Last month, Indiana-resident Kathy Spiegel was snapping photos of bullfrogs at a local pond in her hometown when a flash of turquoise caught her eye. On closer inspection, she discovered an all-teal frog – seemingly oblivious to its striking colouration – hiding amongst the tall grasses lining the pond.

Blue is the new green. Image © Kathy Spiegel
Image © Kathy Spiegel

American bullfrogs range in colour from dark green to earthy brown, so it’s not surprising that this cyan hopper captured Spiegal’s attention. The blue frog owes its unique colouration to a genetic condition called axanthism.

Although it’s rarely witnessed, axanthism is relatively widespread in amphibians and has been recorded in at least 20 species of salamanders and frogs. In some cases, the condition may be difficult to detect as it can leave frogs looking pretty normal, aside from black eyes.

Pigmentation problems are usually the cause of abnormal colouration in animals. Pitch-black servals or dark-coloured leopards owe their inky coats to an overproduction of a pigment called melanin, while a lack of pigmentation results in albinism giving us all-white examples of animals like kangaroos, sharks and alligators.

Interestingly, in the case of blue bullfrogs, its not an overproduction of blue pigment that gives rise to their bold colour, but rather a lack of yellow.

“Colouration in most frogs develops from a combination of a skin-derived underlayer called structural colour, which is blue, and colour that originates embryonically along the spine from neural crest [a structure that gives rise to pigment cells] ,” Dr Michael Lannoo, a Professor at the Department of Anatomy and Cell Biology at Indiana University, explained to us via email. “Neural crest produces xanthophores, which contain yellow pigment.”

In a normally coloured frog, yellow pigments are spread out across the body and when combined with the underlaying blue structural pigments you get a green frog. But in amphibians afflicted with axanthism, formation or migration of the neural crest is absent which results in a lack of yellow pigments. Without yellow to merge with the blue, axanthic frogs appear a striking shade of teal.

Image © Kathy Spiegel

According to Dr Lannoo, the mutation affects one in every 30,000 frogs, and within the US, is reported most frequently in the northern states.

Spiegel has been checking in regularly with her blue discovery and is concerned that its distinctive colouration may make it more vulnerable to predators (or humans eager to turn the frog into a pet). “I'm so afraid, as we have a multitude of red-shouldered hawks and Cooper’s hawks in our area, that he will be a meal one day,” Spiegel told us via email. Daytime predators like great blue herons may pose more of a threat to blue bullfrogs than nocturnal hunters, says Lannoo. Aerial predators also have the upper hand (or talon) as the frogs' bright blue colour is likely easier to spot from above, while risk of attack from underwater prowlers like snapping turtles is about the same as it would be for normal-coloured bullfrogs.

Fortunately for this blue rarity, someone is watching its back: “I am keeping tabs on the blue bullfrog," Spiegal says. "Thus far, he has managed to avoid getting eaten by a hawk or other predator … I will continue to seek him out daily and make sure that he is OK.”

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Header image: ©Kathy Spiegel