Extreme Silk Toughness in <i>Caerostris</i> Spiders Is Limited to Adult Females

Major ampullate (MA) silk, synthesized by spiders, is tougher than most biological and synthetic materials. Orb weavers evolved some of the toughest MA silk, reaching extremes in bark spiders, genus Caerostris (Araneidae). Increased proline content is associated with tougher silk but may increase the metabolic cost. Transitions (phylogenetic/ontogenetic) to larger body sizes are expected to drive coevolution of tougher, costlier silk, because larger prey presents disproportionally higher kinetic energy. Interspecific shifts to tougher MA silk are documented, but intraspecific patterns are unknown, although spiders increase several hundred times in body mass through ontogeny. Small spiderlings prey on small insects and might not face the selection pressure on adults for capturing large prey. Additionally, extreme female-biased sexual size dimorphism in orb-weaving species like bark spiders results in sex-specific selection pressures for small versus large prey. We therefore ask whether species with exceptionally tough silk, like bark spiders, show different patterns in silk toughness between ontogenetic stages and sexes. We posed three hypotheses: H1, constrained silk production hypothesis; H2, sexually decoupled silk production hypothesis; H3, body size selection pressure hypothesis; and tested them by investigating the mechanical properties of MA silk among size classes and sexes in two Caerostris species from Madagascar, C. darwini Kuntner & Agnarsson, 2010 and C. kuntneri Gregorič & Yu, 2025. We found that only large females produce exceptionally tough silk with higher initial stiffness, while juvenile females and all males produce inferior silks. These results imply ontogenetic plasticity in Caerostris silk production and support the third hypothesis.