Animals adjust behaviors to balance changes in predation risk against other vital needs. Animals must therefore collect sensory information and use complex risk assessment process that estimates risks and weigh costs and benefits entailed in different reactions. Studying this cognitive process is challenging, especially in nature because it requires inferring sensory abilities and conscious decisions from behavioral reactions. Our goal was to address this empirical challenge by exploring the considerations used by desert isopods (Hemilepistus reaumuri) to assess the risk of scorpions that hunt exclusively from within their burrows. To do so, we developed a conceptual risk-assessment model and implemented psychophysical principles to field research. In a series of semi-nested field trials, we introduced various combinations of chemical and physical cues in the vicinity of the isopods' burrows. We recorded their detailed reactions upon first encountering the cues and analyzed their full range of behavioral responses using a pre-developed dichotomous decision-tree.
Results/Conclusions
Isopods can detect the mere odor of a scorpion, but reacted defensively only when the odor was accompanied with excavated-soil or other odors typically found near scorpion burrows. Isopods also reacted defensively to piles of excavated soil without scorpion olfactory cues, suggesting that isopods take precautions even against physical disturbances that do not necessarily reflect predator activity. As far as we know, our findings are the first example in which the mere existence of a common physical disturbance was perceived as a threat, dangerous enough to generate escape responses. Simultaneous presence of different cues (chemical and physical) provoked graded responses, possibly reflecting an additive increase in risk estimation. We conclude that wild isopods use defensive reactions toward environmental signals only when the integrated perceptual information implies an active scorpion burrow (i.e., a potential ambush site) or when they lack data to refute this possibility. In summary, our psychophysical based approach enabled us to disentangle sensory abilities from conscious decisions, revealing considerations used by wild isopods to assess scorpion predation risks.