Relevance
- GS 3: Biodiversity and adaptation in unique ecosystems.
- GS 3: Science and technology, evolutionary biology, biomaterials, and bio-inspired innovation.
- Species & Habitat: Darwin’s bark spider is endemic to Madagascar and is known for building the largest orb webs recorded, often spanning up to 25 metres across rivers and lakes.
- Record-breaking Silk: Its dragline silk has a tensile strength of ~1.6 GPa, making it the toughest biological material ever tested, around three times stronger than iron and tougher than steel.
- Key Scientific Finding: Only large adult females produce this ultra-tough silk; silk from males and juveniles is significantly weaker and mechanically indistinguishable across sexes and ages.
- Reason for Female-only Tough Silk:
- Adult females are 3–5 times larger than males, facing stronger evolutionary pressure to support massive webs.
- Tough silk evolved primarily to structurally support huge webs, not to catch specific prey.
- Energy–Efficiency Trade-off:
- Producing high-performance silk is metabolically expensive, requiring costly proteins like proline.
- Females therefore produce less silk overall, rebuild webs more slowly, and invest in quality over quantity.
- Web Architecture Strategy:
- Female webs are sparser, with wider gaps and fewer threads, but each thread absorbs very high mechanical strain.
- Males and juveniles spin denser webs using cheaper, weaker silk.
- Genetic vs Adaptive Traits:
- Elasticity of silk is genetically conserved across all individuals.
- Extreme toughness is selectively “switched on” in large females based on body size and ecological demand.
- Evolutionary Significance:
- Demonstrates sex-specific adaptive evolution, where costly biological materials are produced only when they provide clear survival advantages.
