Can't tell the caterpillars from the trees: countershading enhances survival in a woodland

Abstract

Perception of the body's outline and three-dimensional shape arises from visual cues such as shading, contour, perspective and texture. When a uniformly coloured prey animal is illuminated from above by sunlight, a shadow may be cast on the body, generating a brightness contrast between the dorsal and ventral surfaces. For animals such as caterpillars, which live among flat leaves, a difference in reflectance over the body surface may degrade the degree of background matching and provide cues to shape from shading. This may make otherwise cryptic prey more conspicuous to visually hunting predators. Cryptically coloured prey are expected to match their substrate in colour, pattern and texture (though disruptive patterning is an exception), but they may also abolish self-shadowing and therefore either reduce shape cues or maintain their degree of background matching through countershading: a gradation of pigment on the body of an animal so that the surface closest to illumination is darker. In this study, we report the results from a series of field experiments where artificial prey resembling lepidopteran larvae were presented on the upper surfaces of beech tree branches so that they could be viewed by free-living birds. We demonstrate that countershading is superior to uniform coloration in terms of reducing attack by free-living predators. This result persisted even when we fixed prey to the underside of branches, simulating the resting position of many tree-living caterpillars. Our experiments provide the first demonstration, in an ecologically valid visual context, that shadowing on bodies (such as lepidopteran larvae) provides cues that visually hunting predators use to detect potential prey species, and that countershading counterbalances shadowing to enhance cryptic protection.

Camouflage and Perceptual Organization in the Animal Kingdom

“…However, there are many adaptive reasons to have such a gradient, some of which see the colour only as an incidental by-product of the pigment gradient: for example, protection from UV light, or resistance to abrasion -because melanin toughens biological tissues (Kiltie 1988;Ruxton et al 2004a;Rowland 2009). In fact, recent experimental studies on model 'caterpillars' coloured uniformly, or with countershading or reverse countershading patterns, have demonstrated that countershading helps concealment from birds (Rowland et al 2007(Rowland et al , 2008. However the principle by which countershading patterns achieve camouflage is less obvious.…”

Aposematism: balancing salience and camouflage

“…The experiment followed a well-established paradigm with wild avian predators selectively predating dough, caterpillar-like, baits [18]. Stimuli were approximately 16 mm long (approximately 3 mm wide) cylinders of dough, coloured to produce notionally camouflaged and aversive patterns.…”

Cognition and the evolution of camouflage

“…It is thus not surprising that the vast majority of studies in this field seek to explain how cryptic patterns work in terms of predator vision [4][5][6][8][9][10][11]. This has been a fruitful area of research and has led to the identification of a number of distinct forms of crypsis based on the mechanisms employed to hinder initial detection (box 1).…”

“…For example, caterpillars that resemble twigs can initially remain undetected when viewed against a background of other twigs, and once they are detected as discreet objects they can be mistaken for twigs [7]. Until relatively recently, most work on camouflage has investigated the form and function of cryptic patterning [4][5][6][8][9][10][11]. Understandably, therefore, research has focused on how camouflage patterns exploit the sensory processes of predators (but see [12][13][14][15][16]).…”

“…These definitions are intended to give a brief overview of different forms of camouflage. More detailed definitions can be found in the following papers: [1][2][3][4][5][6]. )…”