Sensory Systems/Birds/MagnetoPerception

Introduction
Sensory magneto-perception is defined as the sense that allows an organism to detect the Earth's magnetic field and to orient itself according to it. Magneto-perception is present throughout the Bacteria and Animalia kingdoms being observed, among others, in honey bees, salamanders, fishes and frogs. Here we will explain and review the current hypotheses on how birds use the Earth's magnetic field to navigate.

During the last decade many laboratories have focused their attention on how aves (i.e. birds) orient themselves. Twice each year, migratory birds travel thousands of kilometers from their breeding region to the overwintering sites and back, finding their way even across unfamiliar territories.

Studies done especially with robins (Erithacus rubecula) and pigeons (Columba livia) have shown that in addition to inclination, intensity and polarity of the geomagnetic field, birds relies on cues such as the sun or the star map to orient. However, the importance of each cue to avian orientation is still debated.





While those studies have clarified the main aspects of birds' navigation, there are still many unanswered questions. Here we give a short introduction to magnetic orientation in animals, explaining the physics behind it, and discuss some of the principal hypotheses on how birds sense the geomagnetic field. The sensory structure, the neuronal circuitry and its mechanisms will be discussed.

Magnetic orientation
Since man cannot consciously sense the geomagnetic field, sensory magneto-reception might appear alien to our understanding. Nonetheless, the ability to sense magnetic fields is common to many animals; among them are mollusks, arthropods and members of all major group of vertebrates. The term magnetic orientation is regarded to the use these animals make of the information coming by a prominent magnetic field, the geomagnetic one, to orient themselves with respect to the Earth in migratory patterns. In this section the Earth's intrinsic magnetic field will be discussed, highlighting the two main classes of information that animals, and birds in particular, may obtain from it.

The geomagnetic field
In a first approximation, the Earth can be seen as a gigantic magnet dipole, with its poles situated close to the geographic, or rotational, poles. Although the magnetic north pole (Nm in the Figure) coincides today with the rotational north pole (Ng), there is no relationship between the two, since the latter is fixed, while the former can change in time.

An intuitive way to visualize magnetic fields is to consider their field lines. They define the direction of a vector field at different points. In a dipole (the arguably most elementary magnet) the northern and southern poles are the sources of the field. Because of the presented dipole approximation, the field lines associated with the geomagnetic field originate from the southern (magnetic, from now on) pole, run around the globe and reach the northern pole. A schematic visualization of the lines is depicted in Figure [fig:field-lines].



The most important aspect to note for the following discussion is that for this reason magnetic field lines point upward on the southern hemisphere, downward on the northern one, while running parallel to the Earth's surface at the magnetic equator (inclined, as the magnetic dipole, about 10 degrees with respect to the geographic equator), showing a fairly regular gradient. The intensity of the field is highest at the poles and lowest at the magnetic equator.

Of course irregularities in the Earth's surface slightly vary the real intensity of the field at different points and the corresponding inclination of its lines. Since these effects are very small, the geomagnetic field represents a reliable and omnipresent source of navigational information. In addition to the orientation of the magnetic field, which acts like a (biological) compass (as is done in human-built tools), the intensity of the field together with the inclination of the associated field line at different points may provide components of a navigational "map" indicating one's position on the globe .

Magnetic compass orientation
A magnetic field can be used as the main source of information to build a magnetic compass. That animals use a biological magnetic compass has been shown in several experiments, mostly involving European robins, Erithacus rubecula. A given migration pattern is in fact chosen by birds, that presents a magnetic characterization which is constant in time. Indeed recreating those fields and just inverting the poles led to analogous behaviors, inverted in direction .

What is most interesting is that while human-built compasses are polarity-based, featuring an orientation information based on the polarity (north/south) of the field lines, birds demonstrated to have an inclination-based compass. The gradient above described of inclinations of the filed lines, from the southern pole to the northern one, passing through the magnetic equator, can be used to detect the position of a given magnetic pole. Surprisingly, birds are not actually able to detect the full inclination of a field vector at a given point, but only of its axial component. The vertical component is inferred by simply realizing their up/down flying asset. This result has been obtained with smartly tuned magnetic fields where same axial component and different polarities led to the same outcome, birds not being able to tell the difference .

Another interesting aspect of birds' biological compass is that it is strictly tuned to only detect tight windows of certain magnetic fields intensities. Even more interestingly, this window may change but not in a shifting nor in an amplifying fashion. It was indeed observed that only already experienced (and orientation-wise efficient) fields are eligible as future recognizable windows ..

Magnetic navigation
A biological compass may be enough to direct navigation, exactly as a human-built polarity-based compass is enough to orient. However, even the earliest experiments showed that birds also utilize information on field intensity. Apparently discording theories have then developed throughout time suggesting that either one or the other approach was actually used by birds for navigation and orientation. Currently, it is generally accepted that the two approaches are both valid, one being prominent over the other under different conditions.

In fact, birds know by experience that in the northern hemisphere the geomagnetic field increases towards north. The difference in intensity between an encountered location and a known one lets them infer whether they are north or south of the known site. The first experimental results suggesting this ability of birds were obtained with pigeons, Columba livia f. domestica .

This is not the only way intensity can be used, though. It could also be used as a "sign-post". Birds, in fact, may show innate responses, both behavioral and physiological, to locations that present a specific combination of field intensity and inclination of the corresponding field line. In an experiment thrush nightingales, L. luscinia, showed extremely rapid changes in weight that correlated to the geomagnetic conditions at which they originated.

Implications
In summary, inclination and intensity are both valid mechanisms that help birds in orientation and navigation. Being so different one from the other, it is clear that no single receptor or sensory system in general might be able to perceive, encode and elaborate the information these two elements represent. This is also the main reason why parallel research theories led to apparently different results when trying to solve the questions about birds' ability to orient themselves.

Magnetic sensory system
Lorenzini ampullae in fishes are electrically sensitive and specialized organs. Previous research investigated a corresponding specialized organ responsible for magnetic detection in birds. However, as it soon turned out birds' detection of the geomagnetic field is more complex than expected and does not rely exclusively on a specialized cell. The difficulty in identifying an underlying physiological mechanisms and a magneto-receptive organ or molecule has been a great obstacle in the study of the avian magnetic perception field.

There are mainly two strong hypotheses on magnetic sensing that are widely accepted in the field and strongly supported by data:


 * 1) Trigeminal iron-mineral based magneto-reception in the upper beak;
 * 2) Chemical light-dependent radical-pair based magneto-reception.

Very recently a third hypothesis has been proposed:

 Inner ear lagena based magneto-reception. 

The review and description of these three hypotheses is the content of the following sections.

Iron-based magneto-reception
The first proposed hypothesis on how birds sense the geomagnetic field relies on iron rich cells which respond to magnetic fields, providing qualitative (directional) and quantitative (intensity) information. Iron rich cells have been found in bacteria [Blakemore, 1975] and bees [Gould et al., 1978], and have been detected in the upper beak of pigeons, finches, robins, warbler and chickens [Falkenberg et al., 2010, Fleissner et al., 2003]. Iron-mineral rich cells, localized in the sensory dendrites [Fleissner et al., 2003], are believed to exist in all birds.

There are two proposed theories about how birds sense the geomagnetic field using iron-based magneto-reception. The first one suggests that iron-based magneto-reception depends only on magnetite ($$Fe_2O_4$$). Magnetite clusters, according to the orientation of the external magnetic field, will attract or repel each other, deforming the dendrite membrane and possibly opening or closing ion channels. On the other hand, this theory has been proposed fifteen years ago, before the discovery of maghemite ($$Fe_2O_3, \gamma-Fe_2O_3$$) platelets in the upper beak of birds.

The second proposed theory suggests that iron-based magneto-reception depends both on magnetite and maghemite. With X-ray and histology methodologies magnetite and maghemite have been detected in the upper beak of pigeons and have been both shown to be necessary for magneto-detection [Fleissner et al., 2007]. Magnetite forms micro-clusters that are attached to the cell membrane, while maghemite crystals are arranged in chains inside the dendrites, as shown in Figure [fig:magnetite-maghemite]. It is believed that maghemite becomes magnetized thereby enhancing the magnetic field of a cell. The magnetite cluster will then experience an attractive (or repulsive) force inducing their displacement and hence the opening of ion channels.



Histology has revealed that both magnetite and maghemite are present within the dendrites of the trigeminal nerve, especially in the branch that transmits sensory inputs form the upper beak to the brain. Besides this finding, researchers have shown the existence of three dendrite fields, each being responsible for coding a specific 3D orientation [Fleissner et al., 2007]. It is hypothesized that the magnetization of magnetite and maghemite caused by the geomagnetic field leads to the opening of ion channels. The information derived from magnetic fields is encoded into action potentials that reach the brain to be correctly interpreted.

There are many behavioral experiments that support this first hypothesis of avian magneto-detection. [Heyers et al., 2010] showed that changes in the magnetic field activate neurons in the trigeminal brainstem complex and that the trigeminal nerve is necessary for magneto-perception. They showed that ablation of the trigeminal nerve or removal of an external magnetic field led to reduced neuronal activation in PrV and SpV, two brain areas receiving primary inputs from the trigeminal nerve. In accordance with this study, disruption of trigeminal nerve or the attachment of a magnet to the upper beak area led to impairment of pigeons orientation [Mora et al., 2004]. Altogether, these findings strongly suggest a strict relation between the trigeminal nerve and the magnetic sensory system. However, more detailed findings are still needed.

The validity of the hypothesis here described, on how birds sense the Earth's magnetic field, has been debated by the recent finding that the believed-to-be iron-mineral structures in the trigeminal branch are in truth immune system cells called macrophages [Treiber et al., 2012]. Attempts by [Treiber et al., 2012] in finding supporting approaches to replicate electro-physiological data showing the presence of magnetite and maghemite in dendrites have failed.

The existence of iron-rich neurons in birds upper beak remains controversial. Nevertheless, the iron-based magnetic theory has not been discarded yet as many behavioral experiments, as the above cited, strongly suggest the involvement of the trigeminal nerve in magneto-perception.

Light-dependent radical-pair magneto-reception
A second hypothesis, also popular in the research field, aims at demonstrating how bird' magnetic orientation system is light dependent. This theory is supported by experiments in which avians' magnetic orientation showed an interesting dependance on a strict range of wavelengths [Wiltschko et al., 2010] . During other experiments in a cage, the utilization of full spectrum light led to birds disorientation. The first research question arising from these experimental results is the origin of a structure in the birds' eye which is able to detect the geomagnetic field. How can visual and magnetic cues be separately processed is also a research topic.

The hypothesis on light-dependent magnetic sensory system states that the direction of a magnetic field is sensed by radical pair forming after photon absorption in photopigments located in the retina. Cryptochrome, a flavoprotein sensitive to blue light, has been suggested as the primary magneto-receptor in birds. Proving the validity of this theory, several cryptochrome family members have been found to be expressed in the retina of migrating birds. Moreover, their activity is highest during migratory behavior.

Light absorption leads to changes in the oxidation state in the cryptochrome pigment flavin adenine dinucleotide (FAD), creating an intermediate state in which the pigment, together with its electron transfer partner (tryptophan), form a radical pair. The electron spin of both radicals makes them sensitive to external magnetic fields. The different states of FAD oxidation are illustrated in the attached Figure. The homeostasis of FAD is extremely important as depending on its reduction state FAD activates different downstreaming signals.



Other important consequences and research questions arising from this hypothesis are how information reaches the brain from the retina and where light-dependent magnetic information are processed in the bird's central nervous system. Ganglion cells are the only ones transmitting information between eyes and brain, therefore magnetic information must pass through them, independently of where in the eye the magnetic cells are active.

Magnetic information gathered in the retina is then transmitted via the thalamus to a forebrain region known as Cluster N, essential to magnetic field processing. Lesions in the Cluster N have been found to affect magnetic compass orientation but not stars and sun compass orientation abilities. This forebrain region is active at night, suggesting that magnetic orientation is a primary nocturnal navigational tool, while during the day other structures are more prominent.

Given that light-dependent magnetic information is detected in the retina, it is central to address how these signals can be separated from normal vision. It is hypothesized that these systems, although in close proximity, are orientated in different directions. Rod and cone cells are oriented approximately perpendicular to the retina, whereas magnetic receptor signals depends on the angular dependence between light, receptor and magnetic field. The maximal signal speed occurs when the receptor is parallel to the geometric field. The reason why birds are able to separate magnetic and visual information is that any magnetic generated pattern moves with half the speed of the surrounding landscape.

It was believed that the magnetic compass senses in birds were strongly lateralized towards the right eye. However recent findings show that cryptochromes are located in both eyes [Mouritsen et al., 2004], Cluster N activation is similar in both brain hemispheres [Zapka et al., 2009] and neuronal pathways between eye and Cluster N are symmetrical [Heyers er al., 2007]. These results then suggest that no lateralization is present.

Despite our far from complete understanding of radical pairs in cryptochromes, they appear to fit the purpose as magneto-receptors, from a theoretical point of view. However, there are still several unresolved issues. First of all, it is unclear which of the four cryptochromes found in the bird's retina is involved in migration, nor it is known if magnetic fields can be detected in vitro by cryptochrome proteins from migratory birds. Lastly, the existence of other brain regions apart from Cluster N, which could be important for signal processing of magnetic information, needs further investigation.

Inner ear lagena
Findings suggest the existence of a third possible magneto-receptor in birds, located in the inner ear lagena organs. The lagena, found in fishes, amphibians, reptiles, birds and monotremes (but not other mammals) is defined as the third otolith organ. In pigeons the lagena lies at the base of the basilar papilla, the avian equivalent of the organ of Corti, with receptors oriented in a sagittal plane [Wu and Dickman, 2011]. The lagena is similar to its neighboring structures, the utricle and the saccule. All three detect changes in head tilt relative to gravity, translational motion and linear acceleration by means of hair cells deflection. Proving the importance of the lagena for magneto-reception, pigeons which had their lagena removed or small magnetic interference inserted into the inner ear showed compromised navigational abilities [Harada, 2002].

It is believed that detection of the geomagnetic field in the lagena, as in the trigeminal nerve, also relies on ferrimagnetic compounds [Harada et al., 2001]. Hair cells are being speculated to contain iron rich cells that senses changes in the geomagnetic field. According to this speculation a recent study [Lauwers et al., 2013] has detected iron-rich structures in both type I and type II cells in the lagena. This in turn suggests that this iron-rich particles, under the influence of the geomagnetic field, can modify the transduction of input stimuli to the brain through the deflection of hair cells leading to opening or closing of ion channels.

Despite the finding of possible magneto-sensory cells in the inner lagena, the neural pathway activated during magneto-reception is still unknown. A study has been carried out with the c-Fos transcription factor, a marker used to highlight activated neurons along the pattern of activation generated by a magnetic field. As expected, activation was detected in brain areas known to be involved in orientation and spatial memory and navigation function. Supporting the theory here discussed, much of these brain regions received information from the lagena receptor organs, while ablation of lagena led to reduced number of active neurons in those regions [Wu and Dickman, 2011].

Research issues with magnetic systems
The difficulties in identifying a magneto-receptive organ contribute to the delay in understanding how the magnetic system developed in birds and, consequently, very few is known about the molecular and genetic factors that determine this kind of sensory system.

Progress in understanding the magnetic sense has been hampered by:


 * The availability of only a small number of techniques that are adequate for analyzing animal behavioral response to magnetic fields. For instance, many studies are performed in bound anesthetized animals, in which the influence of anesthesia on perception is still debated.
 * The difficulty in achieving reproducible results. After the finding of iron-rich cells in the upper beak of birds, many electrophysiological data were replicated but led to different results questioning the validity of the proposed theory;
 * The difficulty in implementing and carrying out new theories that might be more powerful than the ones used today.
 * The human difficulty to understand the avian magnetic perception impedes the development of new and more efficient methods to study avian geomagnetic perception.

Redundancy of the system
It is clearly accepted that birds' orientation relays on the geomagnetic field. Still, as we have seen, no unequivocally magnetic sensitive structure nor a valid explanation to how the brain receives and interpret magnetic field information has yet been found. All the three above discussed hypotheses are plausible and well supported by many behavioral experiments. However, for all of them there are still many open questions and contradictory findings.

Taking into account all the presented evidence, it is difficult to escape the conclusion that birds' magnetoperception does not rely on a single sensory receptor but that it profits from the integration of different ones, presumably one out of the others being more prominent in specific situations.

It is believed that the iron-based magneto-reception provides quantitative or polarity data, such as the intensity of the magnetic field, whereas the cryptochrome receptors detect directional information regarding the Earth's magnetic field. It has also been shown that the iron-based magneto-reception can also control directional behavior when the radical pair process is disrupted [Wiltschko et al., 2010].

Confirming this theory, a study showed that, under blue/green light, birds oriented using cryptochrome-based detection, whereas when green/yellow light was used birds used mainly magnetite based perception [Wiltschko et al., 2012]. Hence, the idea of a possible redundancy in magnetic sensory perception can be proposed, raising a new question on how these two sources of information might be integrated as a single one.

Additionally to the sensory redundancy, studies have also shown that birds orientation does not rely exclusively on the geomagnetic field but also depend on multiple external cues. A hierarchy determining the usage of different orientation strategies has been speculated and showed that the sun or star maps are prioritized over magneto-perception. However, it is believed that information from each system is processed and integrated giving a more detailed, complex and precise representation of the environment. The way none of the elements of this redundancy can be easily shown to be prominent with regard to the other ones shows how magneto-perception can be better explained on an evolutionary basis. Redundant systems are less suitable to outside interference and can overcome situations where one important factor cannot be used or is not fully available. The convergence of multi-sensory cues helps the brain determine precise direction and location information.