Evidence of bird dropping masquerading by a spider to avoid predators

Abstract

Masquerading comes at various costs and benefits. The principal benefit being the avoidance of predators. The orb-web spider Cyclosa ginnaga has a silver body and adds a white discoid-shaped silk decoration to its web. The size, shape and colour of C. ginnaga's body resemble, when viewed by the human eye against its decoration, a bird dropping. We therefore hypothesized that their body colouration might combine with its web decoration to form a bird dropping masquerade to protect it from predators. We measured the spectral reflectance of: (i) the spider's body, (ii) the web decoration, and (iii) bird droppings, in the field against a natural background and found that the colour of the spider bodies and decorations were indistinguishable from each other and from bird droppings when viewed by hymentopteran predators. We monitored the predatory attacks on C. ginnaga when the spider's body and/or its decorations were blackened and found that predator attack probabilities were greater when only the decorations were blackened. Accordingly, we concluded that C. ginnaga's decoration and body colouration forms a bird dropping masquerade, which reduces its probability of predation.

Camouflage

“…Instead, attacking a stone because of the possibility it is a tasty stone‐mimic has no evolutionary consequences for the stone. Mimicry of bird droppings by some spiders and moth caterpillars (Liu et al ., ; Valkonen et al ., ; Suzuki & Sakurai, ), fulfils Endler's criterion for masquerade, not Batesian mimicry, even though erroneous consumption of faeces could be harmful. Birds have not, as far as we know, evolved differently coloured or shaped faeces as a result of mimicry by arachnids or insects.…”

Camouflage in predators

“…For example, the predatory orb‐web spider Cyclosa ginnaga adds a white disc‐shaped silk decoration to its web upon which it positions itself to resemble a bird dropping (Skelhorn, 2015). This strategy could have an anti‐predatory function for the spider in addition to concealment from its own prey (Liu et al ., 2014). Similarly, the South American fish Monocirrhus polyacanthus resembles a floating dead leaf as it approaches its prey (Cott, 1940).…”

Colour matching in an orb‐web spider: a twig‐masquerading species as seen by potential predators

“…In this defensive strategy, species have evolved morphologies that allow them to be mistaken for inedible objects that are common in their environment. These species masquerade as twigs, leaves, stones, bird droppings and other things (Skelhorn et al ., ; Liu et al ., ; Pekár, ). Therefore, the function of masquerade is to lead the predator to misclassify and not to preclude detection as in crypsis (Skelhorn et al ., ).…”