Effect of light-dark cycles with different LD time-ratios and different LD intensity-ratios on the activity rhythm of chaffinches |
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Authors: | Dr George C West Hermann Pohl |
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Institution: | (1) Max-Planck-Institut für Verhaltensphysiologie, Erling-Andechs;(2) Institute of Arctic Biology, University of Alaska, 99701 Fairbanks, Alaska, USA |
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Abstract: | Summary Chaffinches (Fringilla coelebs L.) were kept at light-dark (LD) cycles with two different light intensity ratios (ca. 50 0.1 and ca. 10 1 lux) both with 1.83 hours of twilight at the onset and end of light time. The light time/dark time ratio (LD time-ratio) was varied in the range from 4 20 to 22 2 hours.Birds at the high LD intensity-ratio (50 0.1 lux) increased activity time ( ) with increasing light time from LD 4 20 to 22 2. Time of activity onset, middle, and end of activity corresponded to onset, middle, and end of light time from LD 4 20 to 16 8, but the phase angles of middle and end of activity (related to midpoint of light time and midpoint of artificial dusk respectively) became more positive (advanced) at LD 20 4 and 22 2.Birds at the lower LD intensity-ratio (10 1 lux) increased activity time with increasing light time from LD 8 16 to 20 4 but activity time at LD 4 20 was equivalent to that at LD 8 16. The phase angles of activity onset and middle (related to midpoint of artificial dawn and midpoint of light time, respectively) became more positive at both long (LD 20 4) and short (LD 4 20) light times.By use of different twilight regimes (linear and logarithmic changes in intensity), it was shown that end of activity was more closely related with the rate of change and/or absolute level of light intensity than was onset of activity.During January and February, the birds responded to increased LD time-ratios by an increase of activity time and by strong positive phase angle shifts which correspond to observations of activity patterns of chaffinches exposed to outdoor conditions during the spring breeding season.The free-running circadian periods ( ) of 6 birds measured at 15 lux at the end of the experiments following LD 20 4 averaged 22.17 hours.From the data on precision , i. e. the period-to-period variation in the phase angles of single birds averaged over all birds of one group and condition, as well as from the number of birds not synchronized with the LD cycles, it is concluded that extreme long and short light times per 24 h represent weaker zeitgebers compared to LD cycles with medium lengths of light time. Likewise, LD cycles with low LD intensity-ratios are weaker zeitgebers than LD cycles with high LD intensity-ratios.This study was carried out while the senior author was on sabbatical leave from the University of Alaska and held a fellowship from the Alexander von Humboldt Stiftung. |
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