Rare blue bullfrog spotted in California

Ever seen a blue-green bullfrog?

Image: Jim Wiegand

American bullfrogs usually come in earthy shades of green and brown, so the sight of a bright turquoise frog sitting on the bank of California's Shasta Lake took wildlife biologist and photographer Jim Wiegand quite by surprise.

"I was stunned," Wiegand told Record Searchlight. "I literally went 'Holy crap!'"

Wiegand spotted the frog earlier this month while boating across the lake and observing the local wildlife, and managed to snap some quick photos before the colourful amphibian hopped out of sight.

"These blue bullfrogs [are] incredibly rare," Wiegand told us. "And to my amazement, it is the second one I have come across in my years of exploration." His first encounter happened almost 50 years ago near the Letts Lake campground in California's Mendocino County. Back then, however, Wiegand didn't manage to get a photo.

"Most people do not even know [these frogs] exist," he added on Facebook.

Abnormal colouration in animals often comes down to problems with pigments. A creature with an overabundance of dark pigments will be melanistic, leading to strange sights like an all-black flamingo or a shadowy serval. A lack of pigmentation, on the other hand, is what leads to albinism, giving us stark-white examples of animals like sharks, gators and kangaroos.

In the case of the extremely rare blue lobsters, their unusual appearance is due to an excess of blue-colouring proteins. But this bullfrog's blue hue stems from quite a different issue: not enough yellow.

The condition is called axanthism, and it occurs when a critter is missing the pigments for yellow and orange colouration (sometimes red, too). In some animals, like certain snakes and other frogs, the pigment shortfall simply leaves them a dull grey or brown. But in some cases, like Wiegand's bullfrog, the lack of yellow in their skin lets their blue shine through.

Axanthism may be rarely seen, but it's pretty widespread in amphibians: it's been documented in over 20 species of frogs and salamanders. The condition is often pretty obvious by the animals' unusual colouration, but axanthism can also leave a critter totally normal-looking except for strangely black eyes.

So exactly how often does axanthism show up in frogs? Biologist Len Lindstrand III told Record Searchlight that Wiegand's blue-green bullfrog is "one in a million", but the number seems to vary quite a bit among different frogs. The 2008 book Malformed Frogs states that, overall, axanthism occurs in about one out of 30,000 frogs, similar to the frequency of albinism. Studies that have zoomed in on specific frog populations in Europe and South America, meanwhile, have found frequencies ranging from one in 1,000 to as high as one in 12!

For some reason, axanthism seems to be more common in frogs than other amphibians, and within the US, it's reportedly seen in the northern states most frequently.

The opposite of axanthism exists, too. It's called xanthochromism, and can result in frogs (and other animals) that are shockingly yellow.