Australasian Science: Australia's authority on science since 1938

Out of the Mouths of Snakes

Snakes like this Stimson’s python owe their evolutionary success to their big mo

Snakes like this Stimson’s python owe their evolutionary success to their big mouth and the ability to swallow large prey.

By Alessandro Palci, Mark Hutchinson & Michael Lee

DNA analysis and 3D imaging have revealed how snakes evolved their huge gape independently across different lineages.

“Grandma, what a big mouth you have!” said Red Riding Hood.

“All the better to eat you with,” replied the wolf just before he swallowed Red Riding Hood whole.

The Brothers Grimm wrote one of the most iconic children’s stories of all time, but we wonder whether a big snake would have been a better choice for the villain. Wolves indeed have big mouths, but they certainly cannot swallow a whole person, not even a little girl.

On the other hand, some huge snakes have little problem swallowing humans in one big gulp. Examples include the green anaconda (Eunectes murinus), which inhabits the swamps of South America, or the reticulated python (Malayopython reticulatus), a giant snake (up to 12 metres in total length) from the tropical forests of South-East Asia.

But how can snakes swallow prey items that are so much larger than their own head?

Contrary to popular belief, snakes cannot unhinge their lower jaws from the rest of the skull when swallowing. Their ability lies elsewhere in their amazing skulls, which are composed of a set of loosely connected bones.

Unlike most other animals, snakes can separate the left and right halves of their lower jaws, which are connected only by skin and ligaments at the chin. Also, each lower jaw is extremely long, and has a line of flexion in the middle that allows it to bow outwards.

The upper jaws can also move independently from the rest of the skull, and the teeth on both jaws – together with two extra rows of teeth on the roof of the mouth – can be used to slowly drag a prey item down the throat with well-coordinated alternate left and right movements.

This big mouth – often coupled with potent venom – makes snakes the formidable predators they are today, and is possibly what kick-started the evolution of these amazing animals from long-bodied lizards.

Two of the earliest known fossil snakes, Portugalophis and Parviraptor, which date back to more than 140 million years ago, already possessed some of the breakthrough jaw adaptations that distinguish snakes from their lizard cousins today. These fossil snakes clearly had the same loosely attached, movable upper jaws we find in modern snakes like pythons and boas, although the rest of the skull is still unknown.

However, not all living snakes have a huge mouth. Many small burrowing snakes that feed mostly on small or narrow prey like termites and worms lack the capacity to expand their jaws.

Anatomical studies initially suggested that all big-mouthed snakes were closely related, and that a huge gape thus evolved only once. However, recent studies of DNA sequences in snakes showed that the big mouth appears in different lineages of snakes that are only remotely related. A huge gape thus might have evolved independently in each lineage.

The possibility of independent origins triggered our study of skull and jaw skeletons in a series of newborns and adults of diverse species of snakes. We adopted modern CT imaging techniques to examine the bones inside the heads of a series of snakes, mainly from Australia. We could use these images to produce three-dimensional reconstructions of the skulls on a computer screen, and these virtual skulls could be “landmarked” for points of interest that would allow a precise comparison of the geometry of the skulls between juveniles and adults, and between different species.

It turns out that all snakes that have a huge gape also have a very long lower jaw relative to the skull. This means that their jaw joint is placed further back in the head. We noticed that in the big-mouthed snakes, the jaw’s articulation (or hinge) has been pushed back so far that it actually lies far behind the braincase. This articulation sits at the end of a set of two specialised bones on the skull: the lower jaw is attached to a rod-shaped bone called the quadrate, and this bone in turn is attached to a flat bone called the supratemporal, which sits on the back of the skull.

In pythons, a long supratemporal carries the jaw backwards. However, in file snakes and most other advanced snakes, including all venomous species, an elongated quadrate does the same job. Thus, pythons and file snakes have developed a bigger “grin” in completely different ways.

There’s no obvious advantage with either of the two different ways, so the differences may simply be due to the two different snake lineages independently chancing on different solutions to the same problem of how to make a bigger mouth. Early snakes swallowing large prey would have been a ready target for evolutionary fine-tuning – by just lengthening either of the two skull bones that suspend the lower jaw they could greatly expand their gape and therefore swallow bigger prey. It seems the ancestors of pythons enlarged one of these bones, and the ancestors of file snakes the other.

So how does enlargement of these bones – and thus the jaws – come about?

Actually, similar changes happen during the growth of all animals, including us. The changes in body shape from juvenile to adult involve different bones growing at different rates, a biological phenomenon called “allometry”. By slightly tinkering with the amount and direction of such growth-related proportional changes, evolution can relatively quickly sculpt new adult body shapes without a lot of other genetic change.

By including baby snakes as well as adults in our study we were able to see that the mouths of big-jawed snakes get proportionately (as well as absolutely) larger as they grow. We noticed that baby pythons have a short supratemporal bone and a jaw joint that is level with the back of the skull. However, the supratemporal grows faster than the rest of the skull, so by adulthood the jaw joint has been carried backwards well behind the skull. Thus, a minor increase in the growth rate of this bone leads to a radically different adult skull.

A similar process happens in file snakes, but in this case it’s the quadrate that grows disproportionately faster. Either way, a slight alteration in growth patterns results in a major change in adult shape and function.

Snakes are one of the most formidable success stories of evolution, having survived for more than 140 million years, sailing through the mass extinction that wiped out all non-avian dinosaurs. The ability to swallow large prey items allows these animals to feed infrequently, and thus survive through long periods when food is scarce or absent, as may happen during ecological crises associated with mass extinctions. This adaptation is likely one of the key aspects that drove their incredible success.

Our study suggests that snakes evolved this useful ability multiple times in different ways, and it’s probably not a coincidence that the most diverse and widespread groups of snakes living today all have big mouths.

Alessandro Palci is a Research Associate at Flinders University and a collaborator with the South Australian Museum. Mark Hutchinson is Senior Research Scientist at The South Australian Museum, Adjunct Associate Professor at Flinders University, and Affiliate Lecturer at The University of Adelaide. Mike Lee is Senior Research Scientist at The South Australian Museum, and Professor and Matthew Flinders Fellow at Flinders University.