Spider’s Web is Not Decorated for Fun, It’s a Defense Shield
Was the spider’s web you found very intricate and shiny? Do you know that this could indicate several things about the spider and it’s apprehension of the environment around it?
Animals are known to use conspicuous visual signals too allure prey or deter predators. Using colors, glossiness, ornaments are some of the well known visual signals. Sometimes the colors are used to advertise to the predators about unpalatability or harmfulness. These signals may have a cost, for example metabolic expenditure and generating the color pigment. Visual signals used to attract prey could also land up attracting predators and thus involves a trade off based on the situation.
Unlike most animals which use body coloration for conspicuous appearance, Orb Spiders attach a white silk decoration to their webs. The silk decorations are known to reflect UV light, to which most prey such as huge spiders, insects, and birds are sensitive to. Spiders alter their signal strength by changing the size, shape and number of silk decorations.
Research on the intended purpose of these silk decorations has been highly contradicting in the past. These have been studies which show that the web decoration increases prey success while a few studies show that webs without decorations attract more prey. Similarly, there have been studies which show that web decorations act as a defense against predators while a few studies show that web decorations actually attract predators. Now a researcher from Japan has tested the spiders for both of the cases (prey attraction and predator defense) to see how these situations affect each other.
Perceived level of prey availability was manipulated by feeding or not feeding the spiders (in limited quantities), as spiders are know to estimate prey availability based on past feeding experience. Predator proximity was manipulated using air borne vibrations using tuning forks, which mimic approaching flying insects such as wasps. Spiders in this experiment experienced one, both or none of prey availability and predator proximity manipulations.
The results from experiments, gave a very clear view on the function of the web decorations. It was observed that when exposed to predator proximity, the spiders decreased the thread length of the webs (indicating less time and energy spent, in view of the existing threat) but had longer and more number of silk decorations. On the other hand, when exposed to more prey availability, the spiders only made the threads longer but did not increase the decorations. There was also indication that there was no trade-off in web decoration based on foraging success and predator defense. All of these clearly indicate that the spider’s web decoration serves the primary purpose of defending against predators (specifically, smaller flying insects in this case) and not for attracting prey. However, the variation of this behavior when exposed to bigger predators (such as birds) is left unknown.
