Why do some animals see in black and white?

Question

Do they not have photorecptors?

Answer 1

They couldn't afford color.

Answer 2

Many animals have color vision restricted to the gray scale.  That is the 256 shades from solid white to black.  They just do not have any need to see in color.  Obviously they have evolved to survive without the full range of colors.  

The Evolution of Color Vision

http://www.talkorigins.org/faqs/vision.h...

Answer 3

I at one point heard that it was as a result of their lack of cones, the color discriminating mechanism in animal eyes (humans have them).  Rods allow us to appreciate differening intensities of light.  Humans have both.

I would like to say however that I heard on the news that scientists have in fact come to the conclusion that dogs do see in color.  I cannot say that's true for all animals however.