Dalton and Sam are male squirrel monkeys, about a foot tall. Their ancestors lived by eating fruit and insects and dodging falcons in the forest canopy of Central and South America. Dalton and Sam lead a more protected life in the laboratory of Jay and Maureen Neitz at the University of Washington, Seattle. Recently, the Neitzes endowed them with a new genetic gift: the ability to see the world with full color vision.

Male squirrel monkeys have only two of the color pigments known as opsins, unlike people who have three. The Neitzes, with Katherine Mancuso and other colleagues, used the technique of gene therapy to introduce the gene for the missing red pigment into the cone cells of the monkeys’ retinas. Several months after the therapy, Dalton and Sam were able to see a world in which red hues were visible and oranges no longer looked like lemons, the researchers say in the current issue of Nature.

Although the monkeys could not report that they saw the world with new eyes, their ability to do so was judged by their performance on a color-vision test with a reward of fruit juice.

Jay Neitz said Dalton was named for John Dalton, who not only invented modern atomic theory but in 1794 was also the first person to describe color blindness  his own.