German scientists have recently discovered that rats move their eyes in opposite directions in both the horizontal and the vertical plane when running around.

This surprising discovery means that it's impossible for these animals to fuse the visual information into a single image, like humans do. Their eye movements, however, enable them to  see the space above them at all times. This could be an adaptation to help them deal with the threat from predatory birds, which most rodents face.

In humans, the eyes must continually be aligned in the same direction in order to be able to focus on an object. A deviation of less than a single degree of the field of view is enough to cause double vision. In rats, the opposing eye movements between left and right eye mean that the line of vision varies as much as 40 degrees in the horizontal plane and up to 60 degrees in the vertical plane.  

The researchers from the Max Planck Institute for Biological Cybernetics claim that, although these movements prevent the two visual fields from fusing, and thus the perception of a clear, 3-dimensional image, the permanent visibility in the direction of a potential attack from the sky greatly improves the animal's chances of survival.

Source: http://www.alphagalileo.org/ViewItem.aspx?ItemId=131503&CultureCode=en