Plasticity of the red hourglass in female western black widow spiders (Latrodectus hesperus): urban ecological variation, condition-dependence, and adaptive function

Urbanization provides an excellent opportunity to examine the effects of human-induced rapid environmental change (HIREC) on natural ecosystems. Certain species can dominate in urban habitats at the expense of biodiversity. Phenotypic plasticity may be the mechanism by which these 'urban exploiters' flourish in urban areas. Color displays and condition-dependent phenotypes are known to be highly plastic. However, conspicuous color displays are perplexing in that they can be costly to produce and may increase detection by enemies. The Western black widow spider () is a superabundant pest species that forms dense aggregations throughout metropolitan Phoenix, Arizona, USA. Adult female display a red hourglass on their abdomen, which is speculated to function as a conspicuous warning signal to enemies. Here, I performed field studies to identify how widow morphology and hourglass color differ between urban and desert subpopulations. I also conducted laboratory experiments to examine the dietary sensitivity of hourglass coloration and to identify its functional role in the contexts of agonism, mating, and predator defense. My field data reveal significant spatial variation across urban and desert subpopulations in ecology and color. Furthermore, hourglass coloration was significantly influenced by environmental factors unique to urban habitats. Desert spiders were found to be smaller and less colorful than urban spiders. Throughout, I observed a positive correlation between body condition and hourglass size. Laboratory diet manipulations empirically confirm the condition-dependence of hourglass size. Additionally, widows with extreme body conditions exhibited condition-dependent coloration. However, hourglass obstruction and enlargement did not produce any effects on the outcome of agonistic encounters, male courtship, or predator deterrence. This work offers important insights into the effects of urbanization on the ecology and coloration of a superabundant pest species. While the function of the hourglass remains undetermined, my findings characterize the black widow's hourglass as extremely plastic. Plastic responses to novel environmental conditions can modify the targets of natural selection and subsequently influence evolutionary outcomes. Therefore, assuming a heritable component to this plasticity, the response of hourglass plasticity to the abrupt environmental changes in urban habitats may result in the rapid evolution of this phenotype.