Just because we humans can't sense Earth's magnetic field doesn't mean other animals can't. Although we have plenty of behavioral evidence than many animals, from fruit flies to foxes, have magnetoreceptive abilities, it's tough for us to imagine what it must be like to be able to navigate by some sort of internal magnetic compass. It's been even harder for scientists to grasp what is going on physiologically with this sixth sense, since the mechanism by which animals achieve magnetoreception has remained stubbornly obscure.
But in the past few months, two different studies have reported on a protein found in birds' eyes that seems to allow them to get a visual on the planet's seemingly invisible magnetic pathways. This substance, called Cry4, is a type of protein found in the eyes that helps regulate circadian rhythms, and previous research has suggested it may also be instrumental in magnetoreception.
A study published in the March 26, 2018 issue of the Journal of the Royal Society Interface reports that although zebra finches have three different cryprochrome proteins — Cry1, Cry2 and Cry4 — Cry4 is the only one that showed up consistently throughout a 24-hour period in the birds' eyes, suggesting that it doesn't have as much to do with the animals' circadian clocks as with something that might be necessary no matter what time of day it is. Navigation, for instance.
Atticus Pinzón-Rodríguez, the study's lead author and a doctoral student in the Department of Biology at Lund University in Sweden, told The Independent that their results, "indicate that other animals, perhaps all of them, have magnetic receptors and can pick up on magnetic fields".
Their findings are supported by a separate study published in the Jan. 22, 2018 issue of the journal Current Biology, which looked at European robins instead of zebra finches. The research team, based out of the Carl von Ossietzky University Oldenburg in Germany, found that Cry4 is concentrated in the parts of a robin's eye that receive the most light, suggesting that magnetoreception is most likely a sense associated with eyesight — that birds can probably actually see the magnetic fields rather than, say, hear or feel them.
The Carl von Ossietzky University also compared the presence of Cry4 in the eyes of robins, which migrate to warmer climes each winter, and chickens, which are non-migratory. They found that migratory season was accompanied by boosts in Cry4 levels in the eyes of the robins, where the chicken experienced no such change. This suggests Cry4 is important for navigation in migratory birds.