Evidence for existence of a yearly clock in surgically and self-blinded chipmunks.

By use of simplified technique and constant environmental conditions, I have demonstrated the existence in the chipmunk of a yearly clock. In a blinded chipmunk the clock manifested itself by remarkably consistent changes in running activity, food and water intake, and body weight over 6 1/2 yr. Studies on freshly trapped chipmunks kept in the same laboratory environment but with alternating light and darkness (12 hr each), showed that, when their eyes were covered for much of the light period, they reduced their exposure to light to preserve the activity of the yearly clock. Laboratory-adapted chipmunks that do not shield their eyes from light do not show the clock. The yearly clock has all the characteristics of the 24-hr clock, including sharply defined active and inactive phases, and must likewise play an important part in the animal's survival. Light would appear to be the chief or only cue for the clock. The period lengths did not change with age during the 6 1/2 yr.