The significance of direct sunlight and polarized skylight in the ant's celestial system of navigation

Abstract

As textbook knowledge has it, bees and ants use polarized skylight as a backup cue whenever the main compass cue, the sun, is obscured by clouds. Here we show, by employing a unique experimental paradigm, that the celestial compass system of desert ants, Cataglyphis, relies predominantly on polarized skylight. If ants experience only parts of the polarization pattern during training but the full pattern in a subsequent test situation, they systematically deviate from their true homeward courses, with the systematics depending on what parts of the skylight patterns have been presented during training. This "signature" of the polarization compass remains unaltered, even if the ants can simultaneously experience the sun, which, if presented alone, enables the ants to select their true homeward courses. Information provided by direct sunlight and polarized skylight is picked up by different parts of the ant's compound eyes and is channeled into two rather separate systems of navigation.

Fig. 1.

Experimental setup: orientation of training channels within an earthbound system of reference. The double-arrowed lines depict the orientation of the training channels in Fig. 2 (open arrowheads, outbound courses and nest-to-feeder directions; filled arrowheads, inbound courses and feeder-to-nest directions). The red arc indicates the azimuthal position of the sun (the solar meridian) during the course of the day. 12:00 is solar noon. The angle α defines the angular difference between the solar azimuth and the orientation of the training channel (nest-to-feeder direction), depicted here for one particular azimuthal position of the sun and one particular orientation of the training channel. The azimuthal positions of sunrise (SR) and sunset (SS) are given for the summer solstice, June 21. The experiments described in this account were performed during the time period from June 19 to August 5.

Fig. 2.

Experimental paradigm I. Shown are the directions of homing courses, ϕ, taken by ants during the course of the day under full-sky conditions after the ants had been trained under partial-sky conditions within linear channels oriented in five different directions. Means and standard deviations are given in 30-min bins (n = 731). Bold numbers and dotted lines mark the true homeward directions taken by the ants within the training channels (see filled arrowheads in Fig. 1). The open arrowheads at the top of the graphs indicate training situations with α = 0° or 90° (for definition of α, see the legend of Fig. 1). The orange curves show computed polarization-induced error functions (see Methods).

Fig. 3.

Experimental paradigm II. Shown are the homing courses of ants that experienced only the sun during their channel-bound training. In the open-field test situation, they had access either to the sun plus the polarization of the sky (black symbols, paradigm II/1, n = 161) or to the sun alone (green symbols, paradigm II/2, n = 121). The orange curve shows the computed polarization-induced error function characteristic for the particular orientation of the training channel used in these experiments (homeward direction of 270°; see Fig. 2E). For further explanation of conventions, see the legend of Fig. 2.

Fig. 4.

Experimental paradigm III. Shown are the homing courses of ants that were deprived of direct sunlight in the open-field test situation. The polarization-induced errors (black symbols, means and standard deviations, n = 262) do not significantly deviate from the errors exhibited by ants that were able to experience both the sun and the pattern of polarized light in the test situation (blue squares, data taken from Fig. 2 D and E). For further explanation of conventions, see the legend of Fig. 2.

Fig. 5.

Experimental paradigm IV/1. Shown are the homing courses of ants that were deprived of polarized skylight (by occlusion of their DRAs) in the open-field test situation. Individual data (open circles) and means ± standard deviations in 30-min bins (n = 189) are shown. For further explanation of conventions, see the legend of Fig. 2.

Fig. 6.

Experimental paradigm IV/2. Shown are the homing courses of ants that were deprived of polarized skylight (by optically cutting off the short-wavelength range of the spectrum) in the open-field test situation. Means and standard deviations (n = 163) are shown. The true homeward direction (ϕ = 270°) is indicated by the dotted line. The dashed line marks the directions that the ants should have taken had they behaved in a purely positive phototactic way (i.e., had they moved toward the azimuthal position of the sun irrespective of their training direction).