Over the years, many examples of sensory systems that humans apparently do not possess have popped up in interesting places among the vertebrates. Some are extensions of known senses — echolocation in bats and whales, infrasound and ultrasound detection in birds, and the sex-pheromone receptors (the vomeronasal system) in the nose of most higher vertebrates. Others, such as the electroreceptive organs of sharks and rays, and the infrared detectors of snakes, depend on highly specialized receptor cells. These discoveries generated little scientific controversy.

Not so biomagnetism, which in the late eighteenth century received an early bad rap from the activities of Franz Anton Mesmer and his followers. The mesmerites claimed that they could cure disease by exposing patients to magnetized objects, a claim debunked by a commission (which included Benjamin Franklin) appointed by King Louis XVI. The subject then fell into a long period of disrepute until the 1940s, when experiments suggesting that pigeons might use geomagnetic cues during homing generated great interest. These, however, were difficult to reproduce; likewise, conditioning experiments designed to elicit magnetoreception in the laboratory also failed.

The main stumbling block was in seeing how a geomagnetic stimulus could be converted into a signal that an individual cell could detect. Back-of-the-envelope calculations ruled out most of the obvious methods (paramagnetism, electrical induction, the Hall effect, nuclear magnetic resonance). The simplest strategy — that of having a small permanent magnet — was dismissed on the grounds that there were no physiological ferromagnetic materials2. As discussed in the main text, the discovery of magnetite eventually knocked that argument on the head.

Meantime, behavioural experiments kept on coming up with apparent geomagnetic effects on animal behaviour. But it took nearly two decades to realize that the geomagnetic compass used by adult birds was programmed to be ignored if other orientation cues (such as a Sun or star compass, polarized skylight, infrasound and ultrasound) were present. These orientation cues constitute a complex but consistent web of interacting responses, which are used not only by birds but in all major vertebrate groups and many invertebrates (reviewed in ref. 5).