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Photic Entrainment of Circannual Rhythms in Golden-Mantled Ground Squirrels: Role of the Pineal Gland

Department of Biology, Swarthmore College, Swarthmore, PA 19081, USA

Elena M. Thomas

Departments of Psychology, Integrative Biology, University of California, Berkeley, CA 94720, USA

Theresa M. Lee

Department of Psychology, University of Michigan, Ann Arbor, MI 48109, USA

Kimberly M. Pelz

Departments of Psychology, Integrative Biology, University of California, Berkeley, CA 94720, USA

Steven M. Yellon

Center for Perinatal Biology, Department of Physiology, Loma Linda University School of Medicine, Loma Linda, CA 92350, USA

Irving Zucker

Departments of Psychology, Integrative Biology, University of California, Berkeley, CA 94720, USA

Entrainment of circannual rhythms of body mass and reproduction was monitored for 3 years in female golden-mantled ground squirrels maintained in a simulated natural photoperiod. Both pinealectomized and pinealintact squirrels generated circannual rhythms of body mass and estrus, but only the intact animals entrained these rhythms to a period of 365 days. In the second and third years after treatment, the period of the body mass rhythm was significantly shorter than 365 days for pinealectomized squirrels, and variance in tau among these animals was significantly greater than for intact squirrels. Asimilar pattern was evident in the rhythm of reproduction, which was phase-disrupted in pinealectomized squirrels but entrained in intacts. Seasonal changes in duration of nocturnal melatonin secretion by the pineal appear to be necessary to produce phase-delays required to entrain the circannual clock to a period of 12 months.

Key Words: circannual • body mass • estrus • seasonal • entrainment • pineal

Journal of Biological Rhythms, Vol. 15, No. 2, 126-134 (2000)
DOI: 10.1177/074873040001500207


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