Unveiling key links between behaviour and appearance in the evolution of camouflage

Behaviour is a key yet often overlooked component of animal camouflage and how it evolves alongside colour and morphology remains poorly understood. The repeated evolution of stick-like postures in spiders offers a useful framework for investigating the importance of behaviour for concealment, since matching the environment should rely on specific body forms and postures, not just colouration. We hypothesised that when spiders behaviourally align their body with the background orientation it should influence the shape, posture and colouration that best enhances camouflage. To test this, we used a genetic algorithm and human observers to evolve digital spiders to be harder to find. We evaluated how selection under three behavioural orientation treatments (aligned, random, and evolvable orientation) influenced spider capture time, background match (lightness and colour), posture, and body (cephalothorax and abdomen) dimensions. We found that spiders that behaviourally aligned with the background took substantially longer to find through evolving a better background match, and a more elongated posture and body shape than randomly orientated spiders. Our spiders mirrored the shape and posture adopted by numerous clades, illustrating how behavioural camouflage represents a key concealment strategy in structurally complex habitats, part of an interacting suite of traits that encompass successful concealment.