The Anti-Predator Defense Function Of Silk Decorations Built By Orb-Weaving Argiope Spiders Against Avian Predators

Predation is a strong selective force that drives the evolution of morphological and behavioural defences of prey species to protect them from predators.To avoid predation from predators,orb-weaving spiders have also evolved a variety of defense strategies.All species of the orb-weaving spider genus Argiope decorate their webs with conspicuous,white silk so-called web decorations,and scientists have been interested in their functions since they were first described.Silk decorations have been assumed to serve as an anti-predator function.However,the mechanisms of silk decorations still remain unclear,and the direct evidence for the hypothesis is lacking.Moreover,Argiope spiders have also evolved a suite of secondary defence behaviours against predators such as dropping to the ground,shuttling around the hub of the web,pumping on the web and shifting away from the hub.Whether secondary predator avoidance behaviour in Argiope spiders is associated with web decorations also remains largely unexplored.Here,I used spectrometry and visual modelling and na?ve domestic chicks as the representative of avian predators to test the anti-predator defence hypothesis for the two types of silk decoration built by an orb-weaving spider,Argiope minuta,the discoid decorations of juveniles and the cruciform decorations of adult females.Then I investigated age-and decoration-associated secondary defense behaviour in this Argiope species.My visual model showed that the chicks could distinguish the colours both juvenile spiders from discoid decorations and adults from cruciform decorations against natural backgrounds.My chick predation experiments suggested that chicks attacked more juveniles which were exposed to them than non-exposed.And they attacked the main bodies of adult females much less frequently on the decorated webs than on the undecorated webs,moreover,juveniles were more likely than adults to respond to chick attacks,and both juveniles and adults on webs with decorations responded to chick attacks differently than they did on webs without.Overall,my results showed that the discoid decorations of juvenile spiders function as a visual barrier to prevent them from being detected by predators,while the cruciform decorations of adult Argiope female spiders divert the attention of predators and guide them to silk decoration or the invulnerable body parts such legs.The secondary defense responses to predators are associated with the age of spiders,attacking site and type of decorations.This is the first study to provide direct evidence for the defense mechanisms of different types of silk decorations within a Argiope species,and also the first study to explore the age-and decoration-associated secondary defense behaviour of Argiope spider against real bird predator.This opens a new research opportunity and provides new perspective for the study of such anti-predator strategies in a wide range of animals with decorating behaviour.