A Blog by Ed Yong

To Protect An Endangered Snake, First Protect A Venomous One

If you go for a walk through the rocky hills of Finland’s Aland archipelago, you might come across a medium-size snake with gunmetal grey scales and darker diamonds running down its back. It looks a little bit like an adder, but it can’t be because its head is thin and tapered…

Wait, did that snake just change its head?

You can’t be sure, but now its head is definitely flat and triangular—the defining shape of adders and other vipers. That means it’s venomous. There are people living nearby, with kids and pets. You decide to kill the snake with a rock. That was a poor decision, especially since the dead snake wasn’t an adder. It was a smooth snake—completely harmless, rather endangered, and now very slightly more endangered.

The smooth snake is found throughout Europe but its populations are small and thinly scattered. It could be easily wiped out, so the European Union has listed it as a specially protected species. In Finland, it’s classified as “vulnerable”, and may be bumped up by one degree of concern to “endangered”.

For much of its existence, the smooth snake protected itself by mimicking the far more dangerous adder, and its charade (especially its shape-shifting head) is good enough to fool even trained biologists. But this disguise is now the snake’s undoing. The fear of venomous snakes might compel birds to flee, but it sometimes compels humans to kill the potential threat.

This is doubly problematic for the smooth snake because its brand of mimicry (known as Batesian mimicry) only works if the noxious creature it mimics is plentiful. If an island contains a lot of adders, birds soon learn that attacking a long thing with a triangular head and a diamond back is a very bad idea. That’s good for the smooth snake, whose predators avoid it too. But if an island contains no adders, birds could attack the smooth snakes with impunity. Why wouldn’t they? They’re never come to associate those markings with possible death. Batesian mimics should always be in the minority if their copycat acts are to work.

Johanna Mappes from the University of Jyvaskyla in Finland showed this in 1997, by creating an artificial example of mimicry. She injected mealworm larvae with a foul-tasting liquid, and stuck small sugarballs (the ones used to decorate cakes) onto their heads—these were the models. She stuck the same balls onto other mealworms without the nasty liquid—these were the mimics. She then presented both groups to great tits in varying ratios. Mappes found that if the number of mimics equalled or exceeded that of the models, the benefits of their disguises disappeared.

This is bad news for the smooth snake. On Aland archipelago, they already outnumber adders. If Mappes is right, their defence should already be worthless. “For the successful conservation of smooth snakes in Aland, it seems crucial to also protect adders,” writes Mappes, along with colleague Janne Valkonen. “Our results provide foresight to prevent a potential disaster in a situation where a mimic becomes endangered due to the decreased frequency of its model species.”

This might apply to other species too. Many harmless snakes mimic venomous ones, and many snake populations are crashing all over the world. The smooth snake example suggests that protecting an endangered mimic is when an endangered species mimics a dangerous one, we might need to protect both to save the former—a one-for-the-price-of-two deal.

Reference: Valkonen & Mappes 2014. Resembling a Viper: Implications of Mimicry for Conservation of the Endangered Smooth Snake. Conservation Biology http://dx.doi.org/10.1111/cobi.12368

6 thoughts on “To Protect An Endangered Snake, First Protect A Venomous One

  1. Interesting article and conservation case study. But the mealworm experimental results need not apply to the Smooth Snake. The critical ratio of mimics to models should depend on the severity of the damage the model can do to the predator. Having a bad taste in your mouth is one thing, but nearly dying is quite another. I’d expect evolution to favor predators with a more cautious approach to venomous snake mimics than to mimics of bad-tasting bugs.

  2. But I’d think if the mimics outnumber the adders, many predators won’t have the experience with adders to learn avoidance from no matter how negative the occasional encounter with one is.

  3. Yes, but that assumes all behaviors are learned. If there is a genetic component, caution would evolve even if encounters with adders were rare (since these would be fatal). And I think there is a genetic fear of snakes in birds, chimps and maybe humans, if I recall correctly

  4. Interesting article. I agree though that the example of the worms doesnt seem to apply to the snakes. The avoidance to deadly snakes doesnt seem a behaviour you can learn whereas a new artificial stimulus will always have a learned answer.

  5. Is there any information on snake avoidance in birds being learned vs innate? I thought the same thing as Lou while reading this. There’s a huge difference between death and bad taste.

  6. There’s also the recently described case of a harmless coral-snake mimic whose mimicry became more accurate (in part of its range) after local extinction of the venomous model. That implies stronger selection on the mimic as the model gets rarer and disappears, consistent with what’s suggested is occurring with Vipera & Coronella. But another factor would be that predators with large ranges can encounter both model and mimic even if they no longer overlap at all.

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