Thermal tolerances of different life stages, sexes, and species of widow spiders (Araneae: Theridiidae)

Temperature strongly shapes the physiology and distributions of ectotherms. Environmental extremes and the range of temperatures encountered can limit persistence. Further, thermal tolerance limits are thought to be one of the factors limiting the distributions of invasive species. The critical thermal maximum (CTMax) and critical thermal minimum (CTMin) are metrics frequently applied for defining upper and lower thermal tolerances, respectively. Temperatures contained within the boundaries of the CTMax and CTMin comprise the thermal tolerance range. In this study, we tested whether thermal tolerances differed between two native (Latrodectus hesperus Chamberlin & Ivie, 1935 and L. mactans (Fabricius, 1775)) and one invasive (L. geometricus CL Koch, 1841) widow species (Latrodectus Walckenaer, 1805). We separately tested if thermal tolerances differed by life stage and sex. Using species distribution modelling, we also tested if thermal extremes or temperature range most accurately correspond with current species occurrences in the United States. We hypothesized that Latrodectus geometricus would have lower thermal tolerance range and that minimum temperatures would most influence their present distributions since they presumably originated from a more equatorial climate. We found that CTMin did not vary by developmental stage, but was highest in males. We found that the CTMin and CTMax of L. geometricus (3.2, 51.5 °C) and L. mactans (1.9, 52.4 °C) were higher than L. hesperus (-1.9, 49.6 °C). Males also had the narrowest thermal tolerance range in the three species examined. Lastly, we found that the highest performing distribution model was the one correlating minimum temperature with L. geometricus occurrences.