A Frog of Many Colors

The message given by the brightly-colored patterns on a poison dart frogs body is pretty obvious–eat me, and you’ll regret it. Research shows that predators quickly learn to avoid eating these vibrant amphibians after just one foul experience. Ranitomeya imitator is just one species of poison dart frogs. This frog species is relatively new to science, as they were first discovered and described in 1986 and later reclassified in 2006. These frogs are found throughout northern and northeastern Peru. Their populations are most widely distributed across Perus lowlands, but some populations can be found living at elevations up to 950 meters above sea level.

An interesting aspect of Ranitomeya imitator is that the species has 10 different body patterns, or morphs. Marcel Chouteau, an evolutionary biologist at the University of Montreal, was interested in finding out why there were so many different body patterns within the same species.

To conduct his experiment, Chouteau enlisted the help of his girlfriend to help him make 3600 frog models out of modeling clay. Each model was life size and measured 18 millimeters in length. One-third of the clay models were painted to resemble frogs with a yellow stripe pattern on its back; another third were painted to resemble frogs with green patches on its back; and a final third were used as a control for weight selection and were painted brown to resemble the coloring of non-poisonous frogs in the area.

Three hundred of each model type (striped, patchy, and control), for a total of 900 models, were placed at two sites within the Amazon rain forest. One site was located in the lowland plains. The second site was located 10 kilometers away at a mountainous location. After the models were placed, Chouteau returned to check on them every 24-hours over a span of three days. Chouteau repeated his three-day experiment on three different occasions.

Due to the malleable nature of the models clay bodies, predation marks from bites were clearly visible. Chouteau found that at each site, the frogs that looked least like the local frog had the most bite marks. Accordingly, when predators see targets of a different species, they attack. However, he also found that the predators learn quickly to avoid the novel prey. After the first 24 hours of the study, the number of bite marks significantly decreased for the non-typical frogs.

The results of this research indicate that predators play an important role in the selection of multiple body patterns within the same species of poison dart frog. It appears that avian predators are just as geographically-specific as are their frog prey. The unrecognized frog phenotypes are more easily detected by predators, and hence do not last long in a new territory. In other words, predators keep each pattern going by selecting against any frogs that do not match the skin pattern they have learned to avoid. These results support the idea that natural selection plays an important role in the development of the many different skin patterns found among members of Ranitomeya imitator.

The results of this research were published in the December 2011 issue of the journal The American Naturalist.

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