Aussie island snakes evolved the ability to grow their jaw in mere decades

A population of tiger snakes on an island in Western Australia are able to increase their skull bones to enable them to eat large prey. These findings are surprising because mainland tiger snakes less than 30 kilometres away have no such ability.

“Much like an athlete’s body changes shape with intense training, the [Carnac] Island tiger snake’s skull adapts to the food it needs to survive,” says Adelaide University palaeontologist Dr Alessandro Palci.

“Island tiger snakes start showing shape changes in their skull bones only after prolonged feeding on large prey.”

On Carnac Island, the snakes (Notechis scutatus) eat chicks of seabirds, which are quite large. Tiger snakes on the mainland mostly eat much smaller prey like frogs.

This adaptation to be able to eat them when required is called ‘diet-induced phenotypic plasticity’. It’s important to note that increasing the bones in their jaw isn’t evolution – this change is occurring over the snake’s lifetime.

However, the ability to undertake this phenotypic plasticity at all is something that has evolved in this island population — the snakes offspring share the ability.

But this plasticity didn’t take millions of years to occur – it took less than 100. Snakes were only introduced to Carnac Island around 100 years before, potentially dumped by a snake performer to avoid trouble with the law.

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A 3D model image of a tiger snake skull showing the bones that increase in length in the snakes from Carnac Island after being fed large prey for a prolonged period of time: lower jaws (red) and palate bones (yellow). Credit: Alessandro Palci

This has been looked at before in some detail, but previous studies only looked at external head dimensions. With 3D model imaging the researchers were able to further reveal which skull bones were responding to the drastic dietary change.

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The team took 15 baby snakes whose mums were from Carnac Island and 31 baby snakes whose mums were from Herdsman Lake nearby on the mainland. Half of each were given only small mice, and half were given only large mice.

After a year the snakes were analysed to see how their jaws had held up.  

“The island forms showed diet-induced plasticity within … the mandible and palatopterygoid, which grew longer when the snakes were fed larger prey,” the researchers write in their new paper.

“Importantly, skull plasticity was expressed only after prolonged dietary stress, and was not clearly observable until the snakes approached adulthood.”

The research has been published in Evolutionary Biology.

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