O. G. Dely szerk.: Vertebrata Hungarica 21. (Budapest, 1982)
Adler, Kr.: Sensory aspects of Amphibian navigation and compass orientation 7-18. o.
Tim e -Compena ated Compass Orientation It has been demonstrated for many animals Including amphibians that the sun can be utilized for compass orientation (Fig. 1). If an organism is to use the sun as a compass reference, however, it must take into account the sun' s apparent movement due to the earth' s rotation about Its axis. This requires that compas s-orienting animals must have an endogenous rhythm by which they can determine the time of day and thus interpret the sun' s position in an adaptively appropriate manner. Such dependence upon an "internal clock" can be tested by re-setting the clock using shifted photope rl od s in an environmental chamber In the laboratory and then testing the animals in the normal manner under natural skies. Phase shifts of 6 hours in the animal's time sense should result in angular changes in orientation of about 90° compared to controls, if the animals are compensating in a 15°-per-hour manner (Fig. 2). A phase-delayed animal, for example, when released in the afternoon, is in the early part of Its ptootoperiod (or "morning") and thus moves with respect to the sun as if It is a "morning sun" when in fact it Is an "afternoon sun". Thus, such a phase-delayed animal would move clockwise to the compass heading taken by the control animals. Through a similar series of steps one can predict that a phase-advanced animal would move in a direction counterclockwise to the compass headings of controls. Further details and diagrams are given in ADLER (1976), together with references on time-compensated compass orientation in various amphibian species. Phase-shift experiments such as these lend strong support to the Idea of time-dependence but do not exclusively point to the sun as the reference cue since skylight polarization patterns are directly synchronized to the sun's position and the use of these patterns for orientât!on.would therefore also require time-compensation (von FRISCH 1967, BRINES 1980). Thus, one needs to determine whether amphibians are utilizing polarization patterns or the sun or both in orientation. Use of Skylight Polarization Patterns for Orientation The perception of linearly-polarized light by many species of invertebrates and its use by them for orientation - a process termed polarotaxls - is well documented (see references in von FRISCH 1967, and WATERMAN 1973) but has been demonstrated only recently in vertebrates. In experiments with tiger salamanders (Ambystomatldae: Aroby stoma tigrinum) , animals , were trained under artificial linearly-polarized light and later tested in a circular arena (Fig. 3). The axis of the animals' orientation in the arena could be shifted by turning the transmission axis (or e-vector) of the polaroid filter below the light source, since the animals had been previously trained to move perpendicular to the e-vector. Since the e-vector is a bilaterally symmetrical cue the salamanders cannot distinguish the two sides and, as expected, moved in two opposite directions 90° to the e—vector (Fig, 41. Similar results have been obtained with bullfrog (Ranldae; Rana cate sbeian a) tadpoles, both in laboratory and outdoor tests (AUBURN & TAYLOR 1979). In nature the ability to detect polarization patterns may be adaptive (1) underwater where polarization patterns exist which have a specific relationship to the sun's position; (2) during twilight when the sun's position cannot be directly determined but when skylight polarization is maximal and at which time many amphibians typically make their migratory journeys; and (3) in forested areas where the sun's position can be inferred from polarization information (even though the sun may be obstructed by the canopy), providing that an unclouded patch of blue sky is visible. The availability of a single celestial reference system, used both on land and underwater, would presumably be advantageous especially for amphibious animals (see TAYLOR & ADLER 1973, for further discus sic«). Whether the sun alone can act as a cue for compass orientation is equivocal. Although earlier tests suggested that the sun was utilized (reviewed In FERGUSON 1971), the later demonstration of perception and use of skylight polarization patterns renders those earlier experiments uncertain. Moreover, even in experiments with eyes intact but the pineal body removed or covered (reviewed in ADLER 1976), it is possible that the animals were utilizing magnetic or other cues instead of the sun. Definitive experiments are needed to re-establish this point. Extraocular Detection of Polarized Li ght Orientation experiments with tiger salamanders also showed that the receptor for linearlypolarized light is extraocular, at least in this species (Fig. 4). Animals oriented along the axis predicted from the position of the e-vector even when the animals' eyes had been removed but polarotaxls was eliminated when the tops of their heads were covered with an opaque plastic block. \
