Darwin’s Bark Spider Silk Redefines Natural Material Strength

One of the strongest materials on Earth is produced not in laboratories but by a spider barely a few centimetres long. The Darwin’s bark spider, Caerostris darwini, native to Madagascar’s forests, spins silk that surpasses steel and most synthetic fibres in both strength and toughness. With a tensile strength of about 1.6 gigapascals, its silk is the toughest biological material ever tested, yet scientists now show that such extreme performance is not universal across all individuals.

Silk Strength and Evolutionary Drivers

Across spider species, body size is closely linked to silk quality. Larger spiders tend to produce tougher silk to capture faster or larger prey. In orb-weaving spiders, evolutionary increases in body size have gone hand in hand with larger webs and stronger threads. The Darwin’s bark spider exemplifies this pattern by building the largest orb webs ever recorded, sometimes spanning entire rivers.

Comparative Study Under Controlled Conditions

An international team of researchers studied Caerostris darwini and its close relative Caerostris kuntneri. Egg sacs were collected from Analamazaotra National Park and reared in laboratories to control diet and humidity. The research, published in Integrative Zoology, tested whether silk toughness was uniform across sexes and ages, limited to females, or restricted to large adults with specific ecological demands.

Who Produces the Toughest Silk?

Results were clear. Only large adult females produced exceptionally tough and stiff dragline silk, capable of absorbing far more energy before breaking. Silk from males and juveniles of both sexes was mechanically similar and far weaker. Females appear to activate costly physiological pathways for high-performance silk only when their size and web-building role demand it.

Imporatnt Facts for Exams

• Darwin’s bark spider produces the toughest biological silk recorded.
• Tensile strength of its silk is about 1.6 GPa, nearly three times that of iron.
• Only large adult females spin exceptionally tough silk.
• Analamazaotra National Park is a key biodiversity site in Madagascar.

Energy Trade-offs and Web Architecture

High-performance silk is metabolically expensive, rich in amino acids such as proline. Adult females therefore build sparser but highly effective webs using less silk per area, while juveniles and males spin denser webs of cheaper material. This strategy allows energy to be invested only when it offers clear survival advantages, highlighting how body size, sex, ecology, and behaviour together shape one of nature’s most remarkable materials.