Aposematic coloration is the technical term for the coloration phenomenon found in well-known animals like poison dart frogs (Dendrobatidae) and skunks. If an unpalatable animal makes itself very conspicuous predators may interpret the animal’s flashy coloration as a warning sign. In order for that to work, the predator must be able to recognize the signal and exhibit a corresponding avoidance response.
For predators, this often comes in the form of learning from past unpleasant experiences. Even though some members of the prey species are sacrificed as “test cases” for inexperienced predators, the species as a whole benefits from the predator learning from its mistake.
But what happens when the predator is colorblind? In this case, the predator has to rely on recognition of the color pattern rather than the actual colors.
Now, let’s take things a step further and ask what happens if the aposematically colored organism has two types of predatory aggressors and one of the predators is colorblind and the other is not? Furthermore, what if the colorblind predator isn’t affected by the aposematically colored organism’s defenses at all? Are you still with me?
Accounting for Contingencies
Enter the harlequin bug Tectocoris diophthalmus; a shieldback stinkbug exhibiting a coloration anywhere between solid bright orange to bright orange with iridescent blue patterning. Life is tough for this Hemipteran. It must contend with both avian predators and predatory Hierodula majuscula mantids.
Birds possess the visual capabilities required to detect aposematic coloration and they avoid the flashy harlequin bug like the plague. Birds find the taste of T. diophthalmus repugnant and it only takes one or two exposures before the avoidance behavior is engrained.
The mantids, on the other hand, are unfazed by the same defenses that make Tectocoris diophthalmus unpalatable to birds. They will happily consume these harlequin bugs over and over again.
In order to avoid detection by the mantids, Tectocoris diophthalmus has to rely on a different strategy. Evolution ultimately selected for camouflage (cryptic coloration). But how? These bugs are orange.
When Orange is Also Green
It turns out that orange is a very useful color for countering mantid vision. Mantids possess monochromatic vision with peak sensitivity in the ‘green’ region of the spectrum. To mantids like H. majuscula, a flat orange harlequin bug appears yellow-green in color. That creates little contrast with respect to the surrounding foliage.
Behavioral experiments conducted by Scott A. Fabricant and Marie E. Herberstein support this theory. The researchers determined that H. majuscula would approach iridescent T. diophthalmus harlequin bugs more often than solid orange ones, suggesting that the solid orange coloration has a cryptic effect from the perspective of the mantids.
The researchers noted that the cryptic effect was diminished when the mantids were at close range. If they were on top of the harlequin bugs, the camouflage wasn’t as effective. At longer distances, however, the mantids had difficultly distinguishing between unoccupied leaves and leaves occupied by orange harlequin bugs.
By selecting for a solid orange coloration, evolution gave T. diophthalmus the ability to simultaneously exhibit aposematic and cryptic coloration when the sum of their potential aggressors is considered. This phenomenon also suggests that T. diophthalmus populations in areas heavily influenced by avian predators may exhibit a more ‘patterned’ coloration whereas those populations subject to predation pressures from mantids may exhibit a more ‘uniform orange’ morphology.
References and Further Reading
- Fabricant, S. A., & Herberstein, M. E. (2014). Hidden in plain orange: aposematic coloration is cryptic to a colorblind insect predator. Behavioral Ecology, aru157.
- Fabricant, S. A., & Smith, C. L. (2014). Is the hibiscus harlequin bug aposematic? The importance of testing multiple predators. Ecology and evolution, 4(2), 113-120.