Why might a frog's skin be a bright colour?
Answers
Poisoning your attacker is the second-best option. The best option is to not be attacked in the first place.
That's what the color is for: it lets everybody know that you're a poison frog. It's a threat: "Eat me, and you die. I don't want to kill you, but I will. So let's just leave each other alone."
Now, here's the trick: if the poison frogs can do it, why can't other non-poisonous frogs do it? They can, and do. This little guy (Allobates zaparo) sure looks poisonous, doesn't he:
(Credit: Page on www.dendrobatenwelt.de)
He is, in fact, totally tasty. Unlike this guy, Ameerega bilinguis:
(The remarkable thing is that these aren't the most dangerous frogs in the forest. If A. zaparo wanted to look dangerous, why not mimic the most dangerous ones? The answer, it turns out, is that nobody learns to avoid the toxic frogs: they eat them and die, and then be replaced by a new, ignorant predator. Predators that learn to avoid them create a kind of zone of protection for them. Amusing stuff: Poison Frog Uses Less-Toxic Looks to Survive, Study Finds).