Abstract

Abstract The Octodon degus, or degu, is an excellent animal model for studying the theoretical and neural underpinnings of diurnality. The power of this model comes from their unique evolutionary lineage, long lives, and relative ease of care in the laboratory for a non-domesticated species. We have summarized the field and laboratory data indicating the critical variables that influence the degus' phase preference and the possible mechanisms for the phase flexibility observed in the field and laboratory. We also review studies examining the physiology and anatomy of light and non-photic inputs to the degu circadian system and studies of the circadian pacemaker itself, with particular emphasis placed on characteristics that appear to be convergent adaptations to a diurnal niche. Finally, we begin to seek the origin for the diurnally-phased activity output of the degu, although we conclude that significant work remains to be done. Keywords: chronotypeday-activeHystricognathiperclock genesmasking Acknowledgements We wish to thank Dr. Refinetti and Dr. Waterhouse for inviting us to write this review and for providing valuable feedback on an earlier draft. We also thank the many students who have helped collect data about circadian function in the degu during the past 16 years. They include Drs. Susan Labyak, Namni Goel, Daniel Hummer, and Jennifer Mohawk. Other students that made important contributions include Marcia Governale, Jamie Perryman, and Andrew Vosko. We also thank our animal care staff, Kathy Gimson, Julie Stewlow, and Jim Donner, for their years of excellent care; and the lab managers, Amy Young and Blair Sutton, who helped figure out how to make degus into happy breeders. We wish to recognize the financial support for the research with degus from NIMH, NHLB, NSF, and the University of Michigan; and the support of MHH by the Reproductive Science Program T32 Training grant from NICHD. Notes 1. Chronotype flexibility is not unique to degus, and an extensive review by Kronfeld-Schor and Dayan (2003 Kronfeld-Schor, N and Dayan, T. 2003. Partitioning of time as ecological resource. Annu Rev Ecol Evol Syst, 34: 153–181. [Crossref], [Web of Science ®] , [Google Scholar]) discusses the many ecological variables that may lead to such adaptations. 2. A recent study examining the masking effects of melatonin administration on body temperature in degus found that melatonin produced a hypothermic response in degus, regardless of diurnal or nocturnal chronotype (Vivanco et al. 2007). Therefore, at least some masking responses are definitively the same, and diurnal-typical, in both chronotypes.