A Comparison of Image Statistics of Peacock Jumping Spider Colour Patterns and Natural Scenes

Abstract

The form of arbitrary sexual signals may be driven by the need to be detectable against the background or, alternatively, by selection for efficient processing by the nervous system. This latter alternative is a prediction of the sensory drive hypothesis extended to include efficient coding as a driver of the form of sexual signals. This hypothesis posits that animal visual systems are adapted to process the visual statistics of natural scenes, and that easily processed stimuli induce a sensation of pleasure in the viewer. In support of this, natural scene statistics have been found to be preferred not only by humans, but by the peacock spider Maratus spicatus. Here we test if male peacock spiders of the highly sexually dimorphic Maratus genus generally (a) evolve colour patterns with image statistics that contrast with the natural background or (b) exploit a potential processing bias by evolving colour patterns with visual statistics similar to those of natural scenes. We analyse and compare multispectral images of male and female spiders of 21 Maratus species and of natural scenes similar to the spiders' habitat. We find that the image statistics of male patterns diverge from those of natural scenes, whereas the statistics of female patterns do not. Our results support the idea that colour patterns evolve contrasting image statistics to increase conspicuousness and matching image statistics to be camouflaged. Any processing bias for natural scene image statistics in Maratus thus appears to play little role in the evolution of their sexual signals.

Keywords: Australian peacock spiders; Fourier transform; efficient coding; natural image statistics; processing bias; sexual signalling.

FIGURE 1

Computational filter images (matching the spectral sensitivity of the salticid green receptor that peaks at 530 nm) of the abdomens of male and female Maratus species (one example per species and sex). Squares indicate the area that was cropped from the image (yellow for females and blue for males) and used in the analysis to calculate the spectral slope.

FIGURE 2

Distribution of spectral slopes of male and female Maratus from all species pooled (density plot based on raw data: SD of smoothing kernel = 0.2; females: N _species = 18, N _individuals = 49; males: N _species = 21, N _individuals = 94) compared to the distribution of slopes of natural scene images picturing the ground (N = 41).

FIGURE 3

Distribution of posterior estimates for spectral slopes of male and female abdomens of 18 different Maratus species (Bayesian linear mixed effect model). Means per species and credible intervals are shown in a darker shade. Vertical lines represent means (dotted = males, dashed = females, solid = natural scenes; for boxplots of raw data, see Figure S6; for sample sizes per species, see Table S1).