The Adaptive Significance Of Body Coloration Of Tropical Orb-weaving Spiders And Intra-specific Competition

Evolutionary biologists have been fascinated with animal and plant coloration since Darwin and have tested many evolutionary theories related with colors, including signali ng, crypsis, mimicry and aposematic coloration. In general, animal coloration plays important roles in inter- and intra-specific interactions, including prey attraction, anti-predator defence, species recognition and sexual selection. The function of body coloration in webbuilding spiders has received much attention in recent literature and there is increasing empirical evidence that a spider’s bright body coloration can lure insect prey, thus increasing its foraging success. Many visual oriented predators such as wasps are also known to use color signals to search for spiders. However, most previous studies testing function of body coloration of web-building spiders have exclusively focused at species level. Our field surveys showed that the abundance ratio of conspicuous to inconspicuo us orb-weaving spiders was about 4:1 in the same habitat. However, the evolutiona r y significance of diverse spider color patterns at community level remains unexplored. In this study, using tropic orb-weaving spiders as model systems and considering size, type, orientation(vertical or horizontal), symmetry, decoration and height of webs as confounding factors, we investigated the role of body coloration in the context of foraging intake and predation risk at the community level in the field. 1. Functional significance of body coloration of colour polymorphic orb-weaving spider, Macracantha arcuata(Araneae: Araneidae)Xishuangbanna is biodiversity hotspot in the world and also has the most divers it y species in China. According to our field survey, we found that Macracantha arcuata(Family: Araneidae) have four body color morphs in the same habitat: white, yel ow, red and black. The red and black morphs were vey rare in the study sites and then we used white and yellow morphs to test their adaptive significance by video recording the number of prey intercepted and the number of predators attracted. Our results showed that the white morph had higher prey capture rate than the yel ow morph; compared with yel ow morph, white morph intercepted fewer prey with increasing body size of spiders, but white morph attracted more prey with increasing web area. Moreover, the yel ow morph attracted significantly more predator than the white morph, but no significant different in the predator attack rate was found between the two colour morphs. Base on these results, we further studied the adaptive significance of brightly colored and inconspicuous diurnal, orbweaving spiders at the community level. We also measured some biotic(web capture area; mesh height and total thread lengths etc.) and abiotic(background color) factors as covariance to test prey and predator attraction of body coloration. Totally, we recorded 241 individuals of spiders belonging to three families(Nephilidae, Araneidae, Tetragnathidae) and eight genera. 2. Visual signals: brightly-colored tropical orb-weaving spidersOur results showed that bright body colorations of spiders function as a visual lure to attract more prey, and more predators hit to webs of spiders with bright body coloration. The results showed that a habitat-choice trait, vegetation density as well as other web-traits such as web orientation, web type, and decoration alone had no significant effects on the prey intakes and predator risks. However, body coloration interacting with spider orientation and web decoration, web height, web type, web orientation. Some of web traits had significant effects on the prey intakes and predator risks. Our results provide strong support for the hypothesis that bright body coloration of orb-weaving spider acts as a visua l lure for prey at the community level. 3. A nocturnal sit-and-wait predator visually lures preyThe above results revealed that different body coloration of nocturnal spiders had different fitness in nocturnal system. Base on these results, we further studied the adaptive significance of body coloration of nocturnal, orb-weaving spiders at community level. Our studies demonstrated that body coloration had significant effect on prey interception rate: brightly nocturnal spiders had higher prey interception rate than inconspicuous nocturna l spiders. In addition, spiders with abdomen outward attracted more prey than spiders with dorsal outward. Moreover, brightly-colored spiders had lowest prey interception rate when they oriented their dorsal side outward. Furthermore, there were significant interactio ns between body length, web area and body coloration in the community level. 4. The adaptive significance of body coloration of co-existing nocturnal orb-weaving spidersOur results showed that bright body colorations of spiders function as a visual lure to attract more prey in nocturnal spiders, and there is survive fitness in various body colorations. Bright body coloration use visual uare to attract more prey but increasing predator risks. They are “more risk, more gain.” In contrast, duall color body coloration of siders is “low pain, low risks” strategy at community level.5. Web invasion in nocturnal orb-weaving spidersMeanwhile we found web invasion was widespread among nocturnal orb-weaving spiders. We recorded 77 nocturnal orb-weaving individuals, and found that web invas io n rate was 36% and the web invasion successful rate was 26%. One possible explanation for web invasion is food resource on the invaded webs.In sum, the both color spiders at the same niche and co-existing, compared to dull spiders, bright color are more likely attracted prey but accompanying with higher risks. In contrast, dull spiders are camouflaged themselves and difficult to be detecting, it means lower predatory risks as well as prey intakes. Both survive stregies under different natural pressure, the results from this project are support our hypothesis.