SCHAR, D.W.*; KRUG, P.J.; ZIMMER, R.K.: The scent of danger: Chemical alarm signals and escape from cannibalism in newts

Larvae of the California newt exhibit a chemically mediated anti-predator defense, demonstrating escape behavior and increased refuge use after detecting a cue from cannibalistic adults. Stream water collected near adults was tested in non-recirculating flow-through chambers, and induced alarm responses in 80 to 100% of larvae. Solutions were prepared by bathing adults (in the field), and these stimuli also caused strong alarm reactions. Blockage of the adult cloaca with inert gel did not diminish bathwater bioactivity, indicating the alarm substance was not an excretory product. Swabs of adult backs, sides, and bellies were all highly active in solution, showing the chemical cue was released from adult skin. Reversed-phase HPLC analysis revealed a potent sodium channel blocker, tetrodotoxin (TTX), and related structural isomers present in skin swabs, and in bathwater at a 10^-7 M. A TTX standard was tested in the behavioral assay at concentrations from 10^-7 to 10^-9 M, along with equivalent dilutions of bathwater. At each concentration, bathwater and the corresponding TTX solution triggered an alarm response from the same number of larvae, with no subsequent sublethal toxicity. Two additional sodium channel blockers, saxitoxin (STX) and µ-conotoxin (CTX), were assayed, and neither substance caused an escape reaction. Larvae exhibited a minor sublethal response to CTX distinct from all other solutions, and had no reaction to STX. Taken together, our results show that newt larvae: (1) escape cannibalism by detecting toxins used in adult chemical defenses against snakes and other vertebrate predators, and (2) possess sensitive chemoreceptors for tetrodotoxin, mediating a complex series of behavioral reactions unrelated to any ill physiological effects.