Natural Entrainment without Dawn and Dusk: The Case of the European Ground Squirrel (Spermophilus citellus)

Observational data collected in the field and in enclosures show that diurnal, burrow-dwelling European ground squirrels (Spermophilus citellus) never were above ground during twilight at dawn or at dusk. The animals emerged on average 4.02 h (SD = 0.45) after civil twilight at dawn and retreated in...

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Bibliographic Details
Published in:Journal of biological rhythms 1999-08, Vol.14 (4), p.290-299
Main Authors: Hut, Roelof A., van Oort, Bob E. H., Daan, Serge
Format: Article
Language:English
Subjects:
Animals
Behavior, Animal - physiology
Behavior, Animal - radiation effects
Circadian rhythm
Circadian Rhythm - radiation effects
Data Collection
Female
Light
Male
Regression Analysis
Rodents
Sciuridae - physiology
Spermophilus citellus
Sunlight
Telemetry - methods
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Summary:Observational data collected in the field and in enclosures show that diurnal, burrow-dwelling European ground squirrels (Spermophilus citellus) never were above ground during twilight at dawn or at dusk. The animals emerged on average 4.02 h (SD = 0.45) after civil twilight at dawn and retreated in their burrows on average 2.87 h (SD = 0.47) before civil twilight at dusk. Daily patterns of light perceived by these burrowing mammals were measured with light-sensitive radio collar transmitters in an enclosure (the Netherlands) and in the field (Hungary). The observational data are corroborated by the telemetry data, which show clear daily patterns of timing of light perception including light perceived from the burrow entrances. The first light was observed by the animals on average 3.54 h (enclosure, SD = 0.45) and 3.60 h (field, SD = 0.31) after civil twilight at dawn, whereas the final observed light was on average 3.04 h (enclosure, SD = 0.64) and 2.02 h (field, SD = 0.72) before civil twilight at dusk. Thus, the animals do not perceive the rapid natural light-dark (LD) transitions that occur at civil twilight. Instead, they generate their own pattern of exposure to light within the natural LD cycle. The classical phase response model for entrainment by light or dark pulses cannot explain how the circadian system of this species remains entrained to the external, natural LD cycle while the major LD transitions are created by its own behavior.
ISSN:0748-7304
1552-4531
DOI:10.1177/074873099129000704