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The sun compass of the sandhopper Talitrus saltator: the speed of the chronometric mechanism depends on the hours of light

Alberto Ugolini^1,*, Bruno Tiribilli² and Vieri Boddi³

¹ Dipartimento di Biologia Animale e Genetica, Università di Firenze, Via Romana 17, 50125 Firenze, Italy
² Istituto Nazionale di Ottica Applicata, Firenze, Italy
³ Dipartimento di Sanità Pubblica, Università di Firenze, Italy

View larger version (27K): [in a new window]   Fig. 2. (A) Orientation angles of sandhoppers subjected to an artificial L:D cycle (15h:9h) corresponding to the natural cycle in phase and duration. (B) Orientation angles of individuals subjected to an artificial L:D cycle (15h:9h) inverted with respect to the natural cycle: the sandhoppers were released during their subjective night. (C) Orientation angles of sandhoppers subjected to an artificial 4h:20h L:D cycle; and (D) to an artificial 20h:4h inverted L:D cycle. The diagrams represent the relationship between the mean angle resulting from each release (filled circles) and time of day. White and black bars represent the natural or subjective hours of light and dark, respectively. a, expected direction towards land (i.e. yL) for differential compensation; b, expected yL according to the model of compensation proposed in this study; c, interpolation of the data. In C, curve d represents the expected direction according to the proposed model in the case of clock-shifting affecting subjective noon (clockwise shifting of 6h and 17 min). Line e indicates the same thing in the case of differential compensation. In B and D, the line for yL based on differential compensation is not represented since we do not know the curve of the sun at night. The equations of the regressions are also given.

View larger version (17K): [in a new window]   Fig. 1. Schematic representation of the angular variation in orientation angle for solar orientation in Talitrus saltator and Apis mellifera (A) (modified from Pardi and Ercolini, 1986). The theoretical direction of orientation is eastward. The orientation angle of T. saltator returns to 0° from 180°, passing through south at night. The same data are represented in the two distributions below (B), in which the path of the sun during the day (solid line) and at night (dashed line) is represented. Black triangle, expected direction of orientation. The theoretical angles between the sun and the expected direction of orientation are also reported for some hours of the day. (C) However, since the sandhoppers were released during the real day the sun's path is from east to west. Therefore, the expected direction of orientation varies with the time of release.