Peacock spiders prefer image statistics of average natural scenes over those of male ornamentation

The origins of preferences that drive the evolution of arbitrary sexual signals have been hotly debated for more than 150 years. An emerging but little-tested theory, efficient coding theory, proposes that male visual courtship displays are adapted to pre-existing processing biases shaped by the statistical properties of the natural environment. Natural scenes show strong spatial correlations with average amplitudes of spatial frequencies falling with an average spectral slope of –1 and humans have been shown to prefer random amplitude spectrum images that possess similar slopes. It has been proposed that other animals may also prefer the statistics of their natural environment and that this preference drives the evolution of sexual signaling displays. Here, we measure the spectral slope of the male display pattern of the Australian peacock jumping spider Maratus spicatus and test for a general preference toward that slope. We present spiders (male, female and juvenile) with random images of the male slope of –1.7 compared with: (a) a shallower slope of –1.0 and (b) a steeper slope of –2.3. Spiders spent more time oriented toward the shallower slope than toward the male slope and spent the same amount of time oriented toward the male slope and the steeper slope. Our results indicate that spiders, like humans, prefer the average natural slope of –1, suggesting that this is likely the slope typically found in their natural habitat. Rather than exploiting a potential processing bias it seems that males have evolved slopes that contrast with the visual background to enhance conspicuousness.