Complex sentence: Scientific experiments have demonstrated that non-human animals such as birds can perceive an object's shape from its shading, and it is assumed that most mammals experience this depth cue because of the prevalence of countershading (a type of camouflage) in nature.
Split into simpler sentences: Scientific experiments have demonstrated that obtaining an object's shape from its shading is experienced by birds. It is assumed that most mammals experience this depth cue because of the prevalence of countershading (a type of camouflage) in nature.