Why are Poisonous Frogs so colorful?

Some of the animal world’s most brilliant color combinations can be found on some of its most poisonous creatures. What we as humans see as magnificent and beautiful often has an entirely different meaning for animals, especially the potential predators of tropical frogs. Vibrant colors signify poison and danger, but why and how?

It seems the answer is two-fold: survival and reproduction, the underlying forces of natural selection. Birds and snakes are often the primary predators frogs must avoid and they each sense colors differently. Birds have the most advanced visual abilities of all potential frog predators, so biologists believe that the myriad color varieties that frogs have developed are largely in response to the threat of avian predation.

Many poisonous frogs are not lethal if eaten, but will make a predator extremely sick for some time. Other frogs, such as the Strawberry Poison Dart Frog, can kill a bird or snake with a single drop of its poison. It would appear that over time, birds and snakes learned not to eat the brightly colored frogs after becoming sick. If all frogs were lethal, then the predators that ate them would die, and no “learning” would take place. After many thousands of years, the process of eating dangerous, brightly colored frogs and the associated sickness seems to have become instinctual for many predators and brightly colored frogs are avoided. Additionally, many brightly-colored poisonous frogs will taste terrible due to the poison and are spit back out before enough of the toxins needed to kill the predator have been ingested , further contributing to the “learning” that bright colors are bad when it comes to food. As the brightly colored frogs grew more toxic with the passing of generations, the less toxic frogs became more camouflaged as those that could remain more hidden survived long enough to pass on their genes.

Much like the Galapagos Islands in the Pacific, the island archipelago of Bocas del Toro in the Panamanian Caribbean provides a fantastic laboratory for natural selection. The islands of Bocas del Toro were united about 10,000 years ago, but sea level rise has lead to the formation of nine distinct islands and natural environments. The process of natural selection in poison dart frogs has lead to different coloration and varying toxicity levels between the frogs on each island.

Dr. Molly Cummings of the University of Texas at Austin undertook a study to examine the correlation between coloration and toxicity by using frogs from the different islands with ten distinct color patterns. By injecting mice with the different poisons, the researchers found that the brightest colored frogs did indeed have the most poisonous toxins (no mice died in the experiments).  

So we know that brighter colors mean higher toxicity, but do birds understand this? An additional experiment conducted by Dr. Yoel E. Stuart of Harvard University involved the use of frog decoys made of clay with differing color schemes to see which colors were attacked the most. The scientists placed models of frogs with four different color schemes throughout La Selva Biological Station in Costa Rica. Two of the four models were brightly colored (one of which resembled the local, red and blue strawberry poison dart frog and the other a novel, yellow species resembling a poison dart frog from Bocas del Toro) while the other two models were plainly colored and more camouflaged (one of which resembled a local, brown frog and the other a novel, black species not found at La Selva). The scientists initially tested the clay frogs with real frogs and found out that the clay frogs were just as likely to be attacked, enabling the use of their clay models to conduct the experiment.  

The scientists found that the novel species were attacked more than the familiar species and that the brightly-colored species were attacked less than the more camouflaged species. The models resembling the local, red and blue frogs were attacked the least, suggesting that natural selection will continue selecting frogs with this color scheme to pass on their genes in La Selva.

In addition to coloration playing a role a survival, it also plays a part in reproduction, the other driving force of natural selection. According once again to Cummings, birds can only detect differences in color, but not varying degrees of brightness. It turns out that frogs are adept at detecting brightness and that females are most attracted to males with the most brilliant colors. This finding means that brightly colored frogs are “doubly” selected for in terms of natural selection as their colors warn predators while simultaneously making them more attractive to females and thus more likely to reproduce.