HOLLOWS, PEEPERS, AND HIGHLANDERS: AN APPALACHIAN MOUNTAIN ECOLOGY

17. A Lungless Salamander Trilogy: Mimicry

115 17 A Lungless Salamander Trilogy: Mimicry I n the Great Smoky Mountains, an amazing similarity exists between two salamander species, Jordan’s and Allegheny Mountain. Both include some red-cheeked and some red-legged individuals. It’s as if they are copying each other. The evolutionary cause of this improbable likeness lies in the tastes of salamander predators. Any meat-eating bird that feeds by scratching the forest floor will try to eat a salamander. The blue jay, brown thrasher, and wood thrush are all confirmed salamander predators. All species of woodland salamanders in the genus Plethodon, however, seem to be at least mildly distasteful to birds, and some are downright poisonous. Upon ingesting a “bad” salamander, the birds’ reactions range from gagging to regurgitation. (Skin secretions of salamanders are reported to numb human taste, but I haven’t had the temerity to lick one.) In the continuum of salamander palatability, birds find the Allegheny Mountain tastier than Jordan’s. Although not poisonous, Jordan’s exudes viscous, noxious skin secretions, so experienced birds avoid it. Jordan’s salamander is tastier than the poisonous red eft, but is passed over by most birds because of its bright warning coloration and overt defensive behavior. (You may know the red eft as the familiar , bright red juvenile stage of the red-spotted newt that wanders about boldly in broad daylight.) In the Great Smoky Mountains, some isolates of Jordan’s display red cheek patches. Four other Jordan’s isolates in the Nantahala National Forest south of the Smokies have red legs. Some popula- 116 tions of the Allegheny Mountain possess, with varying frequencies, red legs, red cheeks, or both. Now for the most provocative observations : the red-legged form of Allegheny Mountain occurs only within the range of red-legged Jordan’s; within the range of red-cheeked Jordan’s, the imitator sports red cheeks. There seems to be a lot of copying going on. If they have had previous experience with Jordan’s salamanders , birds avoid the tastier but incognito Allegheny Mountain. Some bird predators transfer their distaste for red-legged Jordan’s to red-legged Allegheny Mountain, avoiding them both. Likewise, birds that have been negatively conditioned by the unsavory red-cheeked Jordan’s avoid red-cheeked Allegheny Mountain salamanders. The protective value of the Allegheny Mountain’s false warning colors of red legs or red cheeks has been demonstrated experimentally. Simply, birds learn to associate red patches with unpalatability. The phenomenon in which a palatable species evolves an exterior appearance or behavior that is similar to that of a distasteful or poisonous species is known as Batesian mimicry. Members of the tasty species thereby gain protection from predators. Allegheny Mountain salamanders with red legs, instead of the usual black, are interpreted as Batesian mimics of the range-sharing red-legged Jordan’s.Analogously,red-cheekedAlleghenyMountainareBatesian mimics of the spatially overlapping red-cheeked Jordan’s. The model is the large, black Jordan’s salamander, which exists as twenty-one geographical isolates, and the mimics belong to the highly variable Allegheny Mountain salamander complex. The mimicry between Jordan’s and Allegheny Mountain salamanders appears to be a weak Batesian mimetic system. The system is based on only one model — Jordan’s — which is only mildly unpalatable to begin with. The success of a Batesian mimetic system with a weak model depends on the availability of alternate food sources for the predators. The greater the availability of other foods, the less predation on the model and the greater the protection for the mimic. Another aspect of this system is consistent with weak Batesian mimicry : natural selection favors variable forms in the mimic — that is, Hollows, Peepers, and Highlanders 117 the mimic occurs in several color forms — when the model cannot support an extensive mimetic population. The red-backed salamander and the red eft carry on another mimicry. In this system, the red eft serves as the model for an all-red phase of the red-backed. A classic example of warning coloration and behavior, red efts are extremely toxic to mammals and birds — their skin contains tetrodotoxin, a powerful neurotoxin and the strongest emetic known. This is why red efts can crawl around fearlessly during the day. As you would expect, where it coexists with red efts, the all-red color phase of the red-backed repels bird predators. The system is not quite this simple, though, because the...