A recent study has suggested a sixth possible way animals might detect the Earth’s magnetic field, a phenomenon known as magnetoreception. Scientists have known for over 60 years that animals can sense magnetic fields, often using this ability to navigate over long distances. However, the exact mechanism by which they detect these fields remains unclear, and there is no single, widely accepted explanation. Magnetoreception is not limited to navigation. It plays a role in orientation for various animals, including invertebrates, fish, amphibians, and reptiles. Mammals also use it in activities like nest building, hunting, and returning to familiar locations. Despite this, there is no obvious sensory organ like the eye or ear that is clearly responsible for detecting the magnetic field. This has led to the idea of a “sense without a receptor,” although scientists argue this is an oversimplification. According to a recent study, there could be as many as six different mechanisms by which animals detect the magnetic field. One of these, proposed in a new study on homing pigeons, involves the liver. Researchers in Germany treated pigeons with a chemical called clodronate, which removes a type of white blood cell called macrophages from the liver. When these pigeons were released under cloudy conditions—when they couldn’t see the sun—they became disoriented and failed to find their way home. However, when released under sunny conditions or without the treatment, they navigated normally. This suggests that the macrophages in the liver might be involved in detecting the magnetic field. Macrophages are part of the immune system and are responsible for clearing old red blood cells, which contain iron. As these cells accumulate iron, they may become magnetically sensitive. The study suggests that these iron-rich macrophages, located in a part of the liver with many nerve cells, could send signals to the brain about the direction of the magnetic field. This is not the first time iron particles have been linked to magnetoreception. For example, the beak of birds contains iron particles that may be involved in detecting the magnetic field in certain situations. Scientists believe that different species may have evolved different ways to detect the magnetic field, depending on their environment and behavior. For instance, a light-dependent mechanism, used by some birds and insects, might not work well in bright daylight. In such cases, another mechanism, like the one involving liver macrophages, may be necessary. Night-migrating birds, on the other hand, might rely more on the light-dependent system. While it’s still unclear which of these mechanisms are truly responsible for magnetoreception, the research has identified six strong possibilities.