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For Poison-Dart Frogs, Markings Matter When It Comes to Survival


The fanciful colored markings of poison dart frogs are a warning to predators: If you eat me, you’ll regret it.

These tiny, colorful creatures secrete bitter toxins in their skin, and birds have come to associate their distinctive markings with danger. The frogs’ chemical defenses can cause swelling, paralysis and sometimes even death. Their markings are so distinctive that it seems any frog trying out a new look would be running a serious risk.

And yet, new markings do crop up. Dyeing poison dart frogs in one part of French Guiana usually are blue and black with yellow markings. But in the nearby Mont Grand Matoury nature preserve, they have white stripes. Scientists curious about how this alternative coloration was working out ran a series of experiments, and reported some surprising results last week in Proceedings of the National Academy of Science.

The white-striped frogs were not as effective at scaring off predators as their yellow brethren, they found. But they still managed to avoid being outcompeted by the fitter, more threatening yellow-striped frogs, perhaps in part because of their location.

The researchers began by setting out more than 2,000 clay models of frogs — some white-striped, some yellow-striped and some that were solid-colored — in both the Matoury nature preserve and in the Kaw Mountains, about 30 miles away, where a population of yellow-striped frogs lives.

When they collected the models later, they looked for gouges and scrapes that indicated a bird attack. They expected that birds in the Matoury preserve would avoid white-striped frogs while birds in the Kaw Mountains would steer clear of the ones with yellow stripes.

They were surprised to find that this was not the case. In Matoury, the white-striped frogs were attacked most, while in the Kaw Mountains frogs of all patterns were attacked about equally.

“This had us scratching our heads,” said J.P. Lawrence, a postdoctoral researcher at the University of California, Irvine, and an author of the new paper.

Hoping to get more insight into the results, the researchers trained chicks in the lab to associate images of either white-striped or yellow-striped frogs with bitter, unpalatable mealworms. They found that the chicks came to dislike yellow-striped frogs much more quickly than white-striped frogs. Once they had learned to be skeptical of yellow frogs, the birds were more cautious about any new color.

That fit with the findings from the forest, Dr. Lawrence said. In the Matoury preserve, white-striped frogs were attacked more because birds had difficulty learning to associate white with a negative experience. In the Kaw Mountains, however, where birds had already learned to avoid yellow-striped frogs, they were equally skeptical of the newcomers with white stripes. Indeed, past research had shown that birds respond most strongly to warm colors like yellow, orange and red; white just doesn’t make the same impression.

If the white-striped frogs were failing to scare off predators with their colors, were they deadlier or at least more distasteful when caught?

Using a test pioneered by Bibiana Rojas, a researcher at the University of Jyväskylä in Finland and an author of the new paper, the researchers mixed oats with extracts of the frogs’ skin and fed them to chickens. They found that the white-striped frogs were less noxious than the yellow ones. In nearly every way that matters when it comes to surviving the ravages of natural selection, the white-striped frogs appeared to be failing.

Yet they have a healthy, lively population. The researchers believe that at least two factors are in play: The two populations of frogs appear to have no contact with each other, judging from limited genetic data. If they lived together, the white frogs would likely be outcompeted. But because they do not, the fact that the yellow-striped frogs are more successful has no effect on the survival of the white-striped frogs. As long as their gene pools do not mix, even a less-fit version can survive.

The other factor is a reminder that appearance isn’t everything. While the team was out collecting frogs in the forest, they noticed a key difference.

“When you come across one of these yellow-striped frogs, they’re just right out in the middle of the forest,” Dr. Lawrence said. “They don’t really care at all that you’re there. The white-striped frogs, they’re much more secretive, much more skittish. You often have to dive into a burrow to catch one.”

By altering their behavior, the white-striped frogs may have increased their chances of success. Even in the cutthroat natural world, where only the strong are said to survive, a counterintuitive new innovation can persist, given the right circumstances.


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