Discoid decorations function to shield juvenile Argiope spiders from avian predator attacks

Abstract

Decorating behavior is common in various animal taxa and serves a variety of functions from camouflage to communication. One predominant function cited for decoration is to avoid predators. Conspicuous, disc-like (discoid) silk decorations spun by orb-web Argiope juvenile spiders are hypothesized, among others, to defend spiders against visual predators by concealing spider outlines on the web, deflecting attacks, shielding them from view, or masquerading as bird-droppings. However, the direct evidence is limited for a specific mechanism by which discoid decorations may deter predators. Here we evaluate the mechanisms by which discoid decorations may defend Argiope juveniles against naïve chicks. Using visual modeling, we show that avian predators are able to distinguish spiders from discoid decorations. Using chick predation experiments, we found that the naïve chicks readily pecked any objects, ruling out the possibility of their neophobia. Significantly more chicks attacked spiders when they were exposed to chicks, regardless of whether their webs had discoid decorations, but few chicks attacked spiders when they were behind the decorations. We also found that significantly few chicks attacked decorations when spiders were absent or behind the decorations. We thus conclude that discoid decorations function to deter avian predators by shielding the spider from view or distracting, not by deflecting attacks, concealing the spider’s outline, or masquerading as bird-droppings. This study sheds light on the study of other similar anti-predator strategies, in a wide range of spider species and other animals that use decorating strategies.

From crypsis to masquerade: Ontogeny changes the colour defences of a crab spider hiding as bird droppings

“…Before trials, we housed chicks in five cardboard boxes (length × width × height: 50 × 35 × 35 cm) with each box containing 30 chicks. We provided the chicks with water daily and fed them with chicken feed (CP feed, CP group, Zhengzhou, China) three times a day (on average 5 g/chick each time; Ma et al., 2020; Wang et al., 2021). In addition, chicks were provided approximately six mealworms Tenebrio molitor twice a day (at 07:00 hr and 18:00 hr, respectively).…”

Conspicuous cruciform silk decorations deflect avian predator attacks

“…The rear inner side of the arena was covered with a sheet of printed pictures of the spiders’ natural habitat, and the other 3 inner sides each were covered with a sheet of white paper as described in Wang et al . (2021) (Fig. S2, Supporting Information).…”

“…2003; Wang et al . 2021). However, this function is most likely to attribute to the solid‐mats or discoid decorations built by Argiope juveniles (reviewed in Eberhard 2020).…”

“…2020) and the visual shield function of discoid decorations of A. minuta (Wang et al . 2021). If the cruciform silk decorations function to deflect the chicks’ attacks away from the spiders, fewer chicks would attack the main bodies of the spiders (cephalothorax or abdomen) on decorated webs than on undecorated ones.…”

Geographic variation in the ontogeny of web decorating by <I>Argiope appensa</I> (Araneae: Araneidae)

“…Discoid decorations are thought to primarily serve a protective function by providing a shield, especially to juvenile spiders, behind which the spider can 'shuttle' to evade would-be predators (Robinson & Robinson 1970;Schoener & Spiller 1992;Wang et al 2021; but see Li et al 2004 for evidence of an additional prey-attraction role). The main predators of orb-web-building spiders, including Argiope, are often wasp hunters and parasitoids (Foelix 2011).…”