Australia’s own SNAKE ISLAND in Western Australia is overrun with large Tiger Snakes that have evolved giant jaws to feed on seabirds! This could be your worst nightmare!These extremely aggressive and deadly reptiles are the fourth most venomous snakes in the world. Their venom if left untreated is so toxic, can kill a human in less than 30 minutes.

On the tiny island called Carnac Island these slithering Tiger Snakes have evolved giant jaws to swallow whole seagull chicks, not in millions of years, but in less than a century!

This rapid evolution has been necessary to survive. 

1. Why have the tiger snakes in Carnac Island evolved huge jaws?

Carnac Island is just 19 hectares in size and is home to a population of over 400 deadly tiger snakes. This is probably one of the densest populations of snakes in the world.

With competition for food being so great, to survive the snakes have had to have a drastic dietary change to eat the food available, The snakes have evolved large jaws to eat the larger seabird chicks and EAT THEM WHOLE!. 

With no smaller prey, like frogs, lizards or mice around, prolonged dietary stress has meant that the plasticity of their skulls changed to grow longer mandibles so that they could eat the only prey which was available to them, seagull chicks. This is known as diet-induced plasticity.

2. Why is this evolution of large jaws so unusual?

It is thought that the tiger snakes arrived on the island having been dumped a hundred years ago, by a snake performer, so that he didn’t get in trouble with the law. The snakes did not have larger jaws when they arrived as on the mainland, they have no such ability! 

Adaptation usually takes millions of years to occur, but in less than 100 years these island snakes have developed larger jaws. Just as unusual this ‘diet induced plasticity’ has occurred over a snake’s lifetime and the snakes’ offspring share the ability. The skull bones changing are the result of a drastic diet change.

This plasticity didn’t take millions of years to occur it took less than 100 years. This ability to grow larger jaws in such a short time is unusual but it has allowed them to survive on a tiny island.

3. Why can a snake swallow prey bigger than their own head? 

Snakes have evolved from lizards and gradually acquired greater and greater flexibility and extensible jaws. In lizards both the lower and upper jaws are inflexible. In the most primitive extinct snakes found a loose joint has developed in the bottom jaw.

In snakes the lower bones of the jaw are called mandibles and they are not connected like they are in mammals. Each mandible is attached by a stretchy ligament and can spread apart increasing the width of the mouth. These mandibles are loosely connected at the back to the skull allowing a greater rotation than most animals have. This is how a snake con open its mouth wider than its body, allowing snakes to swallow bigger than their own head!

4. How does a snake swallow its prey once trapped in its mouth?

The larger mandibles move independently of each other, slowly moving the prey into the throat. At the same time the snakes head does a ‘walking’ forward movement moving the jaws in small steps in a side-to-side motion over the prey’s body (like moving furniture, “walking” it from side to side). The prey is then levered backward as the head moves forward.

The whole process can take some time so backward pointing teeth stop the prey from escaping if it is still alive.

The snake can keep on breathing when it is swallowing a large prey because the glottis can move off to the side so the prey does not stop it from opening allowing the snake to still breath while eating.

5. If snakes don’t chew their food, how do they digest a whole seabird chick? 

Snakes do not have the right kind of teeth to chew their food when they capture the prey whole, so there is no mechanical breakdown at first.

The prey stays in the stomach for days allowing the strong stomach acid and enzymes to digest the tough outside of the prey and bones. Seabird feathers and claws cannot be digested as they are made of a resistant protein keratin which is an indigestible substance. Calcium from the bone is also remains and with the feathers the snake excretes them in the form of pellets.

Unlike many mammals that only keep their food in their stomach for a few hours to a day, the snake’s stomach continues to digest its meal for several days or even weeks, depending on the size of the meal. The liquefied meal makes its way into the intestine, where it is further broken down where a newly thickened lining of the intestine absorbs all these nutrients.

This process is slow, and snakes can often cough up partially digested food.

6. How did researchers study how these snakes have adapted? 

A research team took some baby snakes from Carnac Island and compared them with a group taken from the mainland. They then divided them in to two further groups. To one group they fed small mice and the other large mice.

As the snakes matured and approached adulthood, they looked investigated their heads using a high-resolution version of a medical CT scanner. As they took measurements of the different skull bones, they discovered that the island and mainland groups had responded to different sized prey. 

Regardless of whether they were fed small or large prey the mainland snakes showed the same head shape. But Carnac Island snakes showed that those fed large prey developed a much bigger bite, with longer jaw bones, particularly in the lower jaw and palate!

These skull bones had reacted to drastic dietary change.

7. Why are the snakes from Carnac Island called ‘Easy-gainers’?

Some people struggle to build up muscle from weight training at the gym whilst some people can quickly build up muscle. The ones who build up muscle easily are called ‘Easy-gainers whilst the others who find it harder are called ‘hard-gainers’.

The snakes from Carnac Island are easy-gainers as they have a stronger genetic/intrinsic capability to change shape in response to the stimulus of large prey.

In any population in the animal kingdom some individuals have more plasticity than others. Those trapped on Carnac Island they are forced to survive by having seagull chicks as their food source. The more ‘plastic’ snakes that can develop the biggest bites are more likely to survive and pass down to their offspring their easy gainer genes.

In just a few generations, the whole population of tiger snakes would become easy-gainers in less than 100 years.

8. Why are the tiger snakes of Carnac Island blind?

Weirdly most of the snakes on Carnac Island are blind but they are not blind from birth!

Making the snakes even more unique is that the majority of male adult snakes have had their eyes pecked out by parent seagulls trying to protect their chicks from being their meal.

Strangely the snakes cope well even if they have lost their sight from these aerial attacks. They adapt by sticking their tongues out until they bumped into their prey!

Normal sighted snakes also take-out fast-moving prey like lizards and mice but the helpless seagull chicks are easy prey for the blind snakes.

The blind snakes continue to feed, grow, and reproduce successfully proving that the loss of vision has not reduced their body condition or rate of growth or even their survival. 

9. How do the Tiger snakes survive with very little water on the island?

The tiger snakes of Carnac Island are intriguing because of their ability to survive with almost no fresh water on the island.  Adapting their behaviour to survive in what essentially is a desert island with no running or fresh water source is helped by their ability to have grown larger jaws to capture these silver gull chicks.

With no human inhabitants and now predator’s native to the island the tiger snake population has thrived

10. What is the safest way to visit Carnac Island?

Due to this small 19 hectares having more than 400 tiger snakes, plus numerous juveniles the safest way to visit the island is to do a day trip on a boat.

Renowned for the sea lion population and numerous bird visitors are warned that if they do set foot on the island to stay on the beach and not to venture inland.

With an average of three tiger snakes per 25 square metres, if a sighting of one of them occurred clearly it is best to keep a safe distance from their deadly venom! Tiger snakes are mistakenly thought of to be aggressive, but they do not chase people and will try and flee is they spot someone.

Due to them changing in shape of their jaws, so that they could effortlessly change capture and eat whole seabird chicks is a reason why tiger snakes have been so successful colonising the island.