The yellow-bellied toad is sure to win the hearts of even the most hesitant of anuran appreciators, being an adorable little amphibian with heart-shaped pupils. Exactly what the pupil shape means in terms of the toad’s vision isn’t known for certain, but the staggering diversity of anuran pupil shape has taught us that previously held associations of pupil shape with lifestyle might not apply to amphibians.

Yellow-bellied toads, Bombina sp., are well suited to muddy life, being sediment-brown on top with bright – you guessed it – yellow bellies. They hibernate for a long period over the winter and then emerge in spring for the mating season. 

As they go in search of love, their vision is guided by heart-shaped pupils that are just one of many strange pupil shapes seen in anurans, which is the group of amphibians that encompasses frogs and toads (some of the rarest of which can be seen at ZSL London Zoo’s new Secret Life Of Reptiles And Amphibians house).

A study decided to hop into the strange world of anuran pupil shape by looking at images of 3,200 species. Doing so revealed that the pupil in anurans is a highly plastic structure that’s evolved seven main shapes during the history of the group. They did not find a significant correlation between pupil shape and the animals’ habits, except that aquatic anurans appeared to more often have circular pupils.

“Some authors tested the association of pupil shape and diel activity for some amniote clades,” wrote N. Cervino et al. “They showed a significant correlation between pupil shape and ecological niche (diel activity, foraging mode), where herbivorous (prey) mammals are likely to have horizontal pupils, diurnal predators, circular pupils and nocturnal and arrhythmic ambush predators, vertical slit pupils. However, our results suggest that in anurans, these inferences do not hold, indicating the need to explore other potential drivers of pupil evolution.”

One of those authors was principal investigator Marty Banks of the Banks’ Lab, UC Berkeley’s visual space perception laboratory, who told IFLScience that there was a curious outlier in the animal pupil shapes they studied.

“There’s one very clear exception for which we have no explanation: the mongoose,” said Banks to IFLScience. “It’s a small carnivorous mammal that has eyes on the front of its head. So, you’d expect them to have either a vertical slit pupil or a round pupil, but they have horizontal pupils. That animal does not fit our account at all.”

Heart-shaped or horizontal, it’s clear there’s more to the pupil than meets the eye.

[H/T: Snopes]