Many animals and migratory birds use some navigation system based on magnetic clues. They do not lose their way even when they fly hundreds of kilometres. According to current evidence, a light-dependent “compass” in the retina is one such tool.
But for homing pigeons ( Columbia livia ), scientists have come to accept that supramagnetic magnetite found on the upper part of the beak behaves as a “magnetic sensory apparatus.” Bobolink ( Dolichonyx oryzivorus ) was the first bird in which such a magnetic sensory apparatus was seen.
The magnetite-rich cells in the beak are believed to play a vital role as magnetoreceptors for detecting both the magnetic direction and intensity. Most importantly, these studies showed that these cells are found at six specific locations in the beak of the pigeons.
But a study published recently in Nature has questioned this hypothesis. According to Christoph Daniel Treiber, the first author, the iron-rich cells are not magnetic-field sensitive neurons but macrophages. Iron accumulates as ferritin in the macrophages.
“Our results strongly suggest that the [magnetoreceptor] clusters in the beak of the pigeon are not neurons.” This goes against the grain of previous studies that postulated that magnetoreceptors convert and send the signals to the brain in the form of neuronal impulses.
Most importantly, while the researchers failed to find the six bilateral clusters in the beak as postulated by other authors, they found the macrophages widespread in the body of the birds (stratum laxum of the subepidermis, the basal region of the respiratory epithelium and the apex of feather follicles).
The latest find challenges the hypothesis of magnetite behaving as a sensory system. In fact, it contradicts the work by many researchers who went on to find similar sensory system based on iron-mineral containing structures in other birds.
Found in Robins
For instance, a paper in PLoS One journal (February 2010) found these structures in garden warblers, European robins and domestic chickens. Several studies based on this hypothesis also found certain unique characteristics of the magnetic sensory system.
The role of ferromagnetic materials orienting in response to the earth’s magnetic field comes from the early study of magnetotactic bacteria. A paper published in 2000 in BioMetals journal stated that the “intercellular ferromagnetic materials” in the magnetotactic bacteria “arranged in chains in response” to earth’s magnetic field.
But the Nature paper makes it appear that all previous findings are in fact based on a shaky hypothesis. “Our work necessitates a renewed search for the true magnetite-dependent magnetoreceptor in birds,” the authors write.
They do not exclude the possibility of “sparsely distributed magnetoreceptors” at some “unknown locations” in the upper beak of homing pigeons. They also cite that other locations like the olfactory epithelium may possible be housing the elusive magnetic sensor, as in the case of the rainbow trout.
The study has three major implications, they state. First, it calls for a “re-evaluation” of the behavioural studies that are based on the premise that magnetite-based cells behave as sensors in pigeons in particular and other birds in general. Secondly, there is a need to “re-assess” if the magnetite-based receptors have the required physical and chemical properties to behave like a magnetosensor.
Finally, it is necessary to discover the “elusive magnetic sensor” by looking at alternative sites.