Herpetology is the study of snakes. There are around 2700 snake species, including around 375 venomous snakes with medical relevance. Of the latter, around 200 are potentially lethal. The biotopes vary greatly: from the arctic circle to the equator, and from sea level to 5000 m in elevation. Venomous snakes are not found in Chile, Madagascar, New Zealand, Hawaii and New Caledonia. In Belgium there are a very small number of indigenous vipers (Vipera berus), ringed snakes (Natrix natrix or grass snake) and smooth snakes (Coronella austriaca). The last two snakes are not venomous.

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Snakes are quasi-cylindrical reptiles without limbs. They move using a concertina movement, rectilinear, curvilinear, via "sidewinding" or by a combination of these methods. Some species possess a vestigial pelvic girdle, sometimes with vestigial external spurs, as with boas and pythons. The heart has one ventricle and two atria. The left lung is atrophic, except in boas. The right lung can have an extension in the throat, which is important for the animal because there is airway compression when it swallows large prey. In general, the length of the lung is about one-half of the total body length. The posterior half of the lung can serve as a reservoir while the front part is compressed. In reptiles, the nostrils come out in the mouth cavity, right behind the teeth. They can breathe through their mouth if it is empty. A full mouth blocks respiration. They can tolerate apnea for a fairly long time, because as poikilothermic animals they have a rather low oxygen demand. By exhaling quickly some snakes can produce a hissing noise (cf. the puff adder). Distinguishing between male and female snakes is not easy. To do so it is necessary to examine the cloaca and determine the presence or absence of two hemipenes. There are both oviparous and viviparous species. After birth, the new-born venomous snakes already possess a supply of venom.

The entire skin is covered with scales. Each eye is covered with an immobile, transparent scale. The animals have no eyelids. Blinking snakes only exist in Hollywood. All snakes shed their skin from time to time (ecdysis), e.g. several days before laying eggs or after trauma. Before moulting, the eyes have a somewhat milky appearance, and the snake will be virtually blind. In this condition the snake probably feels quickly treatened and tends to bite more easily. Freshly shed skin has a moist-greasy feel (never slimy). The skin dries after several hours. The scales can be smooth or display a central lengthwise ridge or "keel". Some snakes (e.g. Echis carinatus) can make a warning noise by rubbing the scales over one another. All snakes have 1 row of ventral scales on the belly. In some species, the scales form small horns on the skull, e.g. Cerastes cerastes (North African desert horned viper with protuberances above the eyes), Vipera ammodytes (European horned viper), Agkistrodon acutus ("sharp-nosed viper") and Bitis nasicornis (rhinoceros viper). Yet here too there are variations. The colour of the animals can vary within a given species, and sometimes there is sexual dimorphism. A snake can sometimes change colour over the course of its lifetime (juvenile specimens are generally lighter coloured). Colour descriptions are thus always relative. Colours develop through the presence of pigments, through optical interference (iridescence), and through the Tyndall light scattering effect, i.e. dispersion of light by small intracellular particles (iridophores) composed of purine crystals. Albino, anerythristic, melanotic and amelanotic animals are found, but are not very common in nature.

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The colour patterns have a specific function in helping the animal survive. Many snakes are cryptically coloured and their colour corresponds to that of their natural environment, making them less easily noticed by their prey or a predator. Stripes and/or spots can act as a camouflage, breaking up the visual outline against the surroundings. Countershading (belly lighter than back) make the animal more difficult to see. A harmless snake can imitate a venomous one when both live in the same environment, i.e. Batesian mimicry (1861, Henry Walter Bates, English naturalist). In this way predators avoid the snake, if they have learned earlier that an animal with such coloration is dangerous. We find a typical example of this in coral snakes (Micrurus sp., venomous) and some colubrids (e.g. Lampropeltis sp., not venomous). Müllerian mimicry also occurs in animals (1878, Fritz Müller, German zoologist), whereby two species resemble one another, to the benefit of both. However this phenomenon is more common among insects. A variant of Batesian mimicry is Mertensian mimicry, in which a non-venomous animal resembles a moderately poisonous instead of a highly venomous one. The idea is that a predator will more easily survive a contact with a moderately venomous animal than with a highly venomous animal. A learning process is thus stimulated, without being punished by death. Naive predators will then be less common, which benefits the prey.

Most snakes have poor hearing and limited visual acuity. By contrast, in the roof of their mouth they possess an extremely sensitive organ, known as a Jacobson´s organ. It consists of two openings lined with sensory cells. The animal flicks out its forked tongue and brings it back into the mouth, inserting the tips into the two openings of Jacobson´s organ. The tongue brings molecules from the environment into the organ. In this way the snake can sense its environment. Pit vipers possess heat-sensitive sensors in small pits located between nostrils and eyes. Pythons and some boas also have such sensors, located on their lip scales. Snakes are very good at perceiving vibrations, e.g. of the ground. Some people use this as a means of prevention, by regularly beating a stick on the ground in front of them when they walk in an area with venomous snakes.

Given that practically all snakes lack a retinal fovea, their visual acuity is generally limited. Some tree snakes have rather good sight. The eye contains a non-deformable lens which can be moved forwards and backwards to bring objects into focus. The pupil can be round, oval or slit-shaped. Slit-shaped pupils are customary in nocturnal predators. During the day the slit keeps most of the light out and the retina is not overloaded. At night the iris dilates. This is done more easily with slit-shaped pupils than with round pupils. A thin slit 5 mm long has a circumference of 10 mm. When it dilates to a circle 5 mm in diameter the circumference (16 mm) has increased by a factor of only 1.6. By contrast, when a round iris of 1 mm in diameter has to dilate to a circle 5 mm in diameter, there is a fivefold increase of the circumference, which is mechanically more of a burden for the small iris muscles. A small iris diameter improves the resolution (perpendicular to the slit). Given that they often hunt small animals which move horizontally over the ground, a vertical slit-shaped pupil may give the snake the best sight to e.g. spot a scurrying mouse. This advantage disappears at night, however.

All snakes are carnivorous. Because they do not have to continually maintain their body at a constant temperature, their food intake requirement is a good deal lower than that of warm-blooded animals. The diet differs from species to species and includes snails, earthworms, insects, eggs, lizards, frogs, fish, rodents or other snakes. Most snakes defecate only rarely. Because chronic "constipation" is most pronounced among sit-and-wait predators – animals for which body weight is of great importance – some people assume that these snakes make good use of the extra weight (3 to 22% of their body weight is faecal material). These animals lie still on the ground and use their heavy intestine as a counterweight in order to be able to strike more quickly with the mouth. Most snakes drink water from time to time. Snakes are ectothermic and prefer one particular temperature. Since the environment of the snake is so important for the animal, it is not unusual for a snake to lie at night on a path or road, where the temperature is somewhat higher than in nearby vegetation. Obviously this increases the chances of an accidental bite being suffered by a nighttime walker. In order to conserve heat, they can roll themselves up (small surface/weight ratio). This is also important to limit transcutaneous loss of water. In cold regions snakes can hibernate, individually or in a group. Since snakes do not have to use energy to continually generate heat, but only require food for homeostasis, movement, growth and reproduction, they can get by with very little food. Due to their low metabolism, they cannot maintain a major effort (e.g. pursuit of prey) for a very long time. In this case, they rapidly develop an oxygen deficit. Many snakes have a limited territory. After having bitten somebody, a snake can generally be found within a rather small radius around the site of the incident, even after several hours.

Colubrids have a modified salivary gland (Duvernoy´s gland), which discharges near the fangs at the rear of the mouth. The venom is slowly introduced into the prey via capillary action. Therefore, in order to get sufficient venom into the tissues, a long contact period is necessary. However, this occurs only exceptionally in humans. This explains why most bites by colubrids are harmless. This also explains why occasionally envenomations are described by snakes that traditionally are regarded as non-venomous. In elapids and vipers, by contrast, the venom glands consist of the uppermost labial salivary glands. They can be actively emptied by the musculus constrictor glandulae, so that the animals can actively and very quickly inject venom, or even spit venom (several metres). Accessory venom glands are present in some snakes. Venom evolved before fangs, and even snakes without highly evolved fangs have potent venom. This explains why so many "harmless" snakes can be venomous. They are not necessarily dangerous to humans, but they have enough venom to kill their ordinary prey.

A snake skull is complex. There are numerous small bones and ligaments. The bones of the upper and lower jaw are muscularly and elastically connected with one another and with the skull. The left and right sides of the jaws can move independently of one another. This makes it possible to swallow large prey, yet the animals cannot chew. Snakes have no sternum, so that a large ingested prey does not constitute a mechanical obstacle when it is being swallowed (some prey have a diameter which is greater than the resting diameter of the snake).

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On the lower jaw of a snake there are small teeth on the os dentale ("teeth bone"). In the upper part of the mouth a double row of teeth is present. There is a lateral row on the maxilla and a medial row on the os palatinum and on the pterygoid bone. These small teeth curve backwards, making it more difficult for a prey to escape. In vipers the fangs are joined to the maxillae. The upper jaw of vipers can rotate vis-à-vis the prefrontal bone. This makes it possible for the fangs to be folded backwards when the mouth is closed. When the animal strikes, the maxillae rotate so that the fangs unfold forwards and can be used to bite. In all other snakes the maxillae and the fangs are immobile. Reserve fangs are brought into functional position before the old fangs fall out. Therefore a bite wound can display 1 to 4 fang marks. The puncture wounds are spaced from 5 to 40 mm apart and are 1 to 8 mm deep (even deeper in case of a bite by a gaboon viper).

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In snakes, the teeth are not so firmly attached to the top/inner side of the jawbones (so-called "pleurodont dentition"). This makes it possible for the teeth to be easily replaced throughout a snake´s lifetime. The teeth break off easily. This influences the biting behaviour. Thus vipers bite, inject venom and release again in rapid succession, because a struggling prey could cause injury or break the teeth.

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Note: Infections transferred via snakes:

Pythons can be infested by tongue worms (Pentastomida) such as Armillifer armillatus in Africa or A. moniliformis in Asia. These parasites live in the lungs of the reptiles. The eggs in the snake’s sputum can infect human beings, e.g. through contamination of drinking water or when a snake is prepared as food. Porocephalosis (syn. pentastomiasis) is the result. In general, infection leads to asymptomatic crescent-shaped calcifications in the abdomen. Living parasites are rarely found elsewhere (e.g. subconjunctival). Gnatostomiasis (infection with the nematode Gnathostoma spinigerum) can also follow consumption of undercooked snake meat. A larva migrans syndrome or a very serious eosinophilic meningo-encephalitis can then develop. Spirometra sp. can be transferred via snakes (also via frogs) and cause sparganosis, whereby the immature cestode can be found in the eye. These worms can survive for up to nine years in human beings.