Homing in on vertebrates

Abstract

Imagine being set adrift in a canoe in the middle of an ocean. Which way would you paddle? Most humans would be as lost as lost can be, but creatures such as pigeons, turtles and whales have no difficulty navigating in such circumstances. How they do so remains one of the biggest mysteries in the behavioural sciences, at the centre of which is the question of how organisms might sense the Earth's magnetic field and use it for navigation and homing (a topic with a chequered history — see box, overleaf). The mystery, however, is gradually being solved, and the latest instalment in the story comes in Walker and colleagues' study of rainbow trout (page 371 of this issue). All known sensory systems have specialized receptor cells designed to respond to the external stimulus, and these are always coupled to neurons to bring this information to the brain. In modern times the main objection to claims that magnetoreception is genuine was biophysical — that there was no evidence of appropriate receptors. The first clue towards meeting this objection came in an obscure place, the teeth of small molluscs. The late Heinz A. Lowenstam discovered that the major lateral teeth of the chitons are capped by massive amounts of a hardening agent, the permanently magnetic mineral magnetite (Fe_(3)O_4) (Fig. 1). Later, with the development of ultrasensitive superconducting moment magnetometers, came the recognition that tiny (parts per billion) levels of biogenic magnetite are naturally present in animals as diverse as insects, birds, fish and even humans. The final crack in the biophysical arguments against magnetoreception came with the discovery of the magnetotactic bacteria and protists (Fig. 2), which possess linear chains of either single-domain magnetite or greigite (Fe_(3)S_4). They provide unequivocal examples of biological activity being influenced by the geomagnetic field.