Animal life is largely characterized by movement and high levels of individual mobility. However, an endogenous system for egocentric and allocentric orientation is crucial if any movement is supposed to be goal-directed. Any goal can be reached by the help of more or less advanced navigational capacities. As part of this process, environmental cues are integrated by the available sensory organs. However, the more sensory organs are integrated, the more complex navigation will be due to the weighing of cues. Hierarchies of cues may be established for efficient navigation. Wild mammal navigation research is still in its infancy compared to its avian counterpart, in particular when it comes to long-range navigation and movements of long distances. Animal migration belongs to one of the most complex phenomena, we can observe in nature. To date, observation of long-distance moving mammal migrants is still technically limited, in particular if experimental manipulation of the moving individual is envisioned. Therefore, the navigational capacities and orientation mechanisms of wild species are virtually unknown. A promising example however, are bats. For non-migratory bat species, evidence of a magnetic sense has been provided (Holland et al. 2006, 2008). Further, we know that the calibration of a nocturnal compass system in bats happens at dusk. In this work I address the question of whether migratory bats have the potential for a mammalian model in navigation research. Assuming that non-migratory and migratory bats do not differ significantly in their navigational systems, two further questions can be deduced: Which directional reference or cue calibrates the compass system of bats at dusk? And further: Do migratory bats possess a magnetic sense which they could use for orientation and navigation? Among the migratory species, the Nathusius’ bat, Pipistrellus nathusii (Keyserling und Blasius, 1839), and its sister species the Soprano pipistrelle, Pipistrellus pygmaeus (Leach, 1825), make ideal models for experimental work, as they are relatively small and widely abundant in Europe. Every late summer, thousands of individuals from both species migrate along the Baltic Sea shore. Individuals ringed in the Baltics have been recovered in southern France (~2,000 km) which indicates that they might possess a well-advance navigation system. Here, I studied the orientation capacity of Nathusius’ bats and Soprano pipistrelles after translocation away from their coastal migration corridor. In chapter I, I investigated of polarized skylight calibrates the compass system in Nathusius’ bats. A recently published paper about non-migratory greater mouse-eared bats, Myotis myotis, revealed that these bats would use polarized light for exactly this purpose at dusk. Here, I used the exact same cages including polarization filter windows from that study and compared the departure flight directions of a control and a treatment group after translocation for 11 km. In contrast to the control, the experimental group watched the sky with any polarization cue 90° shifted due to the polarization filters. Animals were later fitter with VHF-tags, released and tracked until they vanished from tracking range. However, after translocation both groups continued flight in a southerly direction. Therefore a calibration based on the band of polarized light at dusk appears non-existent. In chapter II, I applied a self-made circular release box for bats based on eight exits to test whether takeoff orientation would be a reliable proxy for departure flight direction. Using advanced night vision scopes, I found that both Nathusius’ bats and Soprano pipistrelles depart in the same direction as they chose for takeoff. In chapter III, I conducted a mirror-experiment to test whether Soprano pipistrelles would use the solar azimuth (or a 180° deflection of it) at sunset to calibrate their compass system. However, I discriminated between naïve subadult migrants and experienced adult bats. For the measurement of takeoff at night, I built a novel version of the circular release box which enabled bats to take of in any direction. I found that adult Soprano pipistrelle takeoff orientation was dependent on the solar azimuth, with the experimental group departing in the opposite direction compared to the control group. However, naïve subadult migrants took off in random directions suggesting that calibration of a compass system using the solar azimuth bears a learning component. In chapter IV, I used Nathusius’ bats to investigate the hypothesis that the cornea could carry magnetic particles which could be part of a magnetic compass system (Wegner et al. 2006). If so, then bilateral topical anaesthesia using oxybuprocaine would impair magnetic compass-based navigation resulting in random departure flights of bats. In accordance with the predictions, animal with bilateral corneal anaesthesia departed in random directions while controls and unilateral treated animals vanished in seasonally appropriate southerly direction. In conclusion the data collected over the course of my dissertation demonstrate that migratory bats, i.e., P. nathusii and P. pygmaeus are highly suitable model species for studies of mammal navigation and sensory physiology. Both VHF tracking after translocation and release, and the application of the circular release box for bats are useful assays which also generate data that is reproducible. In contrast to findings from non-migratory M. myotis, the band of polarized skylight does not play a role for Nathusius’ bats’ compass calibration at dusk. However, in light of the data collected with Soprano pipistrelles, the solar azimuth appears to be the prevailing calibration reference. Yet, this could only be demonstrated in adult, experienced bats. Data from this thesis further support the assumption that the cornea plays an important role for the orientation capacity of Nathusius’ bats. Observed randomly directed departure flights in bats with bilateral corneal anaesthesia argue for a corneal sense contributing to orientation and navigation which could be a magnetic sense based on iron particles sensu Wegner et al. (2006).
