How dangerous are snakes?

Nov 25, 2006 14:55 GMT  ·  By

Snakes emerged during the Cretaceous period (the last era of the Dinosaurs) from monitor-like lizards. Even today, many lizards lineages developed legless forms, with skinks being the most known group.

Some say that the lack of legs was as adaptation to subterranean life (indeed, even today's most primitive snakes are subterranean), others that this appeared as the first snakes roamed grassy environments, using lateral undulation movements as an easier way of locomotion in these habitats.

Snakes and lizards are the solely groups of prosperous reptiles during the Tertiary, the era from Dinosaur's extinction till now, also named "The Age of the Mammals". In fact, snakes experienced an evolutionary boom during this period correlated by the explosive radiation of rodents (rats and mice) on which most snakes depend.

First thing that occurs on our minds when thinking of snakes is venom! As many snakes kill or eat primates, fear for snake is deeply embedded in our minds from ancestral times when we were just monkeys. But snakes developed their terrific chemical weapon first of all to kill their prey, with the minimum injury risk.

Their venom evolved initially in salivary glands and in many species, the venom still has a digestive role, fact proved by the necrosis of the tissues at the bite site. Of course, due to its lethal power, the venom became a defensive power, so that many snakes warn the intruders about their danger potential through sonic signals (like rattlesnakes or saw-scaled vipers), warning color patterns, like coral snakes and kraits or, like in cobra case, through specific behavior like "hood" broadening.

This is so effective that many harmless non venomous snakes imitate these color patterns, like the shear number of false coral snakes, or even behaviors, like false cobra imitating cobra's hooding.

Of a current number of about 2000 snake species, in fact, only about 400 species possess venom strong enough to produce harm on humans, and even less are deadly. Only three groups of snakes are venomous: Elapids (cobras, kraits, coral snakes, mambas, true sea snakes and all Australian venomous snakes, like taipan and dead adder), Viperids (adders, vipers , rattlesnakes, pit vipers) and Colubrids, which compasses grass snakes, water snakes and related groups.

Colubrids represent the largest and diverse snake group, embracing more than half of the current snake species, and not all are venomous. In fact, the proper grass snakes and water snakes are destitute of venom.

The giant pythons and boa snakes, very known by the public, are also totally devoid of venom. In fact, the giant snakes have primitive forms, with a complete ancestral jaw teeth set, 4 lines on the upper jaw and two on the lower jaw.

Their simple teeth are larger on the front of the jaw (they are proterodonts "big fore teeth") (photo center above), as they are adapted to grab the prey for the giant snake to apply its deadly embrace. But, as the snakes developed venom, they had to develop a mechanism to inject it into their prey. So, their teeth suffered profound changes from this primitive formula. Some enlarged teeth, called fangs, had to inflict the bite, and a groove through the fangs had to provide a way for the venom to flush into the prey's tissues.

In the case of the Elapids (photo center), two fore teeth on the upper jaw enlarged and transformed in fangs. The fangs are immobile on the jaw and their venom groove is usually frontal.

And the groove sides are normally closed, except the tip zone, so the venom enters into the wound like from a syringe. The teeth behind the fangs on the upper jaw disappear or they tend to do so. Some cobras (photo above) can spit their venom towards attackers, as the groove aperture opens perpendicularly on the fang's axis. And their venom can cause severe ocular lesions on people.

Most venomous Colubrid snakes have an opposite structure of their venom injecting apparatus, with fangs developed from the rear teeth of the upper jaw. Here is present a tendency of the fangs to become mobile, but this happens actually in very few forms. The fore teeth of the upper jaw tends to decrease or disappear, contrary to the situation in cobras.

On Viperids (photo center bellow), the tendencies observed on Colubrids went to extreme: their huge fangs are highly mobile, and in fact they stay folded in repose and are risen only when the snake bites. Not only that their fore teeth on upper jaw disappeared, but the fore upper jaw is greatly reduced.

And more importantly, viper fangs do not possess a venom groove but a real closed venom duct. The traits found in both Viperids and rear fanged Colubrids prove an evolutionary common ancestry or maybe Viperids evolved directly from a Colubrid group.

Elapids may share an evolutionary pathway with giant constrictors snakes, as they share the trait of developing the fore teeth, or maybe from non venomous proterodont Colubrids.

But how dangerous really are venomous snakes?

Amongst venomous Colubrids, only one African species, the boomslang (Dispholidus) has fangs big enough and a potent venom to harm a human being, even if it is a non aggressive species difficult to provoke. The rear fanged snakes have to chew their prey while swallowing it in order to administrate the paralyzing venom which initiates the digestion and they usually eat small prey, like lizards, frogs. Moreover, their venom in most cases is weak.

Cobra type venomous snake tends to bite and keep a hold on, as many have shorter fangs and some coral snakes even chew. But vipers, with their highly efficient injecting mechanism, effectuate the bite sometimes in 1/40 of a second and then retreat.

The snake venom varies a lot between different groups. Some venoms are neurotoxic and paralyze the muscles. The victim dies of suffocation and heart attack. All cobra-related snakes have extremely potent neurotoxic venom, but also some rattlesnakes, moccasins and pitless vipers.

Some venoms, like cobra's, produce anaphylactic shock. Other venoms action like a powerful digesting juice, producing necrosis and hemorrhage. Most vipers possess a powerful venom of this type and with their huge fangs can introduce large amounts of venom into the wounds.

The bushmaster (Lachesis muta) (photo bellow), a pit viper from tropical America produces general necrosis, generalized internal hemorrhage and external hemorrhage through all the orifices.

Sea snake venom provokes elimination of mioglobin (muscle protein) through the kidney and necrosis into the muscles, but these snakes are not known to have ever bitten a person.

Some animals are very sensitive to snake venom, like birds, others not, like mongooses that consume venomous snakes and some species, like snake eating snakes (king cobra, king snakes), are totally immune to snake venom. Many snakes have venom specialized for their prey, like an aquatic Colubrid, Fordonia, whose venom is toxic only for the crabs. Ultimately, the aggressiveness is important on appreciating the danger posed by a snake species, and most Elapids and Viperids are both aggressive and highly toxic.

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