Our brains use stereoscopic vision to make the leaves seem transparent, and to highlight the animal.

FORBES: 3D May Be a Box-Office Hit, But Our Eyes Were Made for X-Ray Vision

Stereoscopic vision (twin forward-facing eyes give us slightly different views), says Changizi, offers an evolutionary advantage because it renders clutter non-essential foreground material transparent.

FORBES: 3D May Be a Box-Office Hit, But Our Eyes Were Made for X-Ray Vision