Scaling of the extended phenotype: convergent energetics from diverse spider web geometries

Organisms capture energy to support growth, survival and reproduction in diverse ways. Larger metazoans require less energy per unit time and mass than smaller ones. Thus, structures animals build to capture energy need not scale isometrically with body size. Web-building spiders use silk structures of diverse geometries to capture energy, including two-dimensional orbs in some families or three-dimensional tangles or sheet-and-tangles, in others. Despite this diversity, we show that energy consumption rate per unit mass scaled identically with body size across all web geometries with a less than 1 : 1 relationship to body size, as expected for metazoans from metabolic theory. Spiders thus appear to adjust the size and shape of their webs in precise ways to attain this relationship, including, as we show here, creating a hollow space within certain three-dimensional web types to maintain a constant prey capture surface area per unit spider mass as they grow in size without requiring more silk. Our findings show how the allometric relationship between energetic traits and body size can be mediated by extended phenotypes and suggest an equivalence paradigm akin to the equal fitness paradigm whereby the diverse adaptive strategies of organisms allow them to perform equally well in supplying a unit of mass the energy needed across a lifetime.