Bacteria can respond to environmental factors other than chemicals. A fascinating example is that of the aquatic magnetotactic bacteria that orient themselves in the earth’s magnetic field. Most of these bacteria have intracellular chains of magnetite (Fe3O4) particles or magnetosomes, around 40 to 100 nm in diameter and bounded by a membrane (see Box figure). Some species from sulfidic habitats have magnetosomes containing greigite (Fe3S4) and pyrite (FeS2). Since each iron particle is a tiny magnet, the Northern Hemisphere bacteria use their magnetosome chain to determine northward and downward directions, and swim down to nutrient-rich sediments or locate the optimum depth in freshwater and marine habitats. Magnetotactic bacteria in the Southern Hemisphere generally orient southward and downward, with the same result. Magnetosomes also are present in the heads of birds, tuna, dolphins, green turtles, and other animals, presumably to aid navigation. Animals and bacteria share more in common behaviorally than previously imagined.