Effects of Temperature on Circadian Clock and Chronotype: An Experimental Study on a Passerine Bird

Abstract

Daily schedules of many organisms, including birds, are thought to affect fitness. Timing in birds is based on circadian clocks that have a heritable period length, but fitness consequences for individuals in natural environments depend on the scheduling of entrained clocks. This chronotype, i.e., timing of an individual relative to a zeitgeber, results from interactions between the endogenous circadian clock and environmental factors, including light conditions and ambient temperature. To understand contributions of these factors to timing, we studied daily activity patterns of a captive songbird, the great tit (Parus major), under different temperature and light conditions. Birds were kept in a light (L)-dark (D) cycle (12.5 L:11.5 D) at either 8°C or 18°C with ad libitum access to food and water. We assessed chronotype and subsequently tested birds at the same temperature under constant dim light (LL_dim) to determine period length of their circadian clock. Thermal conditions were then reversed so that period length was measured under both temperatures. We found that under constant dim light conditions individuals lengthened their free-running period at higher temperatures by 5.7 ± 2.1 min (p = .002). Under LD, birds kept at 18°C started activity later and terminated it much earlier in the day than those kept under 8°C. Overall, chronotype was slightly earlier under higher temperature, and duration of activity was shorter. Furthermore, individuals timed their activities consistently on different days under LD and over the two test series under LL_dim (repeatability from .38 to .60). Surprisingly, period length and chronotype did not show the correlation that had been previously found in other avian species. Our study shows that body clocks of birds are precise and repeatable, but are, nonetheless, affected by ambient temperature. (Author correspondence: [email protected])

Keywords:

• Activity patterns
• Parus major
• Phase
• Period length
• Q₁₀
• Repeatability
• Temperature compensation
• Timing

ACKNOWLEDGMENTS

We thank Christa Mateman for EP analysis, Marylou Aaldering and Floor Petit for animal care taking, and Ab and Gilles Wijlhuizen for technical support. Dusty Dowse helped by excellent comments and considerations regarding the circadian data. Thanks to The International Max Planck Research School for Organismal Biology for support. Special thanks go to the three anonymous reviewers for their comments and suggestions, which improved the manuscript considerably.

Declaration of Interest: M.E.V. is supported by a NWO-VICI (The Netherlands Organization for Scientific Research) grant and B.H. is supported by the European Social Fund in Baden-Württemberg.

The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.