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How have snake fangs evolved?

There are few structures in nature that inspire more fear than the fangs of venomous snakes. Their lightning-fast bite is the perfect way to inject venom into their prey, either to attack or defend, and their fangs are the ideal tools for this. Unlike many animals, which use their fangs for stabbing and ripping meat, snakes are long, curved and grooved, suggesting they’re built specifically for venom delivery. But what came first – the venom or the fangs? That’s exactly the question that a team from the College of Science and Engineering at Flinders University, Australia, aimed to answer. Their research, published in the journal Proceedings of the Royal Society B: Biological Sciences, teases apart how snakes’ specialised venom-delivery teeth evolved.

Venomous fangs first developed as grooves at the base of snakes’ teeth. These grooves most likely evolved to keep teeth firmly attached to the jaw, as snake teeth typically have very shallow roots, and their wrinkles (known as plicidentine) give the jaws more surface area to adhere to. According to Alessandro Palci, a research associate who specialises in palaeontology and evolution, fangs then developed from these wrinkles in the teeth. Plicidentine is common in extinct reptiles, but not as well-known in living animals, making snake teeth comparatively unique.

The team studied 3D microCT images of the fangs of 19 snake species and three lizard species, as well as thinly-made slides from a few of the specimens. Before CT scanning, there was no real way to analyse the inside of snake teeth in a satisfactory way – they’re small and hard, so dissecting one is not really an option. In every species they examined – those that were and were not venomous, and those with and without fangs – they found these grooves. This implies that the grooves likely developed in a non-venomous ancestral snake species, and were then co-opted by venomous species, using the pre-existing grooves to deliver venom into their prey.

Snake teeth are an effective weapon, honed over generations to be perfect venom delivery systems

Speaking to Live Science, Palci noted: “The simplest venom fangs only have a shallow groove on their surface. In more advanced snakes (e.g. vipers and cobras) the groove has deepened to the point that its margins meet, sealing the groove and forming a hollow, tube-like structure that resembles the needle of a syringe. These grooves were selected over millions of years of evolution to produce large and highly efficient syringe-like fangs.”

The implication of the study, then, is that venom came before snake fangs. Palci explains: “Venom, in some mild form, is thought to have appeared very early in the common ancestor of snakes and some lizards (a group called Toxicofera). Therefore, venom fangs evolved after venom was already present. The presence of venom was likely an important prerequisite for the evolution of venom fangs.” Once any given snake evolved to produce venom, with evolution favouring snakes that include ever more toxic enzymes in their saliva, natural selection also favoured snakes with larger plicidentine wrinkles to help channel venom to the fang tip.

In May, in the journal Evolution, a team from Monash also explored the evolution of snake fangs, and they found differences between the fangs of different snake families – through evolution, each snake family independently “designed” their fangs and venom delivery systems, leading to slight differences. This is not unusual in the animal kingdom, where variations in tooth shape occur according to diet, and it’s true of snakes, as different species vary in the type of prey they target. Fangs are more robust and blunter in species that target tougher prey, such as lizards and crabs, and more slender and sharp-tipped in species that target prey with softer skins, such as mice. By inspecting a snake’s fangs, it is possible to predict its preferred prey, something that can aid both relocation efforts and the development of suitable protective clothing for snake handlers.

Snake teeth are an effective weapon, honed over generations to be perfect venom delivery systems. But, as it transpires, snake venom predates them – it’s a fascinating look at how two forms of natural defence helped shape each other.

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