Frog and toad pupils mainly come in seven different shapes

Frog and toad pupils come in quite the array, from slits to circles. But overall, there are seven main shapes of these animals’ peepholes, researchers report in the Aug. 25 Proceedings of the Royal Society B.

Eyes are “among the most charismatic features of frogs and toads,” says herpetologist Julián Faivovich of the Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” in Buenos Aires. People have long marveled at the animals’ many iris colors and pupil shapes. Yet “there’s almost nothing known about the anatomical basis of that diversity.”

Faivovich and colleagues catalogued pupil shapes from photos of 3,261 species, representing 44 percent of known frogs and toads. The team identified seven main shapes: vertical slits, horizontal slits, diamonds, circles, triangles, fans and inverted fans. The most common shape, horizontal slits, appeared in 78 percent of studied species.

Mapping pupil shapes onto a tree of evolutionary relationships allowed the scientists to infer how these seven shapes emerged. Though uncommon in other vertebrates, horizontal pupils seem to have given rise to most of the other shapes in frogs and toads. All together, these seven shapes have evolved at least 116 times, the researchers say.

Pupil shape affects the amount of light that reaches the retina and its light-receiving cells, says Nadia Cervino, a herpetologist also at the Argentine museum. But how the shape influences what animals actually see isn’t well-known.

Pupil shapes generally didn’t correspond with animals’ lifestyles and habitats. The scientists plan to continue investigating what drives pupil evolution in tree frogs, a smaller group with fewer types of pupil shapes. And the team will consider other lifestyle factors, including how high frogs climb or whether they lay eggs in water, as well as other eye characteristics, such as iris color, to see if those factors matter to pupil shape.

With a biodiversity crisis caused by a skin fungus that has led to many presumed extinctions of frogs and toads worldwide, researchers may be losing clues to this mystery (SN: 3/28/19). “When we are losing species,” Faivovich says, “we’re actually losing the opportunity of learning a lot about them.”