If you have ever had the misfortune of turning over a log or rock in eastern Oklahoma and grabbing a black salamander with white spots, you know that washing off the thick, sticky liquid that they secrete from their skin in defense is no easy matter. Imagine being a small predator like a snake, biting the salamander, getting a mouthful of that sticky stuff, and then trying to rub it off in the leaf litter ending up with a mouthful of leaves stuck to your mouth! These salamanders, which look pretty much the same from mountain ridge to mountain ridge, and even resemble a lowland salamander, form a complex of species whose identities and relationships are just beginning to be understood. To the layman, these are known as the “slimy” salamanders.
Dr. Don Shepard, a former graduate student from the University of Oklahoma, has combined field studies with DNA sequence data to unravel relationships among these species. Until recently, these salamanders have been considered as four species. One, Plethodon albagula, appears to be widespread in lowlands and the Ozark foothills of eastern Oklahoma, extending up into Missouri. Three other species, P. caddoensis, P. fourchensis, and P. ouachitae, each occur on their own mountain ridges in the Ouachita Mountains of southeastern Oklahoma and southwestern Arkansas. If only it were that simple!
By combining DNA sequence data with known rates of evolutionary change for specific genes, Don has discovered that not only have some of these “species” experienced divergence into genetically different populations, the causes of divergence differ among species. For P. ouachitae and P. fourchensis, genetically different populations are associated with mountain ridges, indicating that populations were isolated on mountain tops as the climate slowly changed over geologic time. Why would they become isolated? The answer is that these salamanders require a specific set of environmental conditions to survive. These include relatively cool temperatures and high humidity, usually associated with mountain tops. The salamanders appear unable to adapt to warmer and drier conditions so they simply live in the same kind of microhabitats that they lived in historically. This inability to adapt is referred to by scientists as “niche conservatism.”
For P. caddoensis, diversification is associated with stream drainages. Four distinct lineages within P. caddoensis have been identified from each of the following four streams; lower Caddo River, Upper Caddo River, Brushy Creek, and Cossatot and Little Missouri Rivers. Diversification in this “species” appears tied to more recent climatic change and specialized niche characteristics of the salamanders, primarily related to thermal and humidity variables associated with stream drainages.
Dr. Shepard is now beginning to examine the lowland and widespread “species” P. albagula. No doubt this species has undergone some interesting divergences in the past, but what the pattern is and how it occurred remains a mystery to be unraveled. So, the next time you pick up one of these so-called “slimy salamanders,” reflect on their evolutionary history and what they tell us about past events underscoring the remarkable diversity that we see today in eastern Oklahoma. Oh, and if you have been slimed, try WD-40 followed by soap and water. WD-40 seems to be a universal solvent and will clean many things!