AJP - Regulatory, Integrative and Comparative Physiology, Vol 250, Issue 5 831-R836, Copyright © 1986 by American Physiological Society
Circannual variations in circadian rhythms of ground squirrels
T. M. Lee, M. S. Carmichael and I. Zucker
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.
