Competition reduces web investment without altering silk properties in the common house spider

Abstract Organisms must be able to adaptively alter their behavior in response to local competition intensity, as this can dictate the optimal strategy for obtaining resources. These behavioral responses may be particularly strong in sessile organisms, such as cobweb (Theridiidae) spiders, which rely upon their web for foraging and safety. Spiders can adjust web structure and silk biomechanical properties in response to their environment, indicating they may be able to respond similarly to other stimuli, including conspecific competition. We quantified the web phenotype and aggregate silk (prey-capture glue) biomechanical properties of Parasteatoda tepidariorum females both before and after cohabitation with conspecific females. We measured the density of silk threads in the upper tangle and gumfoot web sections as a proxy for energy investment into either safety or prey capture. We also tested the adhesive strength of gumfoot prey-capture glue and measured glue droplet volume. We hypothesized that, because silk production is energetically costly, spiders would supplement their investment into prey capture and safety by producing more and stronger glue in lieu of solely increasing the density of safety and gumfoot lines. Surprisingly, we found that spiders produced smaller upper tangles and fewer prey-capture threads following cohabitation while the prey-capture glue properties remained unchanged. These behavioral responses could indicate that spiders reduced investment in safety and foraging, in contrast to our expectations. Our results also suggest that female P. tepidariorum are capable of chemical autodetection, previously found in only 1 other spider.