In the midst of the hot and humid Colombian rainforest a nearly naked man walks silently among the trees, looking for his next meal. Spotting a distracted monkey, the hunter readies his blowgun and darts. One shot will be enough. According to a first-hand account from 1825, the dart is “certain death to man or animal wounded by it”. That’s because it is laced with poison.
Hunters from Colombia’s Embera tribe regularly hunted birds, monkeys and other small animals using poison darts. The poison came from bright yellow frogs just a few centimetres long.A single “golden poison frog” harbours enough poison to kill 10 grown men, making these frogs perhaps the most poisonous animals alive. They are one of many species of similarly toxic frogs, which are known as poison dart frogs. They are all small: the largest are no more than 6cm long, and some are just 1.5 cm. How did these tiny, beautiful creatures become so poisonous, and why?
Poison dart frogs are so lethal, native hunters once used them to make, well, poison darts. Why are these beautiful animals so deadly?
Her team collected strawberry poison dart frogs from western Panama, and measured how toxic the frogs’ skin chemicals were. She also measured their colours precisely, using an instrument called a spectrophotometer, and determined how easily predatory birds could detect them.
As before, the brighter frogs were more toxic, and Cummings’ calculations suggested they were also more conspicuous to the birds. “This relationship informs potential predators, such as birds, just how much of a punch these frogs deliver,” says Cummings. Back in 2006, she also showed that predatory birds quickly learn to avoid the colourful frogs.
Clearly, being poisonous is advantageous to the frogs. The question is, how did they become so lethal?
Poison dart frogs all belong to the same family of frogs, the dendrobatids.
The group was born some 40-45 million years ago, somewhere in the forests of northern South America, says Juan Santos of the University of British Columbia in Vancouver, Canada. “During this time most of South America was warm and covered with tropical forest, and the Andes were not higher than 2500m above sea level.”
The poison dart frogs’ ancestors were not poisonous, and nor were they colourful or small, says Santos.
In a study published in 2003, Santos attempted to trace the ancestry of the frogs by examining their genes. The results are not definitive, but it seems the frogs are descended from something like a true toad, complete with warts.
This common ancestor was probably “diurnal”, meaning it was active during the day, says Santos. Most of the 300 known poison dart frogs are diurnal, whereas most other frogs, including all of the poison dart frogs’ likely ancestors, are active at night. “We expect that diurnality is derived from a nocturnal ancestor,” says Santos.
In that respect, modern poison dart frogs are similar to their last common ancestor. But in another respect, the ancestor was completely different: it wasn’t poisonous at all.
“The origins of toxicity are more complicated,” says Santos. The poison evolved some time after the origin of the poison dart frog lineage, according to Santos’ data, and different groups evolved it at different times.
“There are between 4 and 5 independent origins,” says Santos. The first was around 30 million years ago, while the most recent was just 2.5 million years ago.
The key to the story is that the frogs don’t make the poisons themselves. They get them from animals like ants that they eat. “These prey items are the main source of poison frogs’ toxins,” says Santos.
The ancestors of poison dart frogs may have started eating toxic ants by sheer chance, and begun harbouring the poisons in their bodies. Some of the key chemicals on the frogs’ skin have been traced to ants, beetles and millipedes.
This seems to fit with Santos’s claim that the frogs acquired the ability to make poison on several different occasions. “Most of these origins are associated with locations that were, or are, covered with dense tropical rainforest with enough diversity of ants and mites,” says Santos.
These early poison dart frogs had a big problem: not being poisoned themselves. It is not yet clear how they managed to withstand and retain the poison, says Summers.
One idea is that they had a high metabolic rate, meaning their bodies could process nutrients and other chemicals quickly. “The high metabolic rate could have been crucial in allowing members of this lineage to withstand and process the toxins,” says Summers. In effect, the frogs were “pre-adapted”.
That may explain how the frogs became so poisonous, but why did they do it? Rainforests are dangerous places, with many predators out to eat a tasty frog. But many similarly small animals have found less extreme ways to survive, such as camouflaging themselves.
There may have been something specific about the poison dart frogs’ ancestors that made them predisposed to defend themselves using poison. Or it could be largely down to luck, says Summers.
Whatever the truth, nowadays the frogs are not the only ones benefiting from their poisons. Neuroscientists are studying the toxins in the hope of designing new drugs.
“It’s not that the compounds cause toxic effects that is of interest here,” says Richard Fitch of Indiana State University in Terre Haute. “It’s the way they do it that is useful to the scientist and physician.”
There is precedent for this. Some alkaloids turn out to have anticancer activity, while others serve as stimulants similar to caffeine.
Epibatidine and phantasmidine are prime examples. They may be lethal, but they also both numb pain. They act on the same receptors on our brain cells that respond to nicotine.
You wouldn’t give phantasmidine to someone who’s in pain, but by studying its structure and chemistry it may be possible to design better pain-killing drugs. Fitch and his team are developing upgraded versions of phantasmidine that are similar enough to still ease pain, but without the toxicity.
“If we can cut the key just right, we get the activity we want,” says Fitch. “That’s perhaps a tall order, as we don’t quite know what the lock looks like, but we have a key and that’s a start.”
This new use for poison dart frogs may well have replaced their previous one. The practice of using them to make poison darts was in decline as early as the 1970s. It’s difficult to tell if the Embera people still do it, says Summers, because the area where they live is “remote and very dangerous because of guerilla activity”.
If no one is making poison darts anymore, that’s good news for the frogs: at least they aren’t being pierced with sticks. But like many amphibians, poison dart frogs are vulnerable to extinction. The forests they live in have been hacked back, and a fungus called chytridiomycosis is killing them by infecting their skin.
Might their poisons offer them any protection? The skin of strawberry poison dart frogs can fend off some bacteria and fungi,according to a study published in January 2015. But that doesn’t necessarily mean it can fight off the chytrid fungus.
“At this point, we do not know if the alkaloids in dart poison frogs offer them any protection from chytrid,” says Saporito. “This is something we are beginning to actively study in my lab.”
It seems unlikely that the poisons will be enough to save the frogs, but they might at least buy some time. – BBC Earth