Snake eyes

To many, “snake eyes” is a bad bet at the craps table. To some, it’s a GI Joe villain character. To a very small, select group, it’s a minor addition to the oeuvre of Brian De Palma. ^*

Today, I want to look at the most literal meaning of the term imaginable. But, since this is a biology blog, you could probably guess that I was going to end up talking about the eyes of snakes.

I’m willing to bet that when most people visualize snake eyes, they think of something with a vertical slit for a pupil, like the animal shown here. Such eye look a little scary to humans, because we have circular pupils. Snakes aren’t the only animals to have such pupils; many cats do, too. Why do some animals have vertical pupils, and some have circular pupils?

Brischoux and colleagues set out right away to correct one idea that many people have about vertical eyes that is wrong: it’s not for night vision. They set out to test other hypotheses about the function of pupil shape using snakes as their subjects. One reason for using snakes is that the pupils vary considerably, so compare the animal above to the one below (click to enlarge).

Admit it: it looks friendlier with circular pupils, doesn’t it?

The pupil shape appears to be related to day and night, but in a different way: You can close a slit pupil much tighter than a circular pupil. The yellow snake above has its pupils shut down quite far. This might be advantageous to an animal that normally moves around at night to prevent it from being temporarily blinded by bright light in the day.

The authors also suggest that a vertical pupil may be less recognizable then a round one, making it advantageous for ambush predators who use camouflage. The authors don’t mention that many prey species also use camouflage, and following that logic, might also be expected to have slit pupils.

Brischoux and colleagues coded out about 100 snake species, categorising their pupil shape, type of foraging, and time when the animals were most activity. They found that most snakes fell into a fairly small number of positions in the possible range of values: sit and wait ambush predators that hunted at night tended to have vertical pupils; snakes that actively foraged during the say tended to have circular pupils.

What I don’t get about their figure above, though, is why the data points are scattered. Look at the trio of dots on the lower right corner. Why are they separated? If these are categorical values, all three should be right on top of each other. That implies that they’ve measured something quantitatively, not by category... and if so, where are the value labels for the axes?

Having done all that, the authors admit they still can’t definitively tie pupil shape to any particular adaptive advantage. They suggest some good experiments, such as manipulating pupil shape by retouching pictures to see if this affects how readily detectable an potential predator is. They also point out that many other species vary in their pupil shape, and that similar analyses could be done for those groups, too.

Reference

Brischoux F, Pizzatto L, & Shine R (2010). Insights into the adaptive significance of vertical pupil shape in snakes Journal of Evolutionary Biology DOI: 10.1111/j.1420-9101.2010.02046.x

Yellow snake photo by MrClean1982 on Flickr. Garter snake photo by C. A. Mullhaupt on Flickr. Both used under a Creative Commons license.


^* The film has a special place in my heart, because it was filmed in Montréal when I was living there, and I was able to look in and see the set they had built in the old Montréal Forum when I walked to and from McGill University.