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Mammalian diurnality: some facts and gaps
Laura Smale1, Theresa Lee, Antonio A Nunez
1Department of Psychology, Michigan State University, East Lansing Michigan, MI 48824, USA. smale@msu.edu
Journal of Biological Rhythms
|October 30, 2003
Summary
Mammalian evolution was driven by nocturnal activity. Diurnal activity re-emerged, with similar circadian clock mechanisms but different signal processing in the suprachiasmatic nucleus (SCN).
Area of Science:
- Chronobiology
- Neuroscience
- Evolutionary Biology
Background:
- Mammalian evolution favored nocturnal activity, a previously underutilized niche.
- Diurnality (day-active behavior) has evolved independently multiple times in various mammalian lineages.
Purpose of the Study:
- To review recent data on circadian mechanisms in diurnal mammals.
- To explore emerging themes in the evolution and neural basis of diurnality.
Main Methods:
- Review of behavioral studies on circadian rhythms.
- Examination of molecular and neural mechanisms underlying circadian timing.
- Comparative analysis of suprachiasmatic nucleus (SCN) function in nocturnal and diurnal species.
Main Results:
- Fundamental functions and molecular mechanisms of the circadian system are conserved between nocturnal and diurnal mammals.
- Differences in diurnality likely arise from altered signal processing downstream of the SCN.
- Specific SCN target populations, like orexin and GnRH neurons, show inverted rhythms in diurnal versus nocturnal species.
Conclusions:
- The evolution of diurnality in mammals involves conserved core clock mechanisms but divergent downstream signaling.
- The region dorsal to the SCN may act as a relay, transforming SCN signals for diurnal or nocturnal activity.
- Understanding these neural substrates provides insights into the evolution of day-active lifestyles.