Imagine that you’re deep in Costa Rican lowland rainforest, it’s raining by the bucketload per square metre - and it seems as if the ground is practically bleeding frogs. You stop by a particularly handsome species; let’s say it’s Agalychnis calliehyas, the gorgeous red-eyed tree frog. You gaze into its eyes and… what the? What you don’t see staring back at you are the round pupils you’re used to.
Turns out frogs can sport a wide variety of pupil shapes — from fans and triangles to horizontal and vertical slits, and sometimes all forms are present in the same taxonomic family.
Dr Rayna Bell, a herpetology researcher at the California Academy of Sciences, recently published a paper in the Biological Journal of the Linnean Society looking at why there’s so much diversity in just one animal type. Her conclusions are the scientific equivalent of a shrug emoji. “It definitely asks more questions than it answers, but I think it’s a good foundation, for now at least,” Bell says.
Given humans’ perfectly circular pupils, it might be a little hard to imagine the diversity of shapes we’re talking about here. In dark conditions — when the iris is relaxed or dilated — most frogs’ eyes look pretty circular. But when there’s light, the iris constricts in weird and wonderful ways.
For example, the common eastern froglet’s () pupils become like) looks more like a cat’s eye or diamond shape. Both of these frogs are Australian, are part of the same taxonomic family and live in similar environments. Why one evolved diamonds and one triangles is unknown.
