Size, Spines, and Nests: What Drives Variation in Death-Feigning Behavior in Fungus-Farming Ants?

Death feigning (DF) is a widespread antipredator strategy among animals, yet its expression in social insects is not well understood. Here, we investigate DF in 10 species of fungus-farming ants (Attini) using standardized mechanical stimuli that mimicked different levels of predation threat. Across 5880 behavioral trials, both the probability and duration of DF varied markedly with stimulus type, with forceps eliciting the strongest responses. Generalized linear mixed models revealed pronounced hierarchical structuring, with species, colony, and individual identities each making a substantial contribution to variation in DF. This indicates that substantial variation in DF occurs among species, colonies, and individuals and is influenced by multiple biological factors. At the species level, the propensity and duration of DF were strongly associated with morphological traits. In a principal component analysis, both traits increased toward the high values of PC1, corresponding to smaller-bodied, weakly armored, and more pilose workers. PC2 captured a contrast between armature and pilosity that further differentiated defensive strategies. Nest structure exerted an additional influence: Species with shallower nests were more likely to feign death, whereas deeper-nesting taxa prolonged immobility once initiated. Together, these findings demonstrate that DF in fungus-farming ants emerges from the interplay between perceived threat level, morphological defenses, and nesting ecology, revealing immobility as an integrated component of the defensive syndrome of attines.