"The major difference between stickleback vision and our own is that they can see ultraviolet light, which is invisible to humans. This may be important because carotenoids reflect ultraviolet light as well as the red, oranges and yellows that we can see," Dr Pike explains.

The model developed by Dr Pike and colleagues from the University of Glasgow and Nofima Marine in Norway mimics the stickleback's visual system, allowing the researchers to determine what 'colors' the fish see. "The model tells us how much of the light reflected from a carotenoid signal is actually detected by a female and how this information might be processed by her brain, and so gives us exciting new insights into how females may use color to choose the best mates," says Dr Pike.

Male sticklebacks can fine tune the colors they display to females by varying both the overall amount of carotenoids and the relative amount of the two constituent carotenoids, the red-colored astaxanthin and the yellow tunaxanthin. The model reveals that sticklebacks' visual system and coloration are extremely well co-adapted, and that females are surprisingly good at assessing the quantity of carotenoids a male is able to put in his signal – which previous studies by the authors have shown is linked to his parenting ability.