Magnetoreception - Can birds sense the geomagnetic field?

European robins were one of the first kind of birds which were tested if they could sense the geomagnetic field, using it as a compass. Putting them into an isolated cage, the birds followed a synthetically produced magnetic field, which was later tested with other kinds of birds too. Many positive observations related to this theory lead to the question, how animals could possibly sense the geomagnetic field.

Bird migration

Many species of birds follow a regular seasonal movement called "migration" between their breeding and wintering grounds. Reasons for this process are for example decreasing food supplies and nesting locations which they escape by traveling over long distances to other areas, where they are available. When the time of migration comes, birds seem to be very restless and tend to intuitively follow their inner needs. The question is, how the birds know the exact way to the specific destination over thousands of kilometers.

The geomagnetic field

It all starts with the geomagnetic field, which is made up from the inner structure of our earth:

About 95% of earth's magnetic field is created by the "geodynamo". Hot liquid from the inner core, made out of iron and about 5000 °C hot, floats out to the outer, liquid core and cools down and vice versa, producing convergence streams. The coriolis force is added to these streams, building spiral patterns, parallel to the earth axis. This conductive matter creates inductive loads, which reinforce the magnetic field and this positively affects the inductive load again, leading to a feedback loop. On the average, the strength of earth's magentic field is about 50 tesla strong (tesla is the unit for the magnetic flux density "B"(vector force)). The magnetic north and south pole aren't at the same position as the geographic poles. This ankle shift is called "declination" and is an ongoing process:

Our magnetic poles tend to flip polarity over a certain amount of time. Scientists think, the last time a swift happened was over 700000 years ago. For us, the geomagnetic field is a protection against cosmic radiation and sun winds, which are also a very high energy particle radiation. Regions where more of this radiation can come near earth are a reason for many people to visit countries like finland every year. We know the interactions from the radiaton with the upper atmospheric layer as "polar lights".

The cryptochrome theory

In contrast to a compass which uses the alignment of the field lines in horizontal direction, birds use the inclination of the field lines which are parallel to the equator and vertical at the poles. To use this 6th sense, a special reaction is needed:

Cryptochromes are a class of flavoproteins that are sensitive to blue light and sequentially homologous to photolyase and have got a chain of three tryptohan rests (Trp324, Trp377, Trp400) plus an amino-acid expansion, linked to the convey of the blue light signal.

modell of the structure

They are for example part of the inner clock of an animal and are possibly linked to the magnetic sense as they were found in the photoreceptor neurons of birds. The theory is based on a radical pair reaction (each molecule got a free electron and is strongly reactive but not really stable), affected by the geometric field. If the radical pair is formed, the electrons are entangled and the magnetic field could modulate the orientation of the electron spins, resulting in a change of the bird's visual sense.

Summary of the process

FAD (Flavine Adenine Dinucleotide) is a chromophore which gets activated through light. Chromophores are areas in molecules where visible light get's absorbed by exiting an electron, changing the chromophore from it's ground to exited state. Before the light absorbtion, FAD is in it's oxidised state. Absorbing the blue light photons, FADs state changes to FAD* , meaning it is exited now. After that, the state changes to FADH* because an electron from the tryptophan chain is getting transferred to FAD. Studies also propose the involvement of a superoxide, creating FADH+O2-, ending the reaction cycle showed below:

This whole reaction can be influenced by the geomagnetic field, as the spin state of the radical pair of cryptochrome changes and increases the value of triplet state degradation products, resulting in different levels cryptochrome activity levels: The unpaired electron of FADH can be back-transferred to one of the three tryptophanes included in the electron donating chain. This back-transfer is spin dependent, meaning the spins of the two unpaired electrons on FADH and the tryptophan have to be in an overall singlet (antiparallel) state. As the magnetic field influences the precession of the electron spins, the amount of time they spend in their singlet state where they are able to back-transfer the electron, it changes the amount of time cryptochromes spend in their signaling state.

Red Robin - does the bird have a visual impression of the geomagnetic field?

This different protein activities lead to a modulation of the bird's visual sense, maybe expressed by a black line, changing it's orientation as the bird moves his head. A mathematic view on the changing signals would be a filter function, describing the magnetic field-mediated visual signal modulation recorded on the bird's retina.

The need of blue light to activate cryptochrome matches with the observations that birds need blue, turquoise or green light to find their population's specific route.

Magnetite theory

Another theory is based on the mineral magnetite (Fe3O4). Magnetite is the most magnetic of all the naturally occurring minerals on Earth and found in the beaks of migrant birds. The problem is, the magnetite was in fact found in scavenger immune cells that had nothing to do with the neural system. It was considered, birds would use a combination between the cryptochrome as a "compass" and the magnetite as a "map". How far this theory can be taken as granted has to be observed in studies and isn't really clear yet.

magnetite - a ferromagnetic iron oxide

Have a nice day:)

Source

Text

http://www.ks.uiuc.edu/Research/cryptochrome/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3559131/  (translated)

http://www.diss.fu-berlin.de/diss/servlets/MCRFileNodeServlet/FUDISS_derivate_000000009438/Diss_Ringo_Wenzel__ohne_Lebenslauf.pdf  (translated)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1300674/

https://en.wikipedia.org/wiki/Bird_migration

https://www.allaboutbirds.org/the-basics-how-why-and-where-of-bird-migration/

http://www.esa.int/ger/ESA_in_your_country/Germany/Das_Erdmagnetfeld_Ein_riesiger_Dynamo  (translated)

https://www.nasa.gov/topics/earth/features/2012-poleReversal.html

https://de.wikipedia.org/wiki/Erdmagnetfeld  (translated)

Pictures

https://qph.ec.quoracdn.net/main-qimg-739d90a34ebebc6aceeafc926b0e4d1c

https://upload.wikimedia.org/wikipedia/commons/5/5f/Earth%27s_magnetic_field%2C_schematic.png

https://www.extremetech.com/wp-content/uploads/2015/05/ET-640x353.jpg

https://upload.wikimedia.org/wikipedia/commons/1/16/CRY1Pretty.png

https://upload.wikimedia.org/wikipedia/commons/b/b8/FAD_reaction.png

https://upload.wikimedia.org/wikipedia/commons/f/f3/Erithacus_rubecula_with_cocked_head.jpg

https://3c1703fe8d.site.internapcdn.net/newman/csz/news/800/2016/mysteriousfo.jpg