Conspicuous cruciform silk decorations deflect avian predator attacks

Abstract

Although camouflage as an effective antipredator defense strategy is widespread across animals, highly conspicuous color patterning is not uncommon either. Many orb-web spiders adorn their webs with extra bright white silk. These conspicuous decorations are hypothesized to deter predators by warning the presence of sticky webs, camouflaging spiders, acting as a decoy, or intimidating predators by their apparent size. The decorations may also deflect predator attacks from spiders. However, empirical evidence for this deflection function remains limited. Here, we tested this hypothesis using the X-shaped silk cruciform decorations built by females of Argiope minuta. We employed visual modeling to quantify the conspicuousness of spiders and decorations from a perspective of avian predators. Then, we determined actual predation risk on spiders using naïve chicks as predators. Spider bodies and decorations were conspicuous against natural backgrounds to the avian visual systems. Chicks attacked the spider main bodies significantly less frequently on the decorated webs than on the undecorated webs, thus reducing predation risk. When both spiders and decorations were present, chicks also attacked the spider main bodies and their legs or decorations, and not randomly: they attacked the legs or decorations sooner and more frequently than they attacked the main bodies, independent of the ratio of the surface area between the decoration and spider size. Despite the increase in detectability, incorporating a conspicuous cruciform decoration to the web effectively defends the spider by diverting the attack toward the decoration or leg, but not by camouflaging or intimidating, thus, supporting the deflection hypothesis.

Safety is increasingly important in cobweb spiders based on life history

“…Email: sparassidae@aliyun.com # Haixin Zhang and Rui Zhong contributed equally to this work. shelters (Wang et al 2022). When resources are limited, animals must allocate their resources and energy between defense and foraging to maximize their fitness (McArthur et al 2014;Bonnot et al 2017;Camp et al 2017;Wang et al 2022).…”

“…shelters (Wang et al 2022). When resources are limited, animals must allocate their resources and energy between defense and foraging to maximize their fitness (McArthur et al 2014;Bonnot et al 2017;Camp et al 2017;Wang et al 2022). Therefore, foraging decisions could reflect trade-offs between food intake and safety (Godvik et al 2009;Embar 2014;Javier et al 2018), and could be influenced by an animal's age, life stage, or internal state (Scharf et al 2011;Anotaux et al 2014;Keziah & Dhruba 2015;Pasquet et al 2018).…”

“…2019). In sit‐and‐wait predators, their protective structures serve as foraging devices, mating and breeding platforms, and anti‐predator shelters (Wang et al 2022). When resources are limited, animals must allocate their resources and energy between defense and foraging to maximize their fitness (McArthur et al.…”

Outstanding issues in the study of antipredator defenses

“…The decoy hypothesis, described by Eberhard ( 2020 , p. 169), suggests that these decorations “form a decoy object that resembles the spider and distracts a predator's attention from the nearby spider” (see Table 1 ). However, decoys are not part of the spider's body, so they should not be classified as a distractive marking to the predator (Wang et al., 2022 ). And since it diverts attack to an area that is not part of the spider's body, it should not be considered a deflective marking either.…”

Web decoration is important in mediating foraging site fidelity as well as prey supplement and predation risk in an orb‐web spider

“…It is known that more than 22 genera of web‐building spiders in at least three families (Araneidae, Tetragnathidae and Uloboridae) construct all kinds of decorations in their webs, including silk decorations and detritus decorations (Herberstein et al 2000, Walter and Elgar 2012). The two widely accepted major functions of the silk decorations is prey lure or predator avoidance (Tan et al 2010, Wang et al 2021, 2022) though it is still in dispute (Eberhard 2020). The function of the detritus decorations is predator avoidance (Ma et al 2020).…”