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Magnetoreception is a key element in the sensory repertoire of many organisms, and it has been shown to play a particular role in animal navigation. While the first data to demonstrate a magnetic compass in songbirds through behavioural measures were presented decades ago, studies of behaviour are still the main source of information in learning about the magnetic senses. The behavioural evidence is, however, scattered with sometimes contradictory results. Partly, this is a consequence of a wide spectrum of methods used across multiple research groups studying different model organisms. This has limited the ability of researchers to pin down exactly how and why animals use the Earth's magnetic field. Here, we lay out how a range of methods for testing behaviour spanning from field observations to laboratory manipulations can be used to test for a magnetic sense in animals. To this end, we discuss the principal limitations of behavioural testing in telling us how animals sense the magnetic field, and we argue that behaviour must go hand in hand with other fields to advance our understanding of the magnetic sense. [ABSTRACT FROM AUTHOR]

Virtual magnetic displacements are used to examine the magnetoreceptive ability of animals by changing the local magnetic field to emulate one that exists elsewhere. This technique can be used to test whether animals use a magnetic map. The viability of a magnetic map is dependant upon which magnetic parameters an animal's coordinate system is composed of, and how sensitive they are to those parameters. Previous research has not considered the degree to which sensitivity can change an animal's impression of where a virtual magnetic displacement is located. We re-assessed all published studies that use virtual magnetic displacements assuming the highest likely level of sensitivity to magnetic parameters in animals. The vast majority are susceptible to the existence of alternate possible virtual locations. In some cases, this can cause results to become ambiguous. We present a tool for visualising all possible virtual magnetic displacement alternative locations (ViMDAL) and propose changes to how further research on animal magnetoreception is conducted and reported. A tool for visualising virtual magnetic displacement locations (ViMDAL) is used to analyse existing data on virtual magnetic displacements in animals. This shows the existence of alternate possible virtual locations that may confound existing research. [ABSTRACT FROM AUTHOR]

The exact anatomical location for an iron particle-based magnetic sense remains enigmatic in vertebrates. For mammals, findings from a cornea anaesthesia experiment in mole rats suggest that it carries the primary sensors for magnetoreception. Yet, this has never been tested in a free-ranging mammal. Here, we investigated whether intact corneal sensation is crucial for navigation in migrating Nathusius' bats, Pipistrellus nathusii, translocated from their migratory corridor. We found that bats treated with corneal anaesthesia in both eyes flew in random directions after translocation and release, contrasting bats with a single eye treated, and the control group, which both oriented in the seasonally appropriate direction. Using a Y-maze test, we confirmed that light detection remained unaffected by topical anaesthesia. Therefore our results suggest the cornea as a possible site of magnetoreception in bats, although other conceivable effects of the anaesthetic are also explored. Furthermore, we demonstrate that the corneal based sense is of bilateral nature but can function in a single eye if necessary. Lindecke and colleagues experimentally investigate the effect of corneal anesthesia on bat migratory orientation. Their results suggest that the cornea is a promising organ for the search of biological magnetic compass sensors, and also indicate that a single functional cornea is sufficient for migratory navigation. [ABSTRACT FROM AUTHOR]

Summary From bats to whales, millions of mammals migrate every year. However, their navigation capacity for accomplishing long-distance movements remains remarkably understudied and lags behind by five decades compared to other animals [ 1, 2 ]—partly because, unlike for other taxa, such as birds and sea turtles, no small-scale orientation assay has so far been developed. Yet recently, bats became a model to investigate which cues mammals use for long-range navigation, and, surprisingly for nocturnal animals, sunset cues, and particularly polarized-light cues, appear to be crucial for calibration of the magnetic-compass system in non-migratory bats [ 3–5 ]. This does not appear to hold for a species of migratory bat, however [ 6 ], and thus the nature of the information used by migratory bats for navigation remains unclear. Here, we asked whether the position of the solar disk per se is relevant for compass orientation in a migratory bat, Pipistrellus pygmaeus. Using a new experimental assay that measures takeoff orientation, we tested the orientation of bats exposed to a shifted sunset azimuth using a mirror at dusk. Bats exposed to a 180°-rotated azimuth of the setting sun and released after translocation during the same night shifted their heading direction by ∼180° compared to control bats. However, first-year migrants had no clear orientation either as controls or after the same treatment. This suggests that learning the migratory direction is a key component in the navigational system of naive bats in this species. Our study provides rare evidence for the specific cues and mechanisms that migratory mammals use for navigation. Graphical Abstract Highlights • The first effective manipulation of the compass system in a migratory mammal is described • Adult bats shifted orientation at night by misinterpreting a mirrored sun at dusk • There may be no innate component to orientation in first-year migrants • A novel assay measures bat takeoff orientation based on free directional choice For bats to navigate at night, a compass sense is presumed to be calibrated at sunset. By manipulating the sun's position and recording bats' takeoff orientation at night, Lindecke et al. study the navigational capacity of migratory Soprano pipistrelles. Their results provide evidence that adult bats use the sun's azimuth for compass calibration. [ABSTRACT FROM AUTHOR]