Cryptochrome 4: A Bird’s GPS
The mysterious disappearance of birds through seasons has long fascinated mankind. Aristotle himself thought birds metamorphosed into different species between seasons. The common redstart is a well-known summer visitor in most of Europe, for example, but come winter, its strange disappearance coincides with the European robin’s arrival from the North. This led the Ancient Greeks to believe that redstarts transformed into robins, their somewhat similar plumage reinforcing their point. Pliny the Elder was convinced that some birds hibernated and - although it is well known today that most birds don't - their puzzling disappearance supported the theory. It was still believed in the sixteenth century that swallows dived under water to hibernate. (https://engines.egr.uh.edu/episode/2228) An even more amusing urban legend, of which nobody seems to know the origin, concerns hummingbirds. The myth suggested that hummingbirds hitchhike on the backs of geese, a testimony to the fact that humans found it so hard to believe that a bird so small might be able to achieve such a journey.
One of the reasons people came up with these stories is that they couldn’t observe them migrating and couldn’t imagine how they might get from place to place and navigate. Bird navigation remained a mystery well after we knew about migration - and it has turned out to be even more complex than we could have imagined.
Nowadays, you have probably heard that migrating birds use celestial bodies, amongst other things, to orientate themselves when migrating. However, this idea is a relatively new one. While Charles Darwin described the possibility that birds might use celestial bodies for orientation, nobody could prove this until Franz and Eleonore Sauer’s 1958 planetarium experiments. This experiment involved garden warblers and blackcaps placed in cages and under an artificial starry night sky, that they could manipulate experimentally and observe the effects on the birds. Shifting the position of the stars in the sky caused the birds to fly in a different direction. This was the first official proof that birds did indeed use the stars for nocturnal migration.
Three years earlier, Gustav Kramer, another German zoologist, had been able to prove that birds also use topographical features of the landscape. Birds could use the sun and the stars when these were visible and use the features of the landscape when it is overcast. The puzzle was starting to make sense. However, Roswitha and Wolfgang Wiltschko’s experiments in 1966, involving magnetic manipulation, added a new facet to our understanding of bird migration. The couple placed some robins in cages allowing them to display their usual zugunruhe (migratory restlessness). They then surrounded the cages with coils generating a magnetic field. When the direction of the magnetic field was altered, something strange occurred: the robins changed their zugunruhe orientation. Birds could sense magnetic fields, but how?
In 1978, Klaus Schulten from the Max Planck institute put forth an idea that still stands today as the key to understanding magnetoreception: radical-pair mechanism. Radicals are molecules that have one unpaired electron. This unpaired electron can spin either up or down. This spin gives them magnetic properties: they basically behave like ‘mini-magnets’. When two radicals come together, they form a ‘radical pair’. There are two types of radical pairs: singlets (the two electrons are spinning in the same direction) or triplets (one electron is spinning up and the other is spinning down). These two electrons are ‘connected’ through ‘quantum entanglement’. The internal magnetic field and complex interaction between these two electrons causes them to switch spins regularly, meaning singlets become triplets and triplets become singlets millions of times per second. Most importantly, this intricate and wonderfully synchronised quantum ‘dance’ is affected by external magnetic fields, including the Earth’s.
But, how does this relate to birds, or rather: what triggers radicals to be formed? Birds have a photosensitive protein in the eye, called cryptochrome 4. FAD molecules within the cryptochrome absorb blue light, entering an excited, high-energy state. It transfers one of its electrons to a tryptophan molecule nearby, forming a radical pair. The ratio of the pairs’ singlet to triplet states is affected by the bird’s orientation relative to the Earth’s magnetic field. The radicals also undergo chemical changes which put the cryptochrome into a ‘signalling state’, changing shape and triggering neurotransmitters to be released in the brain. The proportion of cryptochrome in each state depends on the orientation, and informs the direction of the bird’s flight. (The details of this process are too complex to explore fully here, but you can read more in this article.)
Schulten’s research has opened the door to dozens of hypotheses around how birds perceive magnetic fields. Birds could potentially ‘see’ them as patterns overlaid on top of their normal vision. Some people believe that birds also have an internal ‘map’ as a result of magnetite in their beaks.
Magnetoreception is one of these fascinating junctions where quantum physics, chemistry, neuroscience and behavioural biology meet, and it is still a new area of study… If you’re interested in bird migration, you may want to read my previous blog post on the genetics and epigenetics of animal migration (Migration: Unraveling Nature’s Blueprint for Movement)
