One further point worthy of note is the striking similarity between the hybrid photoreceptor identified in this study and a naturally occurring photoreceptor found in ground squirrels. The “rods” of this species have electrophysiologic, molecular, and ultrastructural features of both rods and cones [52–58]. Although these unusual findings have been difficult to interpret under the usual assumptions of “duplicity theory” [56], we would like to suggest that ground squirrels may have experienced a naturally occurring down-regulation or loss of Nr2e3 expression in their “rods” that transformed them into a hybrid photoreceptor cell type. The adaptive significance of such a change, if any, is unknown, and it may simply be due to relaxation of selective pressure for night vision in this strictly diurnal species.