Experimental support for defensive coloration in Eurycea lucifuga

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Scott V. Janis, Biology, University of Louisville, Louisville, KY, Perri K. Eason, Department of Biology, University of Louisville, Louisville, KY and Phillip Sullivan, Biology, University of Louisville, Louisville

Background/Question/Methods
Aposematic coloration is a primary defense that increases the utility of a secondary defense such as poison by making an animal highly identifiable and facilitating the learning avoidance by potential predators. The cave salamander Eurycea lucifuga has bright orange skin and black spotted dorsal pattern but lives in low-light environments such as rock walls and the twilight zone of caves. These salamandars are known to have noxious secretions, particularly on the tail, which has led to the reasonable assumption that this dorsal pattern is aposematic; however, this has not been confirmed experimentally. To do so, we set out a grid of 120 clay model salamanders, 60 models that resembled E. lucifuga and 60 brown-gray models that resembled the cryptic species Desmognathus fuscus. We set up a 60 plots in a grid (7 rows of 9 plots, with 3 unused) outside of Sauerkraut Cave in Louisville, Kentucky; plots were 10 m apart. We placed one orange and one gray model in each plot and at least 0.5 m apart. We checked the sites every morning and evening for 39 days from October 9 – November 17, 2020, for a total of 4680 model days. We recorded when models were damaged or carried away and identified the attacker when models could be recovered; we replaced each damaged/missing model with a new one. We also recorded the date of each model’s damage or loss and whether models were attacked at night or during the day.

Results/Conclusions
A total of 74 salamander models were attacked. Of these, 20 were bitten and recovered, including 8 orange models and 12 gray models, and 54 models were carried away, including 20 orange salamanders and 44 gray salamanders. Using a chi-squared test for the total of 74 attacks, we found that gray salamanders were attacked significantly more often than orange salamander models (χ2 = 4.38, p = 0.036) even though neither model had any secondary defense such as a bad taste. Notably, both orange and gray salamanders were attacked by birds and large mammals like coyotes at comparable rates, but no orange salamanders had small rodent damage. Because of the low number of attacked models that were recovered, this was not a significant difference, but it may suggest that E. lucifuga’s dorsal pattern is primarily effective against rodent predators. These results, though preliminary, support the conclusion that E. lucifuga’s orange and black dorsal pattern is aposematic.