[chris (OP)]

How do poison arrow frogs obtain their toxins?

Incredibly, one frog contains enough toxin to kill 20,000 mice!
 (http://www.pnas.org/content/early/2017/08/29/1707873114.abstract)

But how do poison arrow frogs obtain their toxins in the first place? Sources refer to the toxins (batrachotoxins) as "alkaloids", so are these dietary from plants or algal species?

[chiralSPO]

I remember an article from a few years back that claimed to show that poison dart frogs obtain their toxins from ants (which themselves might rely on bacteria to produce them).

I was able to relocate the article ( https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1187980/ ) from a more accessible news release: https://news.mongabay.com/2005/08/study-discovers-why-poison-dart-frogs-are-toxic/#study

(This article is a dozen years old now, and I don't know if this is still the prevailing theory)

[chiralSPO]

Actually, I will take this opportunity to point out that most of the highly bioactive molecules we have discovered are, in fact, produced by bacteria, algae, and funguses (plants have produced some impressive alkaloids, like strychnine and aconitine, and some animals can make some impressive peptidic toxins, like snake and snail venoms, but they just don't hold a candle to the agents developed by single-celled organisms that have been involved in constant chemical warfare with each other for billions of years). The brevetoxins (red tide), tetrodotoxin (adopted by fugu and blue ringed octopus, among species, see below) and aflatoxins (aspergillus molds) are truly awesome in the biblical sense of the word.

Most of the natural products isolated from marine organisms are believed to be derived from bacteria or algae. These little "bugs" either have a symbiotic relationship with a host animal that benefits from their toxins, or they are eaten by the animal, which then accumulates their toxins.

[chris (OP)]

Thanks @chiralSPO for your insights. I was quite fascinated by this subject and decided to look a bit deeper. There was actually a paper out last week that I decided to write up on why poison dart frogs don't poison themselves.

The literature that I subsequently read when researching the frog story suggested that it was rainforest arthropods, like centipedes, that are the sources of at least some of the batrachotoxins secreted by the poison dart frogs.