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July 2024 - Why are some snakes venomous and others not?

The first snakes evolved more than 120 million years ago. To the best of our knowledge none of them had venom. That came much later – around 80 million years back. Prior to that, snakes largely relied on muscle power to overcome their prey, but that was often problematic – if you grabbed a prey animal and constricted it, there was always the danger that you could be injured in the process.

The first snakes to develop venom were the so-called back-fanged snakes. Some of the teeth in the back of the mouth grew longer than the other teeth and had groves down the front for venom to trickle down. Venom glands were developed in the cheeks and venom was produced – largely a mixture of proteins and peptides. The venom glands of back-fanged snakes, if compared with modern cobras and adders, are quite primitive with minimal muscular control to regulate the amount of venom injected when biting. These glands are referred to as Duvernoy’s glands. If you, for example, observe a Boomslang feeding on a large chameleon, it usually bites its prey, gives it a few good bites and may then release it and approach from a different angle and bite again. The more it chews, the quicker it injects sufficient venom to kill its prey.

The short back fangs of a Boomslang. 

The elapids - the cobras, mambas and their close relatives - have short, fixed fangs in the front of the mouth and it is evident that they have far better control over the amount of venom that is injected in a single bite. Adders have large fangs, mostly hinged, which enables them to deposit venom deep into muscle where it rapidly does damage to kill prey.

While many venomous snakes, like the Mozambique Spitting Cobra and Cape Cobra are generalists and have venom that is effective on a variety of prey species including rodents, birds, frogs and even snakes, other venomous snakes are specialists with prey-specific venom. A good example is the Black-headed Centipede-eater that feeds exclusively on centipedes. Its venom, should a human get bitten, is mild with little effect but it kills a centipede rapidly. The Herald snake is another good example – adults feed largely on frogs, especially toads, and its venom is effective at killing them.

Black-headed Centipede-eaters are mildy venomous for humans but have a potent venom against centipedes. 

From the above it is clear that snake venom was developed to immobilise and kill prey and as no humanoids were around 80 million years ago, had nothing to do with people. This is exactly why snakes do not chase after people or bite people maliciously. Snakes bite in desperation – when hurt, threatened or cornered with no easy escape and if you are five paces or more away from a snake you are safe from being bitten.

There is one exception in South Africa and that is the Mozambique Spitting Cobra. This snake regularly ends up in houses at night when hunting, and bites people sleeping in their beds. In the past it was thought that these snakes accidentally end up in beds and bite when accidentally rolled onto or that they enter beds seeking heat. Having looked at hundreds of bites it is evident that they are smelling a mammal in a bed and biting people in a feeding response.

Mozambique Spitting Cobras are known to enter houses at night and sometimes bite sleeping people. 

Back to snake venom and humans – snakes clearly never developed their venom for self-defense against humans and in confrontations with humans, their venom is of little use as no snake can kill a human in seconds or a few minutes. But there is an exception – five to seven million years ago snakes had to cope with a new threat – neanderthal-like humanoids that could walk on two legs. Around that time some snakes, like the Mozambique Spitting Cobra and the Rinkhals, developed the ability to spit or eject their venom. It is a great strategy – the snake ejects venom towards the eyes of an attacker and the smallest drop of cytotoxic venom that gets into the eyes causes an immediate burning pain. The victim is quick to close the eyes that burn, and the snake has time to escape. The venom that a spitting snake ejects is the same venom that it would use when biting.

The ability to spit or eject venom appears to have evolved with the rise of bipedal humanoids.

More than a third of the snake species in southern Africa have no venom and this includes the likes of the Southern African Python, Brown House Snake, Mole Snake and Spotted Bush Snake. Most of them make use of muscle power and constrict their prey. Most non-venomous snakes have teeth that they use to secure their prey but some, like the egg-eaters, thread snakes and worm snakes, have greatly reduced teeth and cannot bite a human.

Most non-venomous snakes are powerful constrictors like this Brown House Snake. 

Egg-eaters of the genus Dasypeltis feed exclusively on bird eggs. The egg is swallowed whole to as far as the neck region, where bony protrusions from the vertebrae are used to crack the eggshell. The egg is crushed, its contents swallowed, and the crushed eggshell is regurgitated.

Some other specialists feeders include Slug-eaters that feed on snail and slugs and the Shovel-snout snakes that feed largely on reptile eggs.

No matter how we view snakes or think that some are far more advanced and successful than others, they have been around for millions of years and are highly successful. We still know very little and have a great deal to learn about snakes and their behaviour.

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Johan Marais
African Snakebite Institute | +27 82 494 2039
E-mail johan@asiorg.co.za 
Find out more at www.africansnakebiteinstitute.com

Johan Marais is the author of various books on reptiles including the best-seller A Complete Guide to Snakes of Southern Africa. He is a popular public speaker and offers a variety of courses including Snake Awareness, Scorpion Awareness and Venomous Snake Handling. Johan is accredited by the International Society of Zoological Sciences (ISZS) and is a and SASTM (South African Society of Travel Medicine) - approved service provider. His courses are also accredited by the HPCSA (Health Professions Council of South Africa). Johan is a qualified instructor for the Emergency Care & Safety Institute in Oxygen Administration and Wilderness First Aid and a qualified Basic Life Support instructor.
 

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