Behavioural choice and background matching facilitate camouflage in the European garden spider

Visual camouflage via background matching involves a variety of adaptive traits to maintain crypsis, including intraspecific color variation, behavioral choice of substrates, and color change. These non-mutually exclusive solutions frequently act together to conceal and deceive prey and enemies. Here, we combine field observations, image analysis, and laboratory experiments to investigate which processes drive camouflage in the garden cross spider (Araneus diadematus), a species with body coloration that varies in shades of brown. We demonstrate that A. diadematus does not change color significantly, at least within the same instar, when retained on substrates of different coloration. However, there is strong behavioral selection through active substrate choice across spiders for color-matching substrates (dead brown leaves over green leaves) when offered a choice under laboratory conditions. Similar background selection also apparently occurs in nature, where spiders were often observed on brown leaves, even though they are less common than green ones. In general, vision modeling shows that there is a high overlap in the diversity of brown shades (from pale to dark) between spider bodies and dead leaves available in the environment. Image analyses also revealed that spiders fine-tune their camouflage on an individual level by matching the tones (from pale to dark) of their host leaves. Therefore, we demonstrate how behavior coupled with variation in color phenotypes facilitates camouflage at different scales.

Methods
We collected garden cross spiders (Araneus diadematus) and plant leaves (green and brown from host plants; brown from sympatric species) in the field for use in laboratory experiments. In the field study and colour change experiment, we quantified spider colour and camouflage from digital photographs using methods of image analysis and visual modelling. In the behavioural-choice experiment, we recorded spider preferences between green and brown leaves in binomial trials.