Color indicates poison in “poison dart” frogs—honestly!

A strawberry poison dart frog; apparently the San Cristobal color morph. Photo by Wilfredo Falcón.

ResearchBlogging.orgAlmost everyone knows the basic story behind the brilliant coloring of poison dart frogs. These tiny tropical rainforest amphibians secrete toxic alkaloids from their skin, and their bright colors are aposematic signals to warn away potential predators.

You’d expect species that are all sending the same message—Poison! Don’t eat!—to use the same signal to do it. Local studies confirm that birds are more likely to attack poison dart frogs who look different from other poison dart frogs in a given area. Yet not all poison dart frogs have the same color pattern, or even similar color patterns. Far from it—frogs within the same species can look completely different.

One possible explanation is that frogs with different coloration are not, in fact, sending the same signal. Brighter color could indicate greater toxicity. That seems to be the case for one highly variable species, the strawberry poison dart frog Dendrobates pumilio. A paper just published as an online, open-access article in The American Naturalist demonstrates that D. pumilio‘s colors are “honest signals”—and those signals are directed at specific predators.

The many colors of Dendrobates pumilio. Figure 1 from Maan & Cummings (2012).

The new study’s authors, Martine Maan and Molly Cummings, selected a study species that is a veritable rainbow of aposemitism, as you can see from the excerpted figure above. Different populations of Dendrobates pumilio are orange, red, green, blue, and yellow, with or without black spots. Maan and Cummings make sense of that colorful diversity in two major ways: first, by finding out whether there’s a relationship between color and poison, and second, by making an educated guess about how the different color morphs look to D. pumilio‘s many predators.

For the first part, Maan and Cummings took an objective measure of color—reflectance spectrum of frogs’ skin, measured under standardized lighting—and compared it to an objective measure of toxicity—how much discomfort mice exhibited from an injection of frog skin extract. (The mouse injection method is apparently a standard toxicity assay, and I guess it makes sense if you don’t know the specific chemicals that make the frogs poisonous.) The coauthors found a strong relationship between skin brightness and toxicity—frogs with brighter coloring were more poisonous.

Objectively bright coloring isn’t quite the same thing as looking bright to a predator, though. Different animals have different color vision—a frog that looks brightly colored to a frog-eating bird might not be particularly showy to a frog-eating snake, because birds and snakes have different suites of sensory cells in their eyes. So the coauthors then fed the spectral readings from the frogs into mathematical models that estimate how the frogs look to different kinds of animal vision. (This approach has been used elsewhere—for instance, to determine how well brood-parasitic cuckoo eggs blend in with their hosts’.) Maan and Cummings applied models based on the visual sensitivity of crabs, snakes, two kinds of bird vision, and frog vision.

Another strawberry poison dart frog, this time the color morph found on Aguacate. Photo by Drriss.

They found strong relationships between the frogs’ toxicity and their colors as seen by birds, and as seen by other frogs. The crab vision model varied depending on what kind of material the frog would be viewed against—to a crab, the frogs were conspicuous against bark or leaf litter, but not against green leaves. Meanwhile, the snake vision model didn’t perceive any particular relationship between brightness and toxicity. Those results make a lot of sense. Birds are most likely to spot prey from a distance, and make a decision to pursue it or not without getting up close. Crabs aren’t likely to encounter frogs up in the foliage, but on the ground, in the leaf litter. And snakes are less likely to rely sight than on chemical senses—taste or olfaction—in evaluating a potential meal.

This study doesn’t directly demonstrate the action of natural selection, and that leaves a significant question hanging: Why should Dendrobates pumilio signal its toxicity honestly? Certainly, if you’re a highly toxic frog, you’d want to let predators know; but if you’re less toxic than the frogs in the next population, why would you tell the world? Indeed, other species of poison dart frogs have evolved mimicry—bright colors without poison.

That suggests the honest coloration within D. pumilio is be due to more than just selection by predators. Perhaps coloration serves social functions, and then more conspicuous color morphs need to be more toxic to fend off more frequent predator attacks. Or there may be genetic constraints that link bright color and toxicity within the species, and both have evolved local differences due to genetic drift. Finding out how selection and other evolutionary forces have created this pattern would be no small project, but I think it’ll make an interesting story in the end. ◼

References

Darst, C. (2006). A mechanism for diversity in warning signals: Conspicuousness versus toxicity in poison frogs Proc. Nat. Acad. Sciences USA, 103 (15), 5852-7 DOI: 10.1073/pnas.0600625103

Maan, M., & Cummings, M. (2012). Poison frog colors are honest signals of toxicity, particularly for bird predators. The American Naturalist, 179 (1) DOI: 10.1086/663197

Share

Poison dart frogs can’t get too creative

ResearchBlogging.orgBeing a poisonous animal isn’t much help if your predators don’t know about it. That’s why lots of poison-defended critters – monarch butterflies or poison dart frogs, for instance – advertise with bright “warning” colors. This is called aposematism. The idea is that predators will learn (or even evolve) to avoid bad-tasting, poisonous prey if they’re well-marked for future reference.

The trouble with aposematism, though, is that it requires giving up another, more common defensive color scheme: camouflage. If you’re a poisonous critter, and you evolve bright coloration for the first time, predators don’t yet know that you’re poisonous – but you’re really brightly colored and easy to see. How, then, does aposematism evolve from non-aposematic ancestors?


Photo by dbarronoss.

A new study on early release from Biology Letters suggests that it isn’t easy. The authors, Noonan and Comeault, set out to determine whether brightly-colored poison dart frogs are more likely to be attacked when they evolve new color patterns [$-a]. It’s possible that the frogs’ predators avoid all brightly-colored prey regardless of pattern, in which case new frog patterns would be just as good for predator deterrence as the old ones. But it’s also possible that predators only avoid patterns they’ve run across (and spat out) before – so that new, rare patterns would have all the disadvantages of giving up camouflage with none of the benefits of aposematism.

Noonan and Comeault performed an elegant behavioral experiment, setting out clay model frogs in an area where frogs of one color pattern predominate. One set of models matched the local color pattern, another was brightly colored but different from the local pattern, and a third was drab and camouflaged. Birds were much more likely to attack the “new” color pattern than either the “local” version or the drab one. This result is hard to understand at the first pass – if new color patterns are vulnerable to attack, how can aposematism evolve in the first place? The answer is, not by natural selection, but by genetic drift.

Genetic drift is a natural, mathematical consequence of finite populations: imagine a bag full of marbles, half of them black and half white. If you pull a sample of marbles from the bag, you expect them to be half black and half white on average (i.e., over many samples) – but any individual sample might have a very different frequency of white and black marbles, especially if it’s small. If the probability of picking a white marble from the bag is 0.5 (because half the marbles are white), then the probability of picking a sample of four white marbles is 0.5 × 0.5 × 0.5 × 0.5 = 0.0625. That’s a small probability, but not zero. Drift is a very real effect in the natural world, especially during the establishment of new local populations, when the population size is initially quite small.

The key to understanding Noonan and Comeault’s result is that aposematism is frequency dependent – it favors not the old pattern as such, but whatever bright color pattern is most common in the frog population. Birds attacked the “local” color pattern at a low rate, which suggests that they’re always re-learning which pattern to avoid. A new color pattern might be hard to establish within a population of frogs that look very different from it, but if a new pattern pops up in the course of establishing a new population, then – thanks to genetic drift – it may be common enough for predators to learn to avoid it.

Reference

B.P. Noonan, A.A. Comeault (2008). The role of predator selection on polymorphic aposematic poison frogs. Biology Letters DOI: 10.1098/rsbl.2008.0586

Share