From Willink et al, Evolution 2013.
From Willink et al, Evolution 2013.

Poison, Camouflage, and the Rainbow of Evolution

We have a habit of seeing nature in snapshots. We marvel at the adaptation of a species–see Ed Yong today on the maneuverability of cheetahs, for example–and don’t give much though about how it came to be. These snapshots can become downright confusing when we survey the diversity of many different species. Each species may have a radically different solution to the same problem. If one solution is so impressive, how could another one evolve, too?

The cure for this puzzlement is to get away from the snapshots. A species is a blurry, speckled thing. It’s made up of populations spread across a range, and each population is made up of many individuals, each with its own somewhat distinct set of genes. Those genes flow around the range, from individual to individual, mixed into new combinations, some spreading far and wide, some vanishing after a generation.

The picture I’ve reproduced above is a wonderful epitome of that blurriness. It shows frogs that live in Costa Rica at various spots along the Pacific Coast. Some frogs, like the green one at the northern end of the coast, develop colors that help them fade into surrounding vegetation. It’s an impressive way to hide from predators, matching one’s skin to the color of plants.

Other frogs, like the bright red one at the southern end of this picture, are not shy at all. They use brilliant colors, rather than masking ones, in order to ward of predators. These frogs produce poisons in their skin, which can sicken or kill an attacker. By developing a bright color that pops out from the surroundings, frogs can make it easy for birds and other predators to learn to link their appearance to a nasty experience. As a result, the predators stay away.

Same problem–two solutions. Which brings up the question, why would some frogs hide from predators while others tried to get their attention?

And here’s what’s so fascinating about this picture. Those frogs you see are all part of the same species, the granular poison frog, Oophaga granulifera.

In the journal Evolution, Beatriz Willink of the University of Costa Rica and her colleagues take a look at this species’s spectrum from camo green to strawberry red. The frogs don’t just vary in color. They also vary in the way they behave. The green frogs are shy compared to the red ones. Willink and her colleagues found that the green males sang less than half the time that the red male frogs did, for example. The green frogs also spent less time looking for food. In skin and brain alike, the green frogs survive by hiding, while the red frogs fend off death by showing off.

The frogs freely mate with their neighbors, which means that genes from one end of the species range can flow towards the other. And yet those flowing genes have not smeared the species into a single kind of frog. Across just 25 miles or so, they can span the distance from camouflage to conspicuousness.

As effective as those two strategies may be, Willink and her colleagues found that in between the extremes, the frogs were intermediate. Some were yellowish-green, while others were orange. But the intermediates didn’t smoothly grade from one end of the spectrum to the other. They had fascinating jumbles of traits.

There are two ways for frogs to contrast with their surroundings, for example: in brightness and in color. The intermediate frogs had a low brightness contrast, like the green frogs, but a high color contrast, like the red frogs. And instead of having an intermediate level of boldness, they were almost as bold as the red frogs, too.

All the frogs in this species have to cope with predators, and they’re evolving different ways to do so–not just through camouflage or conspicuousness, but in combinations of the two. Willink and her colleagues argue that the low brightness contrast of intermediate frogs is a good way to avoid notice from birds, because their brains seize on changing brightness contrast to notice prey. If that fails, and a bird comes closer to investigate, a frog can still boost its odds of surviving with high color contrast, which is how birds learn to stay away from poisonous prey.

For some reason scientists have yet to work out, the balance of strategies that works best is different at the northern and southern ends of the frog’s range. Ian Wang of the University of California, Davis, has found that the yellow and green populations of frogs evolved from the red one. In other words, they abandoned the showy colors of their ancestors. It’s possible that geography explains why they did so. Northern frogs may face a bigger threat from snakes, for example, which rely on smell instead of vision to recognize dangerous prey.

Wang also found that the green frogs are actually more toxic than the red ones. Rather than waste effort on showy red pigments, perhaps the northern frogs evolved to put their energies into making poison.

Whatever the answer turns out to be, this picture already makes one thing clear: there’s more than one way to be a granular poison frog.