Homing pigeon navigation relies on superparamagnetic macrophages under overcast conditions.

Birds have long been considered an excellent model system for studying navigation. For example, songbirds migrating at night or under cloudy skies can maintain a magnetically calibrated flight direction established at sunset for hundreds of kilometers. Despite decades of intensive research, the mechanisms of magnetoreception remain a mystery and are the subject of intense debate, especially when compared to the sensory systems of other vertebrates. To date, three main theories regarding magnetoreception in pigeons have been proposed: the cryptochrome theory, the magnetite theory, and the theory of an undefined mechanism based on voltage-gated ion channels. A new German study supports a fourth theory, which is based on the collective sensing ability of superparamagnetic macrophages, primarily found in the liver, that enable perception of geomagnetic direction.

To identify tissues in pigeons with magnetic properties, scientists examined various organs, including the liver, spleen, muscles, and the inner and outer beaks, using a technique called vibrating sample magnetometry (VSM). VSM measures the magnetic properties of samples by determining the current induced when the sample moves through magnetized coils. The scientists then used immunohistochemistry with cell type-specific antibodies to isolate the cell type exhibiting magnetic properties. They stained iron(III) with Prussian blue because aside from magnetite crystals, which are made of iron, other forms of iron are the only naturally occurring strongly magnetic elements found in animals. (Although nickel is magnetic, it is incompatible with vertebrate biology.) The scientists isolated the iron-accumulating cell type by exploiting its ability to bind to columns during magnetic cell separation, as previously reported by the same research group. Using gene sequencing and expression patterns, the scientists identified the iron-containing cells and their function. First, they tested clodronate, a toxin specific to the liver macrophages identified in previous steps, in vitro and then on live pigeons to verify whether liver macrophages are essential to the magnetic sense of pigeons.

Thirty-four pigeons were individually trained to fly home over a 19-km route from west to east. After ten successful training flights, the birds were randomly assigned to receive either intravenous clodronate liposomes (n = 18) or control liposomes (n = 16). The following day, the pigeons were released individually under completely overcast skies and tracked using real-time GPS devices.

A strong magnetic blocking effect was observed in the livers and spleens of the pigeons, and a weaker effect was observed in their muscle tissue and beaks. These results suggest the presence of superparamagnetic particles in these organs. However, in the liver and spleen, these signals were masked by significantly stronger ferrimagnetic signals. Previous studies of other animal species, such as horses and mice, have detected ferritin nanoparticles that are predominantly localized in splenic macrophages. In the present study, the magnetic signals indicate a significant accumulation of iron-based nanomagnets in pigeon livers and a significantly lower accumulation in spleens. Prussian blue staining overlapped with MHC-II staining, which is specific to immune cells such as macrophages, dendritic cells, and B lymphocytes. After separating the magnetic cells using a magnetic column, gene sequencing revealed that the cells were macrophages. In addition, high expression of genes associated with iron metabolism was observed. One of these genes encodes a transcription factor that is essential for the development of macrophages in the red pulp of the mammalian spleen. These results suggest that the sorted MHC-II+ cells are involved in erythrocyte clearance and iron(III) and ferritin accumulation. In pigeons injected with the macrophage-specific toxin clodronate, the previously identified highly magnetic liver cells could not be detected 24 hours later when the same tests as above were performed again.

These tests and further investigations confirmed that pigeon livers contain macrophages that accumulate iron, thereby acquiring superparamagnetic properties. Next, the authors investigated how these macrophages might relay (navigational) signals to the brain. Using various staining methods and microscopic techniques, they determined that the brain is directly connected to peripheral nerve fibers in the liver. Within the liver, they frequently found macrophages in close proximity to nerve fibers (≤ 2 μm), which enables paracrine signal transmission or direct cell-to-cell contact.

All of the pigeons that were given the placebo found their way home within 70 minutes. However, none of the birds treated with clodronate returned on days when the skies were persistently cloudy. Instead, they exhibited random spatial orientation. Notably, pigeons with a macrophage deficiency could find their way home as soon as the sun became visible through the clearing clouds.

Taken together, these results suggest that pigeons can perceive magnetic direction independently of a potential visual, cryptochrome-mediated magnetic sensing system. The results also suggest that macrophages are necessary for determining magnetic direction when the sun and landmarks are unavailable. The authors hypothesize that liver macrophages detect changes in the Earth’s magnetic field and relay this information to the brain via the vagus nerve. They assume that magnetoreception requires a signal at the cell population level, rather than the single-cell level, to reach the threshold for neuronal activation. Liver macrophages are well-suited for this role because they accumulate iron(III) during erythrocyte clearance and are closely connected to autonomic nerves.

It is unclear whether macrophages transmit magnetic information mechanically or via a soluble mediator or hormone. However, it is known that macrophages use the paracrine pathway to transmit other types of information to the brain. One advantage of this new model is its potential to explain magnetoreception in animals that lack functional cryptochromes or that inhabit environments with little to no light, including bats, blind moles, and sharks.

Editor’s note:

This new study is refreshing because it draws on the full scientific "toolbox," presents an entirely new and unexpected hypothesis, and largely proves the hypothesis through experimentation. The only remaining questions are how liver macrophages (mechanically or hormonally) transmit magnetic orientation information to the brain and how the brain processes this information. The discovery of a different type of magnetic compass, in addition to retinal cryptochrome and magnetite (mostly found in the brain), raises the question of whether superparamagnetic macrophages (in the liver or spleen) also mediate magnetoreception in other species. Further experiments are needed to determine if this is the actual function of magnetoreception in species that were previously thought to depend on cryptochrome or magnetite. In addition, we must consider which species possess multiple mechanisms of magnetic field sensitivity. (Some species of birds of prey and songbirds, for example, appear to use retinal cryptochromes. Is the homing pigeon an exception or is this the norm?) Finally, we must determine which external electromagnetic field parameters can disrupt this magnetoreception. (AT)