Conspicuously poisonous
"It tastes bad", signal the brightly colored poison dart frogs. But what makes a stronger impression on the robbers - the shrill color of the fright or the horrible taste? Ultimately it depends on which defense is worth investing more in.
Conspicuousness as self-protection? That sounds highly unlikely - if you want to protect yourself, you should probably hide from prying, mostly hungry looks. Unless the respective eye owners remember highly unpleasant experiences with similarly colored prey colleagues - and therefore prefer to withdraw their beak and claw. This is how the principle of Müller's mimicry works, in which several species share a uniform warning costume with which they promise dubious, if not poisonous, enjoyment.
However, such a successful defense does not come free of charge: If you want to spray poison, you first have to produce it - or ingest it through food. But that limits the menu considerably. On the other hand, the colorful color pigments of the warning dress are not necessarily common metabolic products, but usually require extra effort. Which, however, raises the question for users as to which alternative to further perfecting the protection is more worthwhile – even more garish colors? Or even more disgusting taste?
Poison dart frogs of the genus Epipedobates are a great way to examine this dilemma: Their equally red warty back and the yellow stripes on the thighs of their front and rear legs make it clear to any predator - ugh, better give them a bow. There is also a skin toxin that can kill a chick. For the three species E. parvulus, E. bilinguis and E. hahneli from the Amazon lowlands of Ecuador, Catherine Darst and her colleagues from the University of Texas in Austin have now investigated which method the animals rely on more - or whether they invest both defensive measures equally.
On the one hand, they recorded the colorfulness of the animals from a bird's eye view - with a special device that reproduces the visual impression that birds should be given. They offset this against the bird's-eye perception of the foliage background, in which the amphibians usually stay, and thus obtained the contrast as a measure of the conspicuousness with which the four-legged friends betray themselves to potential enemies. They also tested the toxicity of the skin secretion that they isolated from dead frogs and injected into mice while they were sleeping. They rated the toxicity based on how long it took the rodents to go back to sleep afterwards.
And this is how it turned out: The shrillest contrast to the environment is not offered by the most poisonous representative – that would be E. parvulus – but by the more tame variant E. bilinguis, which is harmful to he alth. The toxin E. parvulus remains rather subtle in coloration - so it seems as if it is enough to invest in one of the two defensive measures. E. hahneli, on the other hand, is more reserved in terms of coloration and toxicity.
But are both alternatives - high toxicity and moderate coloring versus moderate toxicity with conspicuous costumes - equally successful? Here young chickens had to serve as test organisms. These certainly eat frogs, as the scientists proved with inconspicuous relatives of the Epipedobates species. When they then presented the shrill specimens to the chicks, they certainly pecked at the hopping morsels – but with one exception (which ended fatally for the hen) they soon ignored them. The victims survived the attacks in four fifths of the cases without major damage.
Here it was noticed: With the particularly poisonous E. parvulus, the chicks learned to avoid possible prey faster than with the other two species. Apparently, the higher toxicity here goes hand in hand with greater inedibility. Coloring, on the other hand, had no influence: when comparing the equally poisonous E.bilinguis and E. hahneli, the researchers found no advantage over E. bilinguis, which is more conspicuous from a bird's point of view.
However, real protection does not depend on the first impression, but on how well it is internalized. So the scientists let their chickens, which were now experienced with disgusting frogs, loose on the colorful victims again - and lo and behold: E. parvulus - the poisonous one - and E. bilinguis - the shrill one - were now equally protected. So even if the learning process was slower, the effect was the same: beak gone. In contrast, E. hahneli, the moderate, remained exposed to the risk of pecking for longer.
So overall, it seems to be the same which measure the frogs invest in to protect themselves - both are having lasting success. Which alternative they choose could depend on environmental factors: if there is an abundance of toxin-producing food, the flavor could be the better choice. If it looks rather meager, it is more tempting to reach into the metabolic paint box. On the other hand, the free riders, who come with every good idea, benefit from the predator training much cheaper: The quite edible distant relatives Allobates zaparo and A. femoralis, which imitate the costume of the epipedobates in the best Batesian mimicry manner, stayed in to a similar extent as E. hahneli was spared from chicken attacks. And that only with the body's own paint box, you save yourself the poison.
