Bird's Internal Compass is Derived From Newly Discovered Protein

Birds can sense Earth’s magnetic field, and this uncanny ability may help them fly home from unfamiliar places or navigate migrations that span tens of thousands of kilometers. For decades, researchers thought iron-rich cells in birds’ beaks acted as microscopic compasses. But in recent years, scientists have found increasing evidence that certain proteins in birds’ eyes might be what allows them to see magnetic fields.

Scientists have now pinpointed a possible protein behind this “sixth sense.” Two new studies — one examining zebra finches, the other looking at European robins — both single out Cry4, a light-sensitive protein found in the retina. If the researchers are correct, this would be the first time a specific molecule responsible for the detection of magnetic fields has been identified in animals.

This is an exciting advance — Cry4 is part of a class of proteins called cryptochromes, which are known to be involved in circadian rhythms, or biological sleep cycles. But at least some of these proteins are also thought to react to Earth’s magnetic field thanks to the weirdness of quantum mechanics. The protein’s quantum interactions could help birds sense this field. To figure out which of three cryptochromes is responsible for this quantum compass, scientists examined the retinas, muscles and brains of 39 zebra finches for the presence of the three proteins Cry1, Cry2 and Cry4. The team found that while levels of Cry1 and Cry2 followed a rhythmic pattern that rose and fell over the day, Cry4 levels remained constant, indicating the protein was being produced steadily.

Scientists assume that birds use magnetic compasses any time of day or night European robins also showed constant levels of Cry4 during a 24-hour cycle, and higher levels during their migratory season. And the researchers in that study found Cry4 in an area of the robin’s retina that receives a lot of light — a position that would help it work as a compass, the study says. CITATIONS A. Pinzon-Rodriguez, S. Bensch and R. Muheim. Expression patterns of cryptochrome genes in avian retina suggest involvement of Cry4 in light-dependent magnetoreception (cryptochrome expression in zebra finches). Journal of the Royal Society Interface. Published online March 28, 2018. doi: 10.1098/rsif.2018.0058 A. Günther et al. Double-cone localization and seasonal expression pattern suggest a role in magnetoreception for European robin cryptochrome 4. Current Biology. Vol. 28. January 22, 2018, p. 211. doi: 10.1016/j.cub.2017.12.003