Defense investments and growth responses under different predation risks and gape-limitation predation threats in a tadpole prey

Abstract

Theories on growth and defense investments in prey have typically revolved around predators’ traits (gape and foraging tactics) and the level of predation risk in the environment, yet how these factors interact on prey responses remains poorly understood. We examined the phenotypic investments and growth rates of Rhacophorus arboreus tadpoles when exposed to “full cues” (predator present in aquaria) and to “chemical cues” (predator rearing water only) from gape-constrained newt and gape-unconstrained large Anax dragonfly larva predators at intermediate food level in a controlled laboratory experiment. Investments in behavioral defense were higher in the presence of dragonfly larvae although both predators were equally dangerous for the tadpoles. Predator identity interacted with cues level to affect defense investments but not growth in the tadpoles. Morphological changes were predator-specific: a larger tail fin in the presence of dragonfly larvae and a thicker tail muscle in the presence of newts. Investments in behavioral and morphological defenses increased from predator chemical cues to full cues. Our results provide weak support to prevailing theoretical predictions: no growth depression in the tadpoles despite strong investments in defense in the presence of dragonfly larvae and no evidence of growth acceleration in the presence of gape-limited newts. The tadpoles, however, accelerated growth when exposed to chemical cues from dragonfly larvae, but this trait disappeared in the full cues treatment when investments in defense increased. Our results indicate that predators’ gape may not necessarily predict life-history responses in prey; instead, local selection regime and the level of predation risk might modulate these responses. Multiple theories predict the evolution of growth and defense in prey in response to predator’s gape and the level of risk in the environment. At high predation risk, an investment in defense at the cost of growth is anticipated in the presence of gape-unconstrained predators, whereas a rapid growth strategy could be advantageous in the presence of gape-limited predators. We exposed tadpole prey to different cue levels of gape-constrained newt and gape-unconstrained aeshnid dragonfly larva predators. Predator identity and cue level interacted on tadpole defensive phenotypes, but not on growth. The tadpoles accelerated growth when exposed to lower concentration cues of dragonfly larvae but prioritized defense at higher cue concentration, indicating that cue level can influence prey decision on whether to invest in growth or defense. Growth responses did not follow any theoretical predictions, but could be more related to the local selection regime than to predator’s gape.