Spiders are underrated. We tend to think of them as creepy crawly cellar dwellers or as Halloween decor, but there is a lot more to spiders than that. Rather than hiding out in dark crevices, many spend their days fully exposed on their webs, enduring heavy rain, howling wind or bright sunlight. Spiders also come in a huge variety of shapes: some have very round, ball-like abdomens, whereas others have an elongated or cylindrical back end. With all this diversity in lifestyle and form, it's not surprising that spiders thrive in every terrestrial habitat, often with the aid of curious adaptations to the peculiarities of their home turf. Interested in the many ways that spiders live their lives and how they cope with environmental challenges, Leonardo Ferreira-Souza from the University of Brasilia, Brazil, and colleagues investigated how a spider's daily dose of sunlight shapes its body.
Ferreira-Souza and colleagues hypothesized that elongated abdomens were more common in spiders routinely exposed to the sun and that this feature protected them from overheating, as some spiders point the end of their abdomen towards heat sources such as the sun. For example, if a spider's abdomen is cylindrical, then this unusual posture could keep most of their body surface out of direct sunlight and help them stay cool. To test their idea, the team scoured museum and scientific archives for photos and illustrations of orb spiders – the most common spiral web-spinning spiders – from the Caribbean, Central America and South America, measuring the total area and roundness of as many spider abdomens as possible. They also classified each spider as either sun exposed or sun protected, where sun-exposed spiders were active during the day, typically soaking up the rays sitting on their webs, while sun-protected spiders were active at night or spent their waking daytime hours in shelters such as leaf litter.
Thousands of measurements later, it was clear that the sun-exposed spiders had more cylindrical body shapes on average than sun-protected spiders. The scientists then turned their attention to the physiological implications of the spiders’ abdomen shapes, calculating how warm different shaped arachnids would get under natural conditions. They paid special attention to how often their simulated spiders exceeded two key temperatures: 35°C, a temperature that most spiders avoid, and 40°C, a temperature where many spiders show symptoms of overheating such as leg spasms or slipping into a coma. If the elongated body shape was adaptive for heat management, then cylinder-shaped spiders should be less likely to reach dangerously hot body temperatures.
As predicted, the cylinder-shaped spiders stayed cool. On a typical 30°C day, the body temperature of an elongated spider would only reach 33°C, whereas a round spider of the same size would hit a dangerous 40°C. By plugging in 10 years’ worth of historical weather data, the researchers estimated that the round spider would experience 85 days per year at or above an uncomfortable 35°C, including 8 days when its body temperature would be critical at 40°C. In contrast, the simulated elongated spider had much fewer heat stress days, only reaching a body temperature above 35°C for 36 days per year and never experiencing a body temperature above 40°C.
Like many animals, spiders have adapted their bodies to survive the challenges of their habitat. Unlike many animals, at least unlike many vertebrates, spiders have literally changed the shape of their bodies as a thermoregulatory strategy, stretching out their abdomens over evolutionary time to protect themselves from overheating. Spiders are way cooler than we give them credit for!
