sea) reverse upon experimental inversion of the magnetic poles. In birds Homing pigeons use magnetic fields as part of their complex navigation system. William Keeton showed that time-shifted homing pigeons (acclimatised in the laboratory to a different time-zone) are unable to orient themselves correctly on a clear, sunny day; this is attributed to time-shifted pigeons being unable to compensate accurately for the movement of the sun during the day. Conversely, time-shifted pigeons released on overcast days navigate correctly, suggesting that pigeons can use magnetic fields to orient themselves; this ability can be disrupted with magnets attached to the birds' backs. Pigeons can detect magnetic anomalies as weak as 1.86 gauss. For a long time the trigeminal system was the suggested location for a magnetite-based magnetoreceptor in the pigeon. This was based on two findings: First, magnetite-containing cells were reported in specific locations in the upper beak. However, the cells proved to be immune system macrophages, not neurons able to detect magnetic fields. Second, pigeon magnetic field detection is impaired by sectioning the trigeminal nerve and by application of lidocaine, an anaesthetic, to the olfactory mucosa. However, lidocaine treatment might lead to unspecific effects and not represent a direct interference with potential magnetoreceptors. As a result, an involvement of the trigeminal system is still debated. In the search for magnetite receptors, a large iron-containing organelle (the cuticulosome) of unknown function was found in the inner ear of pigeons. Areas of the pigeon brain that respond with increased activity to magnetic fields are the posterior vestibular nuclei, dorsal thalamus, hippocampus, and visual hyperpallium. Domestic hens have iron mineral deposits in the sensory dendrites in the upper beak and are capable of magnetoreception. Beak trimming causes loss of the magnetic sense. In mammals Some mammals are capable of magnetoreception. When woodmice are removed from their home area and deprived of visual and olfactory cues, they orient towards their homes until an inverted magnetic field is applied to their cage. When the same mice are allowed access to visual cues, they are able to orient themselves towards home despite the presence of inverted magnetic fields. This indicates that woodmice use magnetic fields to orient themselves when no other cues are available. The magnetic sense of woodmice is likely based on a radical-pair mechanism. The Zambian mole-rat, a subterranean mammal, uses magnetic fields to aid in nest orientation. In contrast to woodmice, Zambian mole-rats do