From butterflies to birds, many harmless animals have evolved to look like their more deadly counterparts. But when a researcher in Thailand picked up what he thought was a non-venomous mock viper, he got a bit of a shock and dropped it when it appeared to turn into its more dangerous opposite number. The discovery was published in Frontiers in Ecology and the Environment.
The common mock viper (Psammodynastes pulverulentus) is well known for looking superficially like the venomous Malayan pit viper (Calloselasma rhodostoma), apart from the tell-tale give away in the different shapes of its pupils. Yet when Colin Strine, from the Suranaree University of Technology, picked one up whilst on an expedition, the eyes of the mock viper physically changed shape to resemble those of the viper, something which has never been observed before.
It is usually believed that venomous snakes tend to have vertical pupils, while their non-venomous counterparts tend to favor rounded ones, although this fails to take into account cobras, mambas and taipans, which all have rounded pupils. The reasons behind the differences are not entirely understood, but one analysis of 127 Australian snake species found that the shape of the pupil tended to be associated more with the hunting behavior of the species, rather than the time of day at which they are active as has traditionally been thought.
They found that there must be some strong driver for the difference between the eyes, as vertical and rounded pupils evolved multiple times in different lineages of snakes. By looking at the hunting behavior and time at which the animals were active, they found those species of snake that ambush their prey at night were more likely to have vertical pupils, while the hunters which actively chased their prey during the day tended to have rounded pupils. They argue that a slim vertical pupil may make the image of prey in the distance sharper, allowing the predator to sit and wait, but with the cost of letting less light in, reducing their night vision.
As sit-and-wait ambush predators, all vipers have vertical pupils. And so the ability of the mock viper, which tends to have rounded pupils, to alter them to look like the deadly vipers shocked the researchers. Whether or not the snake has evolved to do this specifically to increase their mimicry and confuse potential predators, however, is still not understood. It may be, for example, that the shift improves the snake’s vision during attacks, increasing the accuracy of their defensive strike.