Monday, August 19, 2013

The bullfrog blues

Way back in 1969, folk singers Peter, Paul, and Mary recorded a song called "I'm in Love with a Big Blue Frog." It was a pretty silly song (with an important social message as well), as you can see from this verse:  

Well I'm not worried about our kids, 
 I know they'll turn out neat. 
They'll be great lookin' 'cause they'll have my face, 
great swimmers 'cause they'll have his feet!

To a kid who had already spent much of his life in the company of frogs, it seemed even sillier, for reasons that should be obvious: Falling in love with a frog was one thing, but a big blue frog? I mean, c'mon. Everybody knows frogs come in green or brown, with maybe a touch of red or yellow on the legs and belly.

Later I learned about Dendrobatidae, a family of tropical frogs with skin secretions so toxic (in some species, at least) that Amazonian Indians used them to poison the tips of their arrows. In an adaptation known as aposematism, or warning coloration, these frogs have evolved bright colors, making them easy to see and giving potential predators a clear message that they are not to be messed with. They come in a variety of colors – reds, oranges, yellows, and, yes, even blue. But Dendrobatids are tiny, with most species less than an inch long. Maybe the song should be called "I'm in Love with a Little Blue Frog."

You can imagine my surprise, all these decades later, when I found myself face to face with a big blue frog.

(Note to my many friends who are photographers, printers, or graphic designers: Yeah, I know – the frog's not blue, it's cyan. Get over it. Of all the people who have seen these photos, or who saw the actual frog, I didn't hear even one of them exclaim, "Wow! Look at that cyan frog!")

Now, if you're thinking this encounter happened in some exotic, unexplored wilderness, you're wrong. It was right here in Northern California, and the frog in question was none other than Rana catesbeiana, the common American bullfrog. Commonly green, that is.

(OK, now a note to my friends who are biologists, copy editors, or just language geeks: Here in California, bullfrogs really are exotic – meaning they're not native. But calling such a common species exotic just sounds wrong to most people.)

So what's going on here? Why, in a pond with hundreds of ordinary green bullfrogs, would there be one that seems normal in every way except for its striking and improbable color?

The short, simple answer is that it's a rare mutation. How rare? I really don't know. I found one article that called it "one in a million," but that phrase is so overused that I just take it to mean "very rare," without any specificity. I do know that I've seen thousands of bullfrogs in my life, and this is the first blue one I've come across.

The details – how a frog gets its color – are surprisingly complicated. Frog skin has three layers of pigment cells, collectively called chromatophores. The deepest layer, the melanophores, contain melanin, which gives a frog its black or dark brown pattern and can make the overall color lighter or darker. At the surface are the xanthophores, which contain yellow pigment. In between are the iridophores, which don't really have any pigment at all – they contain mirror-like crystals that reflect blue light.

When light hits a frog's skin, it passes through the xanthophores to the iridophores, which reflect blue light back up through the xanthophores – having the effect of adding a yellow filter to the blue light. If you remember finger-painting in kindergarten, you know that mixing blue and yellow will make green, and that's more or less what's happening in a normal frog. Without the xanthophores' yellow filter, we would see the blue reflected by the iridophores.

In a blue frog, the xanthophores are missing – or maybe they're not. Scientists Michael W. Berns and K. Shankar Narayan analyzed the skin of normal and blue frogs in great detail, but their results were inconclusive. The blue frogs' xanthophores might never have developed properly, or they might have atrophied for another reason, or they could have been replaced by another type of cell. In spite of the obvious importance of this line of research, the question remains unanswered.

And it gets even more complicated. Biologists have always assumed that the blue mutation was genetic, but when researchers tried breeding blue and normal adults in various combinations, all they got was a whole bunch of little green frogs. That doesn't rule out the possibility of a genetic origin, but, as Berns and Narayan put it, "it appears that expression of the characteristic either does not follow simple Mendelian lines, or is greatly influenced by environmental factors (or both)."

Whatever the explanation, it all adds up to one very special frog. I think I might be in love.

Song lyrics copyright © Leslie Braunstein

Thanks to Gary Fellers for finding the scientific publications. 

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