeophiidae. The sister clade to the Acrochordoidea is the huge Colubroidea, with its vast and diverse array of ratsnakes, watersnakes, treesnakes, cobras, seasnakes, and vipers. This superfamily comprises 11 families and more than 3,000 species — almost 82 percent of all living snakes.
CHAPTER 2WHAT IS A SNAKE?
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.
SKELETON AND LIMBS
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 sea snake.
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.
SENSE ORGANS
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.
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.
SHEDDING
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.
CHAPTER 3PREY & HUNTING
All snakes are carnivorous, but as a group they hunt a wide diversity of prey types and sizes. Some snakes are generalist feeders, with a catholic diet, while others are specialists that concentrate on one type of prey.
INVERTEBRATE PREY
The smallest living snakes, the scolecophidian threadsnakes (Leptotyphlopidae) and blindsnakes (Typhlopidae), have tiny mouths and limited dentition. They feed on small, soft-bodied prey such as ant or termite larvae and eggs, although some of the larger species (Acutotyphlops) take earthworms.
Vermivory, or feeding on worms, is a common feature of the diets of snakes across many families. Earthworms are the prey of the Asian shieldtail snakes (Uropeltidae) and spine-jawed snakes (Xenophidion), but numerous advanced snakes also feed on earthworms. There are even a few venomous snakes that eat earthworms — the Fiji Snake (Ogmodon vitianus) and Papuan worm-eating snakes (Toxicocalamus) in the Elapidae, and the Udzungwa Mountain Viper (Atheris barbouri) in the Viperidae.
Biologists refer to snakes that feed on "slimy" prey — earthworms, slugs, and snails — as "goo-eaters." Molluscivorous snakes require specialized oral glands to neutralize the excessively sticky secretions produced by their prey, and have specially adapted jaws to enable them to extract the snails from their shells. Slugs and snails, and the snakes that eat them, are common in the tropics, with Dipsas and Sibon species found in tropical America, Duberria in Africa, and Pareas in Asia.
Specialized centipede-eaters are found in Central America (Scolecophis) and Africa (Aparallactus), while the American hooknose snakes (Ficimia and Gyalopion) prey on spiders and scorpions in addition to centipedes.
The Crab-eating Mangrove Snake (Fordonia leucobalia) and Gerard's Watersnake (Gerarda prevostiana), both found in Southeast Asia, feed on freshly molted crabs and mud lobsters. And in North America, crayfish snakes (Liodytes) have specially adapted skulls to enable them to feed on hard-shelled crayfish.
PISCINE (FISH) PREY
Fish feature in the diets of many snakes, especially the freshwater aquatic keelbacks and watersnakes (Natricidae), which hunt by sight and touch. The tuberculate skin of the Indo-Australian filesnakes (Acrochordus) enables them to grasp a slimy fish while they maneuver prey into the mouth, while the Tentacled Snake (Erpeton tentaculatum) detects fish using the curious tentacles on its head. Venomous freshwater piscivorous snakes include the Aquatic Coralsnake (Micrurus surinamensis) in Amazonia, the Banded Water Cobra (Naja annulata) in Africa, and the Cottonmouth (Agkistrodon piscivorus) in the USA. Seasnakes prey on a variety of gobies, moray eels, catfish, and pufferfish, while sea kraits (Laticauda) specialize in eels. A few seasnakes eat only fish eggs — the Mosaic Seasnake (Aipysurus mosaicus) takes the eggs of benthic gobies in their seabed burrows, while the Southern Turtle-headed Seasnake (Emydocephalus annulatus) uses its enlarged lateral lip scales to scrape the eggs of blennies and gobies from coral.
(Continues…)
Excerpted from "The Book of Snakes"
by .
Copyright © 2018 Quarto Publishing plc.
Excerpted by permission of The University of Chicago Press.
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Table of Contents
Introduction Evolution & diversity of snakes What is a snake? Prey & hunting Enemies & defense Reproductive strategies Snakes & humankind The snakes SCOLECOPHIDIA ALETHINOPHIDIA: AMEROPHIDIA ALETHINOPHIDIA: AFROPHIDIA: HENOPHIDIA ALETHINOPHIDIA: AFROPHIDIA: CAENOPHIDIA Glossary Resources Index of common names Index of scientific names Index of taxonomic groups AcknowledgmentsCustomer Reviews
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