Among frogs and New Yorkers alike, those wearing loud colors are = assumed to=20 have very poor taste. As researchers have long observed, the brightest = frog=20 species in nature are often the most poisonous, and for good reason. =
Why else would a creature coveted by everything from snakes and birds = to=20 Jacques P=E9pin evolve an extravagantly colored skin, except to warn any = would-be=20 predators of bitter toxins embedded therein?
Now it turns out that it is no mean feat for a frog to earn its mean = feet,=20 and that one of the surest routes to optimal toxicity is through a = highly=20 specialized form of ant eating.
Writing in the current issue of The Proceedings of the National = Academy of=20 Sciences, researchers report that poison frogs in Africa and the = Neotropics of=20 Central and South America appear to have converged on the same difficult = method=20 of harvesting the toxic chemicals they need to defend themselves against = predators.
Both the famed poison-dart frogs of the New World and the Mantella = poison=20 frogs of Madagascar dine largely though not exclusively on ants, and = many of=20 those ants, the researchers have determined, contain toxic chemicals = called=20 alkaloids.
Through entirely independent pathways, it seems, the two unrelated = groups of=20 frogs evolved a similar capacity to store, or sequester, the ingested = alkaloids=20 in their skin sacs without being harmed by the pungent substances = themselves.=20 And once the unrelated amphibian clans had succeeded in caching the ant = bane in=20 their glands, they autonomously evolved bright coloration to broadcast = to=20 potential frog-eaters their possession of distasteful alkaloids.
The evolution of chemical protection and concomitant advertising gave = the=20 poison frogs a considerable leg up on the competition. Whereas most = frogs dare=20 only emerge at night to feed and will skitter for cover at the slightest = breeze,=20 a majority of poison frogs are diurnal, brazenly hopping around the = sunlit=20 forest floor like scattered gems on pogo sticks.
The new work offers a striking example of convergent evolution, of = different=20 species confronting a similar challenge or opportunity and taking such a = similar=20 stepwise approach to the task that, despite their distinct genetic = backgrounds,=20 they end up looking like close kin.
More significant still, the scientists said, the convergence in this = case did=20 not begin with the frogs. The ants, too, in both Africa and the = Neotropics, seem=20 to have independently evolved their possession of alkaloid chemicals, = which they=20 either synthesize directly, or somehow wrest from plants they eat.
Most of the 4,500 known ant species are not thought to traffic in = alkaloids,=20 and the scientists are still unsure whether the endowed insects under = study use=20 the chemicals for defense, communication or something else.
Whatever the incentives, said Valerie C. Clark, a graduate student in = chemistry at Cornell and the lead author on the new report, the = autonomous=20 advance in antly alkaloid management on each continent very likely = served as the=20 requisite precursor to the emergence of the far-flung families of = flamboyant=20 amphibians.
"Without the presence of the alkaloid ants," she said, "that nice = little=20 evolutionary niche of becoming diurnal and colorful would very likely = never have=20 opened up for the frogs."
The power of convergent evolution has fascinated naturalists from = Charles=20 Darwin onward, and it helps explain the appearance of the many aesthetic = and=20 functional d=E9j=E0-vus that abound throughout nature: the sleekly = hydrodynamic=20 silhouettes of sharks and dolphins, the spindly wings of bats, birds and = pterosaurs.
As evolutionary biologists see it, the underlying principle of = evolutionary=20 convergence - that often there is one right tool for the job, and that = selective=20 pressures will reinvent the bio-utensil whenever the need arises - = exemplifies=20 just how non-random and ostensibly purposeful natural selection can be, = and how=20 readily it may be mistaken for evidence of supernatural "design."
In the case of poison frogs, specific palettes and patterns seem to = be so=20 useful for warning off predators that they pop up again and again. On = both=20 continents can be found frogs of pure bold Velveeta gold, frogs with = glaring=20 spots of red on black.
"Sometimes the resemblance really is striking," said Christopher J.=20 Raxworthy, a curator of herpetology at the American Museum of Natural = History=20 and an author on the report. "You'll see two unrelated species of frog, = one=20 Madagascan, one Neotropical, and they'll be almost the same size and = shape,=20 they'll move in a similar style, and they'll have the same vivid = markings in=20 virtually the same place on the body."
The frogs range in their relative toxicity from the merely acrid to = the aptly=20 named Phylobates terribilis, which packs enough venom in its skin to = kill at=20 least a dozen people, and which the Indians of western Colombia use to = poison=20 the serrated tips of their blow darts. And then, there are the = inevitable=20 mimics: frogs that have evolved the lurid packaging of their poisonous = peers,=20 but without the means to sequester alkaloids.
Sure, the copycats may have no ants in their glands, but what wise = hunter=20 will take that chance?
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