Of toads and snakes: Evolution at work

A recent article in the Proceedings of the National Academy of Sciences caught my attention. A research group has investigated the snake population of two Japanese islands -- namely Ishima and Kinkazan -- and has made some interesting findings about their behaviour with respect to toads.

Those snakes, a species of keelback snakes called Rhabdophis tigrinus (Tiger grooved-neck keelback, or simply Tiger keelback), exhibit a different reaction when attacked. Whereas the snakes on Kinkazan flee, those on Ishima stand their ground and fight, relying on their poison. However, this particular species of snake is unable to synthesise its own venom. But R. tigrinus found on Ishima have been demonstrated to harbour bufadienolide compounds (toad toxins) in their neck (nuchal) glands.

The investigation of the origin of the toxins indicated that they were obtained from the snakes' diet. Whereas most animals, including snakes, avoid eating toads because of the toxins contained in their skin, R. tigrinus on Ishima island not only survive the ingestion of those toxins, but also store them for their own defense. This was confirmed by feeding the snakes a toad-free or toad rich diet and measuring the amount of bufadienolide contained in their nuchal glands.

The research team also showed that baby snakes benefit these toxins too. Snake mothers with high toxin levels were shown to pass on the compounds to their offspring, thus allowing them to enjoy the toad-derived protection.

So snakes of the same species living on different islands exhibit different behaviours that confer them a different survival advantage. Whereas the snakes on Kinkazan will keep struggling and fleeing, those on Ishima will thrive on a toad diet that will allow them to face predators more efficiently. Like Darwin's account of finches' beaks on different islands, this observation confirms once again the (so-called) theory of evolution.