Oecologia (2004) 138: 350–359 DOI 10.1007/s00442-003-1412-3 POPULATION ECOLOGY Emily May LaFiandra . Kimberly J. Babbitt Predator induced phenotypic plasticity in the pinewoods tree frog, Hyla femoralis: necessary cues and the cost of development Received: 3 June 2002 / Accepted: 17 September 2003 / Published online: 12 December 2003 # Springer-Verlag 2003 Abstract Predator-induced defenses can result from non- contact cues associated with the presence of a feeding predator; however, the nature of the predator cue has not been determined. We tested the role of two non-contact cues, metabolites of digestion of conspecific prey released by the predator and alarm pheromones released by attacked conspecific prey, in the development of inducible defenses by exposing pinewoods tree frog (Hyla femor- alis) tadpoles to non-lethal dragonfly (Anax junius) larvae fed either inside experimental bins or removed from the bins for feeding to eliminate alarm pheromones. The costs associated with the development of the induced morphol- ogy were also investigated by providing the tadpoles with two food levels intended to provide adequate or growth limiting resources. The generalized morphological re- sponse of H. femoralis tadpoles to predators included the development of bodies and tails that were both deeper and shorter, smaller overall body size, and increased orange tail fin coloration and black tail outline. Metabolites of digestion were sufficient to initiate development of inducible defenses; however, the combination of metabo- lites and alarm cue resulted in a greater response. Furthermore, growth and development were slowed in tadpoles that expressed the induced morphology; however, this growth cost was insufficient to preclude the develop- ment of the induced morphology when food resources were low. These results indicate that two aspects of the indirect predator cue work together to trigger a morpho- logical anti-predator response. Keywords Anax . Anuran . Inducible defenses . Multiple stressors . Pheromones Introduction Phenotypic plasticity is the ability of an organism to change its phenotype during the course of development in response to environmental conditions (Van Buskirk et al. 1997). This ability allows organisms to develop traits precisely suited to the environment without becoming genetically specialized, thus making them capable of inhabiting a wide variety of habitats. The ephemeral habitats occupied by many larval anurans are characterized by heterogeneous environmental conditions, including variability in hydroperiod, predator regime, and compe- titor community composition (Skelly 1997; Tarr 2000). These environmental conditions induce phenotypic plas- ticity in the life history, morphology, and behavior of larval anurans (e.g., Van Buskirk et al. 1997; Van Buskirk and McCollum 1999; Relyea and Werner 2000). In larval anurans, the traits developed in response to the predator regime, inducible defenses, have been suggested to reduce detection rates by predators and/or increase the potential for escape when detection occurs (e.g., McCollum and Van Buskirk 1996; McCollum and Leimberger 1997). Tadpoles generally develop deeper tail fin and tail muscle, and smaller body length and width in response to predators (Smith and Van Buskirk 1995; McCollum and Leimberger 1997; Van Buskirk and Relyea 1998; Relyea 2000, 2001a, 2001b; Relyea and Werner 2000). For some species, the response to predators also includes development of bright red-orange or black tipped tails (Caldwell 1982; McCol- lum and Van Buskirk 1996). Inducible defenses are expressed only in response to a predator stimulus and must have an associated fitness cost or they would be constitutive, or genetically fixed (McCollum and Leimberger 1997; Relyea 2002). Fitness costs are typically expressed in terms of reduced repro- duction; however, for larval anurans this cost is widely separated from expression of the defensive phenotype, making assessment of the true fitness costs associated with inducible defenses in larval anurans difficult. The growth rate during the larval stage is, however, assumed to impact adult fitness as it determines the timing of and size at E. M. LaFiandra . K. J. Babbitt (*) Department of Natural Resources, University of New Hampshire, 226 James Hall, Durham, NH 03824, USA e-mail: kbabbitt@cisunix.unh.edu Tel.: +1-603-8624287 Fax: +1-603-8624976