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-web spiders.

We estimated body elongation and area of spiders by measuring dorsal images extracted from taxonomical articles on Neotropical species. These images were processed using ImageJ software (v. 1.8.0). We drew over the abdomen with the polygon feature and recorded two outputs: total area and roundness [38]. Roundness may vary between 0 and 1, with the latter being a perfectly round shape (equal minor and major axes; Supplementary Material eq. 6).

We classified spider genera as either sun-exposed or sun-protected primarily based on the scientific literature. We classified diurnal spiders that sit on the web hub during the day as sun-exposed and nocturnal and diurnal spiders that hide from the sun (by shelter construction or waiting on vegetation) as sun-protected.

We built a heat transfer model to estimate the equilibrium temperature of animals of varying elongations (roundness), volumes, and angles of exposure to sunlight. We coupled classical models on heat exchange of ectotherms with a model on thermoregulatory postures of orb-web spiders plus a body elongation parameter.

Further, we estimated the number of days per year sun-exposed orb-web spiders would reach ≥35°C and ≥40°C. We applied the heat transfer model to 10 years of meteorological data (2011-2020) collected hourly (wind speed, solar radiation and air temperature) in automatic stations (-34° to 0° Lat. and -36° to -73° Long.).

Code and analyses were run in R. Package 'brms' needs to be installed to open statistical models. Open RDS files using the readRDS function. Data are stored in model_name$data.