COS 137-4 - Müllerian mimicry in camouflaged species: a case study using Gray treefrogs (Dryophytes versicolor) and pickerel frogs (Lithobates palustris)

Background/Question/Methods

While evolutionary ecologists have been studying animal coloration for over 150 years, the way we tackle questions about crypsis and conspicuousness is slowly changing. Aposematism, the use of conspicuous colors to advertise secondary defenses, is perhaps one of the most famous uses of body coloration. This strategy is so common that mimicry often evolves along with warning signals. We usually tend to think of mimicking species as brightly colored (e.g., bees and wasps, poison dart frogs, etc.). Yet, mimicry is seldom investigated in species that are thought of as cryptic, using coloration primarily for camouflage. We have therefore decided to study pickerel frogs (Lithobates palustris) and Gray treefrogs (Dryophytes versicolor), two species of anurans that have an overall cryptic color pattern, yet share a noxious taste, breeding pools and an antipredator threat display. Using digital photography and visual modelling, we sought to quantify and compare the seemingly conspicuous inner thighs of both species, primarily revealed during threat posturing. Color patterns were investigated in both human visible and ultraviolet spectra, along with the frogs’ ability to produce biofluorescence. We then used an unsupervised machine learning algorithm to assess whether both displays are distinguishable under bird, snake, and mammal vision.

Results/Conclusions

We sampled individuals from a population of L. palustris and from two populations of D. versicolor, one that was sympatric with the L. palustris and one that was not. The inner thighs of the allopatric treefrogs were not biofluorescent, while the inner thighs of both the sympatric treefrogs and pickerel frogs did produce biofluorescence. Additionally, all three groups had statistically distinct levels of ultraviolet reflectance, with sympatric treefrogs reflecting the least UV and pickerel frogs reflecting the most UV. K-means clustering performed on hue, luminance, and saturation data in the 400-700nm range yielded similar results for all three predator visual systems. On average, allopatric treefrogs and pickerel frogs were distinguishable 75% of the time, allopatric and sympatric treefrogs were distinguishable 24% of the time, while sympatric treefrogs and pickerel frogs were only distinguishable 8% of the time. Taken together, these results provide support to the hypothesis that D. versicolor and L. palustris have converged towards secondary Müllerian mimicry, but only when co-evolving in sympatry. We propose that sympatric D. versicolor manage to produce an inner thigh coloration closer to that of pickerel frogs by reducing their reflectance of UV-blue light and instead converting it into longer wavelengths through biofluorescence.