Panama and Madagascar home to similar killer = ants, frogs=20
They evolved 9,000 miles apart yet have 13 of same = poisons

David Perlman, Chronicle = Science=20 Editor

Monday, August 22, 2005

Printable=20 Version=20
Email=20 This Article

Beware the poison frogs of Madagascar and Panama!=20

Separated by nearly 9,000 miles across the globe and by = millions of=20 years of evolution, both are deadly -- if you're a predator.=20

And if you are, it's best not to sample the diets of those = hopping=20 denizens of the misty rain forests, for both frog tribes thrive on = poisonous ants, both have incorporated the ant toxins into their = own=20 bodies and both have developed a patchwork of brilliant skin = colors to=20 warn their enemies that they are dangerously inedible.=20

The vivid yellow, purple, orange and green frog bodies = virtually shout=20 a warning to would-be predators that they are killers defending=20 themselves.=20

Two biologists, a 25-year-old frog fancier from Cornell = University and=20 a 40-year-old ant fanatic at the California Academy of Sciences in = San=20 Francisco, are studying the two widely separated groups of frogs = and ants=20 to reveal a striking example of what science calls convergent = evolution --=20 the development over many millennia of similar body forms or = behaviors in=20 animals that are unrelated to each other and often widely = separated=20 geographically.=20

Unrelated now, such animals may in fact have descended from = common=20 ancestors millions upon millions of years ago, when continents and = islands=20 now separated were once part of the same land mass but eventually = split=20 away from each other and drifted to the regions where they stand = now.=20

Valerie C. Clark, a Cornell graduate student in biological = chemistry,=20 and Brian L. Fisher, chairman of the California Academy's = entomology=20 department, together with colleagues in Madagascar and the United = States,=20 published details of their unusual frog-and-ant studies last week = in the=20 Proceedings of the National Academy of Sciences.=20

In two months of frog collecting during the rainy season along = the=20 banks of streams in the tropical forest of Madagascar's Ranomafana = National Park, Clark focused on the poisonous skins of three = species of=20 colorful Mantella frogs, all less than 2 inches long, all native = to=20 Madagascar and none found anywhere else in the world.=20

Together, she and Fisher also deftly wielded specialized = entomological=20 forceps to rummage through the leaf litter of the forest floor and = gather=20 hundreds of native Madagascar ants, each no more than 5 = millimeters long=20 -- barely one-tenth the size of the tiny frogs -- and each filled = with=20 similar alkaloid poisons.=20

In the laboratory, Clark analyzed both the captured ants and = the=20 stomach contents of the frogs to determine which ant species = dominate the=20 frogs' diets. She then identified which of many poisonous = alkaloids the=20 frogs and insects sequester.=20

Another research team, led by Ralph Saporito of Florida = International=20 University and John W. Daly of the National Institutes of Health, = had=20 already discovered that the skin of brightly colored poison frogs = in=20 Panama hold the same toxic chemicals as the unrelated Madagascar=20 amphibians, and that those frogs, too, are hungry eaters of an ant = species=20 unrelated to those favored by the Madagascar frogs.=20

Comparing their findings with the Saporito team's report, Clark = and=20 Fisher and their colleagues found that, despite existing more than = a third=20 of the world apart, the frogs and ants shared 13 identical poison=20 compounds. Only three poison compounds found in the subjects were=20 different.=20

What was most striking to the researchers was that the = unrelated frogs=20 in both regions have evolved in nearly identical ways: Both not = only eat=20 virtually the same diets of ants containing the same toxic = alkaloids, but=20 they have also developed skins containing many of the same poisons = emblazoned with varieties of vivid skin color patterns warning any = predator that might be stupid enough to try eating one that they = are=20 indeed poisonous.=20

During Clark's field work, she made another interesting = discovery: The=20 skin of one species of Madagascar frog, Mantella baroni, contains = the=20 toxic alkaloid nicotine, which is known to be synthesized by many = plants,=20 including tobacco and the nightshade family of deadly poisonous = flowering=20 weeds.=20

Clark concedes it's just speculation, but says that the finding = suggests at least the possibility of a singular evolutionary = progression:=20 from a plant containing the poison to an ant that eats the plant = and=20 sequesters the poison to a frog that eats the ant and then = advertises=20 itself as poisonous and dangerous to any predator on the hunt for = tasty=20 cuisses de grenouilles -- frog's legs.=20

On the other hand, Fisher suggests that the original source of = the=20 toxic alkaloids in the ants' bodies might be the even tinier mites = on=20 which they feed.=20

It's a mystery that awaits curious researchers.=20

Whether or not the plants or the mites play a role in this = evolutionary=20 saga, the connection between poison ants and poison frogs is = clear, both=20 Clark and Fisher agree. To Clark, the lesson seems evident.=20

"The frogs have evolved ways to keep the poison down without = harming=20 themselves when they eat the ants," she said in a telephone = interview. "So=20 they've developed a way to protect themselves by their distinctive = coloration. And in a region where selective pressures are high, = they've=20 opened up a totally different niche for themselves. Instead of = hiding from=20 predators or hunting their own prey in the dark of night the way = many=20 other frogs do, there in Madagascar they can just hop around in = the=20 daylight, knowing that predators have learned to stay away."=20

As for Fisher, who is now seeking to create a new center for=20 biodiversity research in Madagascar, this striking observation of = parallel=20 developments in unrelated species of frogs and ants reveals "an = obvious=20 example of natural selection."=20

"Ants have been around for 120 million years, and here's a = fascinating=20 instance of biological and chemical evolution," he said during an=20 interview in his Howard Street laboratory. "The frogs in both = regions have=20 faced competition in their dangerous world, and they've evolved a = way to=20 defend themselves by sequestering toxic chemicals from the ants, = and then=20 letting predators know it. It's a beautiful story."=20

Aiding the research were Valerie Rakotomalalaof the University = of=20 Antananarivo in Madagascar, Christopher Raxworthy of the American = Museum=20 of Natural History in New York, and Petra Sierwald of Chicago's = Field=20 Museum of Natural History.=20

Similar examples of convergent evolution are widespread = throughout the=20 animal world.=20

The giant pangolin of Africa and the giant anteater of South = America,=20 for instance, have developed elongated snouts to suck up entire = ant=20 colonies in a single gulp; they resemble each other, and if both = had a=20 common ancestor at all, it must have lived millions of years ago, = when=20 Africa and the Americas were joined together before continental = drift=20 moved them an ocean apart.=20

Or take Africa's serval cat and South America's maned wolf, who = is not=20 a wolf at all, but a member of the dog tribe. They have uncannily = similar=20 builds, with tremendously long legs for their slender bodies and = large=20 upright ears for acute hearing; neither runs particularly fast, = but both=20 can leap and pounce on their similar rodent prey to impale a meal = with=20 their long, sharp teeth.=20

E-mail David Perlman at dperlman@sfchronicle.com.=20

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Panama and Madagascar home to similar killer = ants, frogs=20 They evolved 9,000 miles apart yet have 13 of same = poisons

Panama and Madagascar home to similar killer = ants, frogs=20
They evolved 9,000 miles apart yet have 13 of same = poisons