The eyes of frogs and toads have been dubbed one of their "most striking morphological characteristics." Take the red-eyed treefrog, so-named for its brilliant flame-hued eyes, and the yellow-bellied toad, which has appeared across social media because of its uniquely enduring "heart-shaped pupils."
Hey uhh did you know that some frogs have heart shaped pupils? pic.twitter.com/6PgYlm4JYH
— Louise! (@themouseyouknow) April 16, 2020
Rayna Bell, a herpetology expert and curator at the California Academy of Sciences, told Snopes it's true that some frogs have heart-shaped pupils, adding that the photo in question appears to be a species of fire-bellied toad (genus Bombina).
"It's still unclear what the potential benefits [or] purposes of different pupil shapes might be," Bell told Snopes, referring our newsroom to a recently published overview of pupil shape diversity in amphibians, which was published in the Biological Journal of the Linnean Society in November 2022.
As part of the 2022 research, Bell and her colleagues surveyed pupil shape across nearly 1,300 amphibian species to "explore the anatomical, physiological, optical and ecological mechanisms underlying the evolution of pupil shape." Of all the amphibians analyzed, anurans — the order comprising frogs, toads and treefrogs – exhibited the greatest diversity in pupil shape and size.
The study built on 2021 research published in the journal Proceedings of the Royal Society, which surveyed the pupil diversity of more than 3,200 species to determine how their habits and habitats may influence, or be influenced by, their pupil shape. Seven main pupil types were identified, yet the only correspondence with lifestyle changes had to do with whether the anuran species was aquatic or terrestrial.
(Proceedings of the Royal Society)
Species whose pupils were mostly circular were those that burrowed, also known as fossorial, or who lived in aquatic environments. This, the researchers speculated, is likely because these environments see less variation in light levels than terrestrial landscapes influenced more directly by the sun.
"Animals that operate in a wide range of light levels, either because they are active both at night-time and during the day or because they move between aquatic and terrestrial environments, tend to have large pupillary ranges," Bell and colleagues wrote.
Elongated pupils, for example, can constrict more than those that are circular in shape. Generally, the pupil is circular when dilated in scotopic (low-light) conditions to allow for more light. Pupils can also take on a variety of shapes when contracted in photopic, or well-lit, conditions, including horizontal, erect or vertically elliptical, round, rhomboidal, triangular and, yes, even heart-shaped.
Frogs and toads are diverse in their lifestyles and reproduction habits, so understanding how their pupil shape is related to their environment may help provide more information about their evolutionary history and modern lifecycles. All said, the researchers agreed in 2022 that their work provides opportunities for future research into and understanding of anurans.
"Amphibians provide an exciting group for future research exploring the anatomical, physiological, optical and ecological mechanisms underlying the evolution of pupil diversity," they concluded.