American cockroaches prefer four cardinal geomagnetic positions at rest

A specific behaviour based on the ability to perceive the magnetic field has been described in several species: when resting or grazing animals take up a position they place their main body axis parallel with the North-South or East-West geomagnetic axes, which is referred to as magnetic alignment. The adaptive significance of this behaviour remains an enigma. No experiments have been made to date to demonstrate conclusively whether that orientation will adequately change in response to an experimental rotation of geomagnetic axes which is a key step to prove the use of exclusively magnetic cues for orientation. In our study, we used automatic photographing to monitor body positions of American cockroaches (Periplaneta americana) placed individually to Petri dishes both in a natural geomagnetic field and in a field where the magnetic North has been rotated by 60 degrees. We identified a preference regarding the four cardinal magnetic axes, i.e., a quadrimodal alignment both in natural and in 60 degrees rotated fields. The study gives the original evidence that quadrimodal alignment is a type of animal behaviour specifically related to the cardinal magnetic axes of the Earth.