Abstract
Body temperature can strongly influence fitness. Some Sun-exposed ectotherms thermoregulate by adjusting body posture according to the Sun's position. In these species, body elongation should reduce the risk of heat stress by allowing the exposure of a smaller body area to sunlight. Therefore, selection should favour more elongated bodies in Sun-exposed than in Sun-protected species. Diurnal orb-web spider species that sit on their webs are more likely to be Sun-exposed, on average, than nocturnal or diurnal shelter-building species. We measured the body elongation of orb-web spiders (Araneae, Araneidae) across 1024 species and classified them as Sun-protected or exposed based on the literature. We found that Sun-exposed species evolved more elongate bodies than Sun-protected ones. Further, we built a model combining traditional heat transfer models with models of thermoregulatory postures in orb-web spiders and meteorological data. The model indicates that body elongation in large orb-web spiders decreases the risk of high body temperatures. Overall, our results suggest that Sun exposure influenced the evolution of body shapes of orb-web spiders.
Highlights
Internal body temperature can have a major impact on an individual’s fitness [1,2,3]
We evaluated whether the evolution of body elongation in Neotropical orb-web spiders of the Araneidae family is explained by the likelihood of Sun exposure
Data on Neotropical orb-web spider species suggest an effect of Sun exposure on body elongation
Summary
Internal body temperature can have a major impact on an individual’s fitness [1,2,3]. We built a heat transfer model to estimate the effect of body size and elongation on the body temperature of Sun-exposed and Sun-protected spiders. We estimated body elongation and area of spiders by measuring dorsal images extracted from taxonomical articles on Neotropical species (electronic supplementary material, table S1).
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