Variability of behavioral chronotypes of 16 mammalian species under controlled conditions

• Mammals ranging in body size from mice to cattle exhibited variation in chronotypes.
• The interindividual range of chronotypes varied from 43 min in sheep to 23 h in cats.
• Chronotype variability was greater between species than within species.
• Chronotype variability was also greater between individuals than within individuals.

Human chronotypes (differences in preference for early or late rising each day) have been extensively studied in recent years, but no attempt has been made to compare human chronotypes with the chronotypes of other animal species. We evaluated behavioral chronotypes in 16 mammalian species along a body size gradient of five orders of magnitude (from mice to cattle). Individuals of all species were studied under a 12 L:12D photoperiod in a thermoneutral environment with food and water available at all times. Rhythms of locomotor activity were analyzed for onset time, acrophase, and robustness. Neither of these rhythmic parameters was significantly related to body size, but onset time and acrophase varied considerably from species to species, thus characterizing diurnal and nocturnal species. Chronotype spreads ranged from less than an hour in sheep to almost 24 h in cats, thus extending both below and above the human chronotype spread of 6 h. The variability of chronotype (as quantified by the standard deviation of group means) was much larger between species than within species and also larger between individuals of a species than within individuals on consecutive days. These results help situate the matter of human chronotypes within the broader context of variability in the phase angle of entrainment of circadian rhythms in animals.