Circannual variations in circadian rhythms of ground squirrels.

There was a circannual rhythm in the times of activity onset (psi o) and activity termination (psi T) and in the duration of the active phase (alpha) in female ground squirrels (Spermophilus lateralis) maintained in a light-dark (LD) 14:10 photoperiod for 22 mo. Activity onset occurred 2.3 h after light onset in the 4 mo before estrus, 0.5 h before light onset during the month encompassing estrus, and progressively later during each of the ensuing 4 mo. Termination of activity occurred later during the month of estrus than in the 4 mo preceding estrus; alpha nearly doubled during the month of estrus and decreased gradually over the next 3 mo. The psi o and psi T remained entrained to the daily illumination cycle, but the phase angles of activity onset and offset and the duration of the active phase varied seasonally and recurred with a circannual period of 9-11 mo. The LD 14:10 photoperiod entrained the circadian activity rhythm throughout the year; however, circannual variations in activity as well as in estrus and body mass were not entrained by this photoperiod but free-ran with periods of less than 12 mo.