Researchers have identified as many as six distinct biological mechanisms through which animals may sense the Earth's magnetic field — a navigational ability observed across species from insects and fish to birds and mammals, yet still lacking a clearly confirmed sensory receptor. The latest addition to the list comes from a German study published in Science, in which homing pigeons whose liver macrophages (immune cells that accumulate iron from degraded red blood cells) were chemically removed became disoriented when navigating under overcast skies, suggesting the liver may play an unexpected role in magnetic sensing. Scientists believe different species may have evolved different solutions depending on their ecology — for example, night-migrating birds may rely on a light-sensitive molecule called cryptochrome, while daytime navigators like pigeons may depend on iron-based mechanisms — though direct proof remains elusive for all six proposed systems.