The trade-off between fleeing and tonic immobility behaviors in an ectothermic animal

Abstract

When a prey is perceived and attacked, it may adopt direct anti-predator strategies, such as fleeing and tonic immobility (TI). During this latter, individuals remain in a motionless posture, which may reduce chances of being preyed, but retain close distance to the predator. Alternatively, fleeing increases distance from the threat, while enhancing chances of predation by visually guided predators. Thus, there is a trade-off between these two opposite direct anti-predator strategies. Here, we characterize the balance between these two behaviors in Zabrotes subfasciatus, a beetle that when stimulated displays binary responses: TI or running away. We investigated which factors determine TI frequency, probability, and duration. We examined the roles of period of the day (3 AM, 9 AM, 3 PM, and 9 PM), environmental temperature, sex, body size, and consecutive stimuli (three successive). None of these factors influenced TI duration. However, TI exhibition was higher at 3 PM, with no effects regarding sex or stimulus. As temperature raises, TI probability increases. This is paradoxical because ectothermic animals are expected to present increased locomotive activity under higher temperatures due to metabolic dependence. As far as we know, this is the first report of temperature positive relating to TI, especially in an ectothermic animal. At hotter periods of the day, while visually guided ectothermic animals are more active, Z. subfasciatus TI frequency is higher. As TI success is based on the lack of movement, which triggers killing behaviors of visually guided predators, this association may increase chances of survival against ectothermic predators.