Induced Morphological Plasticity in Lowland Leopard Frog Larvae (Rana yavapaiensis) Does Not Confer a Survival Advantage against Green Sunfish (Lepomis cyanellus)

—Tadpoles can generally increase their probability of survival in the presence of known predators by reducing their foraging activities or modifying their tail shape to increase swimming speed or lure attacks away from the head. However, it is unknown to what extent tadpoles can induce such behavioral and morphological plasticity in response to introduced predators. Lowland Leopard Frogs (Rana yavapaiensis) are native to Arizona and are currently declining because of a variety of factors including introduced predators such as the Green Sunfish (Lepomis cyanellus). We reared Lowland Leopard Frog tadpoles in the presence of tadpole-fed Green Sunfish or in control tanks and tested whether Lowland Leopard Frog tadpoles alter their behaviors or body shape in response to the visual and chemical cues of this predator. We found that tadpoles reared in the presence of Green Sunfish were 90% less active and had significantly different body shapes (including 5% deeper tail fins, 3% larger tail muscle height, and 3% smaller tail muscle area) than tadpoles reared in control tanks. In a subsequent survival experiment with sunfish predators, however, the survival rates did not differ between the two groups of tadpoles. Thus, our results suggest that Lowland Leopard Frog tadpoles perceive Green Sunfish as potential predators, but the induced morphological changes and the experience of prior exposure do not confer a survival advantage. Tadpoles of many species face a low probability of reaching metamorphosis because of competition for food resources and high predation, both of which can be highly variable and unpredictable (McDiarmid and Altig, 1999). To cope with this uncertainty and the conflicting demands of foraging and predator avoidance, tadpoles have evolved a suite of behavioral and morphological traits—called inducible defenses—that are expressed in the presence of known predators (Harvell, 1990). Behaviorally, tadpoles will decrease their swimming activity levels and spend more time in refuges to minimize the chances of fatal encounters with predators (Petranka et al., 1987; Kats et al., 1988; Kats and Dill, 1998; Hoff et al., 1999). Tadpoles of some species will also alter their tail and body morphology when raised in the presence of predators. The induced morphological changes are likely speciesand predator-specific, but a common response observed in many families including Ranidae (Van Buskirk, 2002b; LaFiandra and Babbitt, 2004; McIntyre et al., 2004; Relyea, 2004; Steiner, 2007), Hylidae (Van Buskirk et al., 1997; Lardner, 2000), and Myobatrachidae (Kraft et al., 2005) is an increase in tail fin depth, which may provide greater propulsion surfaces and increased burst speed (Van Buskirk and Relyea, 1998; although see Van Buskirk and McCollum, 2000; Dayton et al., 2005). In addition, larger and more brightly colored tails (Caldwell, 1982) may serve to deflect attacks away from the head toward the tail region (Van Buskirk et al., 2003; Johnson et al., 2008). Collectively, this suite of morphological and behavioral changes can confer a survival advantage when faced with a known predator (e.g., Lima and Dill, 1990; McCollum and Van Buskirk, 1996; Van Buskirk and Relyea, 1998; Alvarez and Nicieza, 2006). However, the mechanisms and specificity underlying predator-recognition in tadpoles remain poorly elucidated (Relyea, 2004; Smith et al., 2008b). As a result, it is difficult to predict how a species will react to the presence of a nonnative predator during initial encounters. In some instances, prey may be naı̈ve to the danger represented by an introduced predator (Cox and Lima, 2006; Salo et al., 2007; Smith et al., 2008a). For example, Red-Legged Frog (Rana aurora) tadpoles experienced 56% mortality in experimental enclosures with introduced American Bullfrogs (Rana catesbeiana)—versus 4% mortality in control enclosures without the predator—when they came from populations that had historically never been exposed to 1 Present address: Department of Natural Resources Management, Texas Tech University, Lubbock, Texas 79409 USA 2 Corresponding Author. Present address: State University of New York, College of Environmental Science and Forestry, Syracuse, New York 13210 USA; E-mail: [email protected] Journal of Herpetology, Vol. 43, No. 3, pp. 460–468, 2009 Copyright 2009 Society for the Study of Amphibians and Reptiles