Heavy metal pollution is pervasive in urban soils, and it can negatively impact the fitness of arthropods. Arthropod populations can evolve to become more tolerant or resistant to metals, but the mechanisms of these processes are understudied. Here, we tested the hypotheses of fixed adaptation and facultative adaptation in a lead (Pb)-exposure experiment using the progeny of field-collected Pardosa milvina spiders from urban (elevated soil Pb) and rural (background soil Pb) habitats. We predicted that spiders from both populations that were reared in the elevated Pb treatment would exhibit decreased body size in comparison to individuals reared in background Pb soil due to a facultative shift in metabolic investment from growth to Pb resistance, or, alternatively, that urban spiders would exhibit no change in body size across treatments, supporting fixed adaptation as a consequence of a legacy effect of urban Pb pollution. We found no evidence that exposure to elevated Pb soil had an effect on P. milvina’s adult body size or survival, regardless of population origin. Our results indicate that the amount of Pb accumulated by the spiders was not sufficient to induce a measurable change in body size, or that there was a fitness cost that was not measured in the experiment, such as changes in fecundity or body condition. Overall, these findings do not provide support for fixed or facultative adaptation to Pb pollution in P. milvina; as such, future studies should investigate other possible tradeoffs, including changes in egg size, body weight, and foraging effort.

Offspring from field-collected spiders (Pardosa milvina) were reared on soil substrates with three concentration ranges of Pb. The spiders came from two populations (rural and urban) for a total of six experimental treatments. The spiderlings were reared on a diet of springtails and crickets until they matured and eventually died. We recorded the survival time of adult spiders (n = 41-61) and measured the carapace width of a subset of specimens in each group upon death (n= 7-15). Measurements were made with an ocular micrometer (eye piece units) and converted to milimeters.