Study discovers why poison dart frogs are toxic
Rhett Butler, mongabay.com
August 9, 2005
Poison poison dart frogs are small, colorful frogs found in the tropical forests of Central and South America. The brilliant coloration of these amphibians warns predators of their extraordinary toxicity — the golden poison frog (Phyllobates terribilis) of Colombia is said to be lethal if held in one’s hand.
Scientists have long speculated on the origin of their toxins, but now, a new study published in the current issue of The Proceedings of the National Academy of Sciences reports that poison dart frogs, as well as the Mantella poison frogs of Madagascar, derive their toxicity from the ants they eat. Specifically, both groups are frogs are capable of storing ants’ toxic alkaloid molecules in their glands without being harmed. Ants either synthesize these alkaloids themselves or acquire them from the plants on which they feed.
The conclusion that poison frogs’ toxic alkaloids are derived from ants explains why frogs reared in captivity lack the toxic defense of their counterparts in the wild. It also supports the observation that when poison frogs are introduced to non-native places like Hawaii their toxic alkaloids are different. Evidently, introduced frogs are feeding on different species of ants which have their own set of alkaloids.
Golden mantella (Mantella aurantiaca) on the Indian ocean island of Madagascar. Like poison arrow frogs of the New World, Mantella frogs of Madagascar are capable of storing ant poisons in glands along their back. Their bright coloration wards of predators who have learned to avoid these conspicuous amphibians. Madagascar is home to an estimated 300 species of frogs.
The alkaloids found in poison arrow frogs have applications beyond being a deterrent to frog-eaters. In the early 1990s, Abbott Labs began working with a frog toxin-derived compound for the treatment of pain. Abbott Labs eventually created ABT-594, a non-toxic, nonaddictive painkiller potentially effective for treating several types of pain. Unlike an opiate, ABT-594 “promotes alertness instead of sleepiness and has no side effects on respiration or digestion,” according to E.O. Wilson in his The Future of Life.
The creation of ABT-594 almost didn’t happen. The area of Ecuadorian rainforest from which the frog was originally collected in 1974 was cleared shortly thereafter for banana plantations. Luckily, a second collection site still housed the frogs and scientists were able to collect a sample of the poison which would later serve as the template for the painkiller.
This near miss with ABT-594 illustrates the importance of conserving biodiversity, especially in the tropics. Frog conservation is of particularly concern given the recent worldwide decline in amphibians species. The Global Amphibian Assessment, a survey of the planet’s amphibian species, found that nearly a third (32%) of the world’s amphibian species are threatened. No one has yet pinpointed the ultimate cause for the demise of amphibians — global climate change and the emergence of deadly chytrid fungal disease are often cited as prime culprits — but it is evident that we may be losing a lot more than just some colorful frog species.
Mites are the primary source of poison arrow frog toxins
Release from the Proceedings of the National Academy of Sciences: Convergent Evolution of Poison Frogs and Ants
A steady diet of ants may have driven the convergent evolution of
poisonous frogs in Madagascar and the Americas, researchers report.
Painted mantella (Mantella madagascariensis) in Madagascar.
Some frog species in both Madagascar and the Neotropics secrete a
variety of toxic skin chemicals, called alkaloids, for protection
against predation. These “poison frogs” do not produce the alkaloids,
however, but instead attain them from their insect-rich diet. While
Neotropic frogs are well-studied, the alkaloid sources for Malagasy
frogs are unknown. Valerie Clark and colleagues extracted alkaloid
samples from both Malagasy frogs and their food sources, which were
determined by examining the frogs’ stomach contents. The authors found
that Malagasy frogs, like their New World counterparts, acquire their
alkaloids from a diet rich in ants. Thirteen of the 16 Malagasy
alkaloids detected are also known to exist in insects and frogs in the
Americas. Neither the frogs nor the ants in these two regions are
closely related, which suggests that the evolution of acquisition
mechanisms for protective alkaloids in these ant species was likely
responsible for the subsequent convergent evolution of the frogs that
preyed on them. Additionally, the researchers found the well-known plant
alkaloid nicotine in one Malagasy frog species, suggesting a possible
plant-insect-frog toxin food chain.
Article #03502: “Convergent evolution of chemical defense in poison
frogs and arthropod prey between Madagascar and the Neotropics” by
Valerie C. Clark, Christopher J. Raxworthy, Valérie Rakotomalala, Petra
Sierwald, and Brian L. Fisher
MEDIA CONTACT: Valerie C. Clark, Cornell University, Ithaca, NY; tel:
212-864-1123; fax: 607-255-1227; e-mail: