Extreme silk toughness in Caerostris spiders is limited to adult females

Spider silks are protein fibers, renowned for their remarkable intrinsic mechanical properties. Dragline or major ampullate (MA) silk is particularly noteworthy for its combination of high tensile strength and extensibility, resulting in exceptional toughness, i.e., the amount of the absorbed energy before breaking, outperforming most biological and synthetic materials. While other spider silks, from fibers to adhesives, also exhibit intriguing mechanical properties, research has predominantly focused on MA silk due to its potential applications in biomimetics.

Orb weavers evolved some of the toughest MA silk, reaching extremes in bark spiders, genus Caerostris (Araneidae). Darwin’s bark spider (Caerostris darwini) represents a particularly interesting case of web evolution, uniquely inhabiting the air column above rivers and lakes of Madagascar, where it builds the largest known orb webs. Its congeners seem to utilize similarly tough silk, with variation in toughness attributed to differences in gene expression among species.

Theory predicts that metabolic costs to synthesize silks may vary due to differences in costs of synthesizing different amino acids. For example, MA silk of C. darwini is rich in proline, an amino acid predicted to be metabolically expensive. Transitions (phylogenetic/ontogenetic) to larger body sizes are expected to drive coevolution of tougher, costlier silk, because larger prey presents disproportionally higher kinetic energy. Shifts to tougher MA silk are documented among species, but within-species patterns are almost unknown.

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.

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