Spectral sensitivity curves of diurnal squirrels

Abstract

Spectral sensitivity curves were constructed for four species of ground squirrel, two species of tree squirrel and one species of chipmunk using the stimulus intensity required to produce a constant etoctroretinogram as the criterion. All these species have an apparently pure-cone retina although the retinal structure of the two tree squirrels differs somewhat from that of the ground squirrels and chipmunk. There was no evidence of a Purkinje shift. The spectral sensitivity curves of all seven species were very similar. The curves showed two maxima, one at about 535 mμ, the other at about 490 mμ. The relative heights of these maxima varied from one experiment to another but the factors responsible for this variation could not be determined. Adaptation to blue light depressed the maximum at 490 mμ while adaptation to green or orange light depressed that at 535 mμ. Evidence was obtained that the mechanisms responsible for the two maxima were to some extent antagonistic. The possible nature of these two mechanisms is discussed in the light of recent work on the photosensitive substances extractable from the retina of the common gray squirrel. There was some evidence for a further mechanism maximally sensitive between 450 and 460 mμ at least in some of the species investigated.