Migration and orientation in anuran amphibians

Abstract

In anuran amphibians migratory behaviour is constrained by the demands of water balance and thermoregulation. Therefore, the migratory range of anurans amounts to 15 km at most. Adult anurans perform migrations, if important habitat resources (e.g. sites for reproduction, nutrition and hibernation) are spatially separated. Site fidelity to these spatial units is a common feature of most anurans. These general considerations are exemplified in the common European toad Bufo bufo. Directed movements such as migrations towards a breeding or home site require mechanisms of orientation. The spatial range of the homing ability coincides widely with the natural migratory range of each species. Many sensory systems are involved in the perception of orientation stimuli: the auditory, the olfactory and the visual system as well as the still unidentified system of magneto-perception. Vocalization of conspecifics, odours of ponds, landmarks, the positions of sun, moon and stars, polarization patterns of the sky and the earth's magnetic field may serve as orientation cues. Initial orientation following displacement is based on the processing of different sensory information in a hierarchial order which depends on the relative availability of the potential cues in the habitat of each species. The fact that initial orientation requires at least two independent cues suggests that the map-compass concept of orientation is applicable to anurans. Nevertheless, there is evidence for redundancy in this multisensory system: olfactory and some visual cues are probably related to the map, magnetic and celestial cues to the compass orientation, and acoustic cues and fixed visual landmarks can be used for piloting.