Summary. Ground squirrels have dichromatic color vision. The spectral sensitivities of the two classes of cones found in the retinas of two species of ground squirrel were measured using ERG flicker photometry. The spectral sensitivity curves for these cone classes were closely fit by curves from wavelength-dependent visual pigment nomo-grams. One cone type had an average peak sensi-tivity of 518.9 nm (California ground squirrels, Sperrnophilus beecheyi) or 517.0 nm (thirteen-lined ground squirrels, Spermophilus tridecemlineatus). The second type of cone found in these ground squirrels had an average peak sensitivity of 436.7 nm. An examination of the variation in spec-tral sensitivity among individual animals suggests that the sensitivity peaks for the middle-wave-length cone cover a range of not greater than 4 nm
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