Effects of Size, Body Mass and Body Condition on Tonic Immobility Occurrence in Lissotriton vulgaris

Tonic immobility is a state of natural paralysis that species across the animal kingdom exhibit as an instinctive response to an external threat. In ectotherms, it has been linked with temperature, but the effects of individual size, body mass and body condition are still poorly understood. We hypothesize that under natural conditions, size, body mass and body condition play an important role in tonic immobility occurrence in smaller ectothermic animals such as the Smooth newt. In this study, we photographed, measured and weighed 357 adult Lissotriton vulgaris from four populations across Bulgaria in the period December 2025–June 2026. All observed cases of tonic immobility were noted and subsequently analyzed against the effects of newt size, body mass and body condition, as well as ambient air temperature. Our results indicated that smaller newts, with lower body mass, were more prone to displaying the behaviour than larger newts, while sex and ambient temperature had no observable effects.