Yagua (Yahua) Blowpipe. Iquitos. Peru - © Jialiang Gao
The long held belief that ants were the primary source of the defensive skin alkaloids in frogs has recently been found to be wrong. It now appears that mites and not ants are the primary source of these skin toxins. These skin secretions are so toxic that they are used to poison the tips of hunting blow pipe darts and dart frogs are one of several groups of animals capable of sequestering deadly compounds from dietary sources without being harmed.
Screening ants, mites, and other arthropods collected from the native habitat of the Central American poison frog Oophaga pumilio for various alkaloids, researchers led by Ralph A. Saporito, a biologist at Florida International University, found more than 80 alkaloids present in free-living, soil-dwelling oribatid mites. Of these toxic compounds, 42 were present in the skin glands of O. pumilio, suggesting that mites are the dominant source for frog poison.
"It turns out that mites are the primary dietary source of alkaloids," Saporito said. "The dogma for years was that ants were the primary source. In fact, ants were thought to be evolutionarily significant to frogs, but now it turns out that mites are probably more important, certainly in terms of the number and diversity of alkaloids."
The researchers also found 40 novel alkaloids in the mites that were not present in the frogs. Saporito says the findings may have shed light on the origins of toxicity for other frog species.
"Our research has implications for all poison frogs worldwide, including mantellids from Madagascar, certain bufonids from South America, and certain myobatrachids from Australia," he explained. "Many of the alkaloids that we identified in mites are common to all poison frogs, in addition to Oophaga pumilio (the strawberry dart-poison frog)."
Overall, Saporito and colleagues found the range of alkaloids present in the skin of O. pumilio "echoes the variety found in mites at the same collection site," indicating the importance of biological diversity for the chemical defence strategies of frogs.
"Reduced arthropod diversity or differences in arthropod diversity among locations likely affects the composition of alkaloids that are found in frogs," Saporito explained. "It is possible that these differences may affect frog toxicity."
The conclusion that poison frogs' toxic alkaloids are derived from their prey explains why frogs reared in captivity lack the toxic defence of their wild counterparts. 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 mites and ants which have their own set of alkaloids.
Dart Frog Medicine?
There are several species of snakes in South America that eat these frogs without any harmful effects. This has prompted a number of scientific research projects to understand how certain animals can be resistant to batrachotoxin.
The question being asked is how certain animals can be resistant to this usually fatal batrachotoxin. Detailed research on these powerful toxins are now revealing why nervous impulses are transmitted in animals, and why not in the humans. Scientists have discovered that batrachotoxin does not immediately block the nervous system; at first, it makes the contractions of the heart muscle stronger. Researchers claim that the pumiliotoxin released by Dendrobates auratus might be used as a cardiac stimulant after a heart attack. According to the American National Institute of Health, poison dart frogs offer over 300 alkaloid components–chemicals that are similar to cocaine and morphine and can be used for medical purposes. Some medicines produced on the basis of batrachotoxin are already being used as anesthetics in surgery.
Where do frogs get their ‘poison’?