Differential use of visual and chemical cues in predator recognition and threat-sensitive predator-avoidance responses by larval newts (Notophthalmus viridescens)

For prey under the threat of predation, the ability to distinguish between different levels of danger can have important fitness consequences. Larval central newts, Notophthalmus viridescens louisianensis, distinguished between predatory (Ambystoma tigrinum tigrinum larvae) and nonpredatory (Hyla chrysoscelis/versicolor complex tadpoles) heterospecifics, but only when chemical cues were available. When only visual cues were present, larvae responded to both predatory and nonpredatory stimuli by reducing activity (fright response), but did not distinguish between the two types. Fine-scale discrimination of visual stimuli may have failed to develop because larval newts typically live in aquatic habitats in which chemical cues may be more reliable than visual cues, owing to large amounts of sediments and vegetation or possibly to myopia. Late-stage newt larvae that were approaching metamorphosis were unpalatable to A. t. tigrinum, and histological examination of the skin revealed that granular (poison) glands were present in the skin of late-stage but not early-stage larvae. Late-stage larvae did not distinguish between chemical stimuli from predators and nonpredators, which suggests that fright responses of larval newts are plastic and can be modified according to the level of perceived threat.