A Bird's Internal Compass Is Made Out Of A Light-Sensitive Protein

Do migrating birds use their cryptochromes as an internal compass? Anjo Kan/Shutterstock

A new piece of research has backed up the theory that birds use a specialized protein within their eyes to detect Earth’s magnetic field.

Although it has been long-established that birds definitely have the ability to sense the planet’s magnetosphere, the precise mechanism that allows them to do so remains somewhat elusive. Chemical reactions involving this protein are the primary suspect, but there’s no direct proof to currently support this – only strong circumstantial evidence.

“The principle that chemical transformations can respond to very weak magnetic fields… is unquestionably genuine,” Peter Hore, a biophysical chemist at Oxford University and coordinating author of the study, said in a statement. “What is not yet proven is whether this mechanism lies at the heart of avian magnetoreception.”

However, as reported in a novel study in the New Journal of Physics, computer models back up the idea that proteins within birds’ eyes do indeed react to Earth’s magnetic field lines, and not some other mysterious chemical.

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Cryptochromes may be the key to their internal compass. Triff/Shutterstock

Magnetoreception describes the ability of an organism to detect the magnetic field of the Earth. Humans, sadly, do not have it, but research is revealing that many other animals and lifeforms do, including bats, turtles, ants, sharks, dogs, wolves, foxes, bears, bacteria, and even some primates.

Birds are perhaps the most well-known magnetosensitive creatures, but there’s much about the avian compass that still remains a mystery. This new study explores the idea that birds can navigate the world using an in-built magnetic compass that is dependent on the chemistry of their eyes.

Within their retinas, proteins known as cryptochromes are involved in light-induced, magnetically sensitive chemical reactions, and it is these reactions that may allow the birds to “activate” their compass. These chemical reactions outside of birds are well-documented, but just because they also occur within birds’ retinas, it doesn’t necessarily mean they actually make use of them.

A more significant potential problem is that Earth’s magnetic field may not be strong enough to actually alter the chemistry of the cryptochromes found in birds; if it isn’t, then these key magnetoreception-generating reactions will not occur within their retinas. Perhaps birds are magnetoreceptive thanks to another, currently unknown protein or compound in their bodies.

In order to find out, the team ran a series of computer simulations designed to replicate the chemical reactions that would occur when this type of cryptochrome is exposed to varying magnetic field strengths and changing light levels. Indeed, it appears as if the magnetic field strength of the Earth is sufficient enough to induce magnetically-dependent reactions within cryptochromes.

In fact, the reactions are shown to be strongly dependent on the direction of the magnetic field at the time. This adds yet more evidence to the idea that not only do birds use the Earth’s magnetic field to navigate, but that their cryptochromes are the key compound involved.

In any case, cryptochromes aren’t the only way that creatures can detect magnetic fields, and even if they possess these proteins, there’s no guarantee that they actually use it.

For example, two species of macaque monkey – who have only recently been found to contain light-sensitive, magnetically-sensitive cryptochromes – may not be able to use theirs to detect magnetic fields, and there’s no evidence yet to suggest that they do. Red foxes, on the other hand, certainly appear to be able to use their cryptochromes to increase the accuracy of their lethal, prey-killing pounces.

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