• Becca Dzombak

So That's Neat! Why poison dart frogs are poisonous

I came across an article that I thought would be cool to cover about how a Panamanian poison frog species' chemical defenses change throughout the year as they undergo a "vertical migration" from the forest floor to the treetops. "Neat!" I thought. But then I realized I had no idea how any poison dart frogs get their poison—the species in the article gets its toxins from the bugs it eats, but I wasn't sure if other species produced their own or if this diet-based poison was the norm. And how did the diet-based toxins evolve? And where do they store it? I imagine there's just a little Poison Pouch somewhere with a tiny "Do Not Puncture" label, but I'm sure that's not it. (Mostly.)

'Poison frogs' are perhaps most recognizable from their bright colors, with their vibrant hues warning of the dangers their skin holds for would-be predators. The colloquial name 'poison-dart or poison-arrow frog' comes from Indigenous peoples' use of the frogs' toxins to tip arrows or blowdarts, although the term is commonly used to describe a broader group of frogs than it actually represents (three species in Phyllobates). The frogs pack a punch: the deadliest of the family, one dose from a golden poison frog (Phyllobates terribilis) is commonly said to have the strength to kill 10 men. But how do they get their poison?

Until relatively recently, scientists still weren't sure if the frogs developed their own toxins or took them in through their diet. The toxins found on frogs' skin are chemical compounds called lipophilic alkaloids, and more than 850 variations of the chemical have been detected on various species. I was pretty close with my "Poison Pouch" guess. The toxins are stored in glands beneath frogs' skin, and this compartmentalization may help prevent "auto-toxicity" (frogs poisoning themselves). The complex relationships between species, diet, habitat, and toxins has meant that discerning how different factors influence each other can be tedious.

Developing their own would, in some ways, be easier: the frogs would always be toxic regardless of what their lunch was, so if insect availability changed (say, due to climate or environmental change), they'd be okay. But as early as the 1990s, the frogs' diet and toxicity were being examined together. By comparing the frogs' toxin levels in captivity to wild levels, biologists were able to say that diet is probably the source of poison frogs' poison. And the new study I saw specifically tracked how Panamanian poison frogs' diet and toxicity tracked each other throughout the year, as the frogs migrated from the forest floor up to tree canopies. Different insects along the way, different chemical defense systems throughout the year. So that's neat.

Not so neat: The authors of the study also found that the frogs' toxin levels dropped during the dry season due to a paucity of prey. Because climate change in the tropics could lead to longer dry seasons and extended periods of limited food, poison frogs could risk losing their poison. (Climate change also threatens frogs with the spread of deadly fungi and simply getting too toasty.)

Extra deets: Even with these concrete experiments, there are still some outstanding questions about poison dart frogs and their toxicity. While we know what they generally eat (ants, mites, and other arthropods), herpetologists are still working to figure out what specific diet makes for the most poisonous frogs. Out of those 850 or so compounds, fewer than 10% have been clearly identified in arthropods, raising questions about chemical sources. And, as you might imagine, identifying a species of insect once it's been partially digested by a frog is pretty difficult, so scientists struggle to identify frogs' diets at a high level of detail.

There are some evolutionary questions, too: relying on a subset of insects to maintain your toxicity means evolving to be a very specialist species (surviving in a relatively strict ecological niche) rather than a more flexible, adaptable generalist. While there are clear evolutionary benefits to having poisonous skin, being a specialist makes you. more vulnerable to losing your habitat and snacks. Equally interesting are questions about the coevolution of the frogs' diet, toxicity, and colorful skin patterns. Since they're meant to warn predators (aposematic), you might think more toxic means brighter, but that's not always the case.

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