Looking at unrelated animals with similar eye colors is one way to test the idea that eye darkness and behavior are correlated. I can do unsystematic (eyeball) tests by looking at my database of eye colors for 5620 species of land vertebrates. Of course that is only a sample of all land vertebrates and not all taxonomic classes and orders are represented equally. However, the sample is more than adequate to generate meaningful observations or viable hypotheses.

    At this lightest-eyed end of of the distribution, all four classes of land vertebrates--amphibians, reptiles, mammals and birds--are represented. By contrast, at the darkest-eyed end of the distribution and at the midrange one does not find large families  of amphibians or reptiles.

    Given the taxonomic diversity of the eight families, it is noteworthy that all these animals are carnivorous predators and all hunt by means of ambush. All have anatomies that can be “spring-loaded” to result in a sudden, explosive strike or pounce toward the prey. Frogs use a coiled tongue; herons use a long neck folded back into an S shape.

    In my view, the behavioral similarity of these families is far beyond what one would expect by chance.