Shaped by the sun: the effect of exposure to sunlight on the evolution of spider bodies

Body temperature strongly influences fitness. Some sun-exposed ectotherms thermoregulate by adjusting body posture according to the sun's position. To evaluate the impact of body shape, size and posture on body temperature, we first built a model combining traditional heat transfer models with models of thermoregulatory postures in spiders. The model indicates that both body size and shape determine thermoregulation efficiency by affecting heat gain via solar irradiance. These estimates corroborate previous empirical studies on spider thermoregulation. We then coupled meteorological data to our heat transfer model. The model predicts that body elongation in large orb-web spiders decreases the risk of high body temperatures. Lastly, we measured the elongation of orb-web spiders across 1,024 species and found that sun-exposed species evolved more elongate bodies than sun-protected species. Overall, our results suggest that thermoregulation influenced the evolution of body shapes of orb-we...