COS 205-3 - CANCELLED - Body mass, not web geometry, shapes mass-specific energy intake among web-building spiders

Background/Question/Methods

Web-building spiders use traps composed of silk to capture insect prey. Metabolic theory predicts that organisms should need less food per unit of mass as they increase in size. Spider webs, therefore, need not increase isometrically with spider size. It is unclear, however, how the allometric relationships between spiders, their webs, and the prey they capture are affected by web geometry. To assess how spider webs and the energy they capture scale with spider mass across web geometries, we estimated web, body size, and prey capture metrics (including prey order and size, number of prey per hour, and prey biomass per hour) on over 300 web-building spiders in Pacific Spirit Park, Vancouver, British Columbia.

Results/Conclusions

We found that a web’s standing silk content per unit of spider mass indeed declined as spider mass increased in orb, tangle, and sheet-and-tangle building spiders. Sheet-and-tangle webs, however, contained around two orders of magnitude more silk for a given spider mass than the other two web types, thus being potentially less efficient prey capture devices per unit of standing web mass. Prey capture surface area per unit of spider mass, however, was constant as spider mass increased in sheet and tangle webs- potentially allowing them to overcome the lower efficiency of their prey capture webs- whereas it declined in both orb and tangle building spiders. As a result, hourly energy intake per unit of spider mass declined with identical slopes and intercepts as spider mass increased across all three web geometries. Our findings suggest that, despite striking differences in their web geometries, spiders build webs that are large enough to supply them with the energy they need, and no bigger.