Data S1. Laboratory behavioral data. The composition is summarized in figure S1.

Autotomy, the self-amputation of body parts, serves as an anti-predator defense in many taxonomic groups of animals. However, its adaptive value has seldom been quantified. Here, we propose a novel modeling approach for measuring the fitness advantage conferred by the capability for autotomy in the wild. Using a predator-prey system where a land snail autotomizes and regenerates its foot specifically in response to snake bites, we conducted a laboratory behavioral experiment and a 3-year multi-event capture–mark–recapture (CMR) study. Combining these empirical data, we developed a hierarchical model and estimated the basic life history parameters of the snail. Using samples from the posterior distribution, we constructed the snail’s life table as well as that of a snail variant incapable of foot autotomy. As a result of our analyses, we estimated the monthly encounter rate with snake predators at 3.3% (95% CI: 1.6–4.9%), the contribution of snake predation to total mortality until maturity at 43.3% (15.0–95.3%), and the fitness advantage conferred by foot autotomy at 6.5% (2.7–11.5%). This study demonstrated the utility of the multi-method hierarchical modeling approach for the quantitative understanding of the ecological and evolutionary processes of anti-predator defenses in the wild.