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Fig. 3. A comparison of body patterns displayed by 20 juvenile cuttlefish (A) in response to pebbles under 5 mm Perspex, with responses to photographic images of these pebbles, suggests that they are sensitive to real visual depth in pebble backgrounds (B). Placing 10 real pebbles on the photograph (C) suggests that a small number of light, but not dark, 3-D objects give a similar response to a natural substrate. (A) A photograph of the pebble background, and illustrations of body patterns that typify the three PCs identified in this study (see Fig. 2A). Positively weighted behavioural components for each PC are numbered as in Figs 1 and 2. (B) Mean weights (+s.e.m.) of PCs 1–3 for real pebbles under Perspex or photographs of the pebbles at three contrast levels. The photographic backgrounds were presented at three contrast levels: (i) to match the natural pebbles; (ii) enhanced by 25% in Adobe Photoshop; or (iii) enhanced by 50% in Adobe Photoshop. Weights of all three PCs increase with image contrast in the photographs, whereas real pebbles give the strongest weights of PC1 (disruptive pattern, Fig. 2) and negative weights of PC2 (mottle/stipple). This indicates that no level of image contrast in a photograph could produce a response matching that to real pebbles. A repeated measures ANOVA showed a significant effect of the background and amplitude of PCs 1–3 between the natural pebbles and all three photographic backgrounds (PC1, F_3,57=19.61, P<0.05; PC2, F_3,57=8.556, P<0.05; and PC3,F_3,57=6.614, P<0.05). The ANOVA also revealed an interaction of background type (3-D vs 2-D) and the expression of PC1 and PC2 (F_1,19=27.95, P<0.05). (C) Evidence that placing 10 pebbles at arbitrary locations on the 2-D background (area 0.07 m²) affects the body pattern. The presence of 10 light stones on the 2-D background enhances the expression of PC1 (disruptive; Student's t-test: t₁₉=–3.547, P<0.05) and suppresses PC2 (mottle/stipple). With 10 dark stones there is no significant effect on PC1 (Student's t-test: t₁₉=–1.811, P>0.05) but a suggestion that the expression of PC2 is suppressed. A mixture of five light and five dark stones gives similar responses to 10 light stones.

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