Natural entrainment without dawn and dusk: The case of the European groundsquirrel (Spermophilus citellus)

Observational data collected in the field and in enclosures show that diurn al, 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 Nether lands) and in the field (Hungary). The observational data are corroborated by the telemetry data, which show clear daily patterns of timing of light p erception including light perceived from the burrow entrances. The first li ght was observed by the animals on average 3.54 h (enclosure, SD = 0.45) an d 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 (fi eld, SD = 0.72) before civil twilight at dusk. Thus, the animals do not per ceive the rapid natural light-dark (LD) transitions that occur at civil twi light. Instead, they generate their own pattern of exposure to light within the natural LD cycle. The classical phase response model for entrainment b y light or dark pulses cannot explain how the circadian system of this spec ies remains entrained to the external, natural LD cycle while the major LD transitions are created by its own behavior.