FIGURE 8 - uploaded by Henrik Mouritsen
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1 The Earth’s magnetic field (the geomagnetic field). Notice that the southern and northern magnetic poles and the magnetic equator do not coincide with the geographical poles and the geographic equator. Also notice that the magnetic field lines intersect the Earth’s surface at different angles depending on the magnetic latitude (blue-green lines and vectors). The intersection angle is called the magnetic inclination. Magnetic inclination is +90° at the Magnetic North Pole (red vector), c. +67° at the latitude of Germany (yellow vector), 0° at the magnetic equator (dark blue vectors), c. −64° at the latitude of South Africa (orange vector), and −90° at the Magnetic South Pole (magenta vector) (Adapted with permission after Wiltschko and Wiltschko (1996) and Mouritsen (2013).) The magnetic intensity varies from c. 60,000 nT near the magnetic poles to c. 30,000 nT along the magnetic equator.
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The Earth's magnetic field provides potentially useful information, which birds could use for directional and/or positional information. It has been clearly demonstrated that birds are able to sense the compass direction of the Earth's magnetic field and that they can use this information as part of a compass sense. Magnetic information could also...
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Context 1
... Earth’s magnetic field provides potentially useful information, which birds could use for directional and/ or positional information. It has been clearly demon- strated that birds are able to sense the compass direction of the Earth’s magnetic field and that they can use this information as part of a compass sense. Magnetic information could also be useful as part of a map sense, and there is a growing body of evidence that birds are able to determine their approximate position on the Earth on the basis of geomagnetic cues. In addition to direct uses for orientation and navigation, magnetic information also seems to be able to influence other physiological processes, such as fattening and migratory motivation, as a trigger for changes in behavior. Although the behavioral responses to geomagnetic cues are relatively well understood, the physiological mechanisms enabling birds to sense the Earth’s magnetic field are only starting to be understood, and understanding the magnetic sense(s) of animals, including birds, remains one of the most significant unsolved problems in biology. It is very challenging to sense magnetic fields as weak as that of the Earth using only biologically available materials. Only two basic mechanisms are considered theoretically viable in terrestrial animals: iron-mineral-based magnetoreception and radical-pair based magnetoreception. On the basis of current scientific evidence, iron-mineral-based magnetoreception and radical-pair-based magnetoreception mechanisms seem to exist in birds, but they seem to be used for different purposes. Plausible primary sensory molecules and a few brain areas involved in processing magnetic information have been identified in birds for each of these two types of magnetic senses. Nevertheless, we are still far away from understanding the detailed function of any of the at least two different magnetic senses existing in some if not all bird species, and, at present, no primary sensory structure has been identified beyond reasonable doubt to be the source of avian magnetoreception. This is an exciting but challenging field in which several major discoveries are likely to be made in the next 1–2 decades. Moving electric charges such as electrons produce magnetic fields. On the microscopic scale, electron (and nuclear) spins can generate magnetic fields. On the mac- roscopic scale, a magnetic field, B , is, for instance, generated around a wire when current runs through it. The magnetic field at a given location can be described as a three-dimensional (3D) vector for which the strength, B , is measured as magnetic flux density using the unit “Tesla” (T), 1 T = 1 (V*s)/m 2 = 1 (N*s)/(C*m) = 10,000 Gauss (V =Volt, s = second, m = meter, N = Newton, C = Cou- lomb). Some materials, which are called “ferromagnetic,” can be permanently magnetized by a magnetic field, and this magnetization remains after the magnetizing field has been removed. Magnetite (Fe 3 O 4 ), an iron oxide, is a well- known example of a ferromagnetic mineral (Mouritsen, 2013). The Earth generates its own magnetic field (the geomagnetic field), which is mostly caused by electric currents in the liquid outer core of the Earth (the “dynamo effect”). The magnetic field measured at the Earth’s surface is similar to the magnetic field one would expect to see if a large dipole magnet was placed in the center of the Earth (see Figure 8.1). The Earth’s magnetic field currently has a magnetic field South Pole near the Earth’s geographic North Pole (referred to as “Magnetic North” or “Magnetic North Pole” in biology). Throughout this chapter, I will follow the convention used in the bird orientation research literature and use the term “Magnetic North” or “Magnetic North Pole” to refer, not to the physical magnetic North Pole, but to the magnetic pole located closest to the geographic North Pole. Likewise, the magnetic field North Pole near the Earth’s geographic South Pole will be referred to as “Magnetic South” or “Magnetic South Pole” (Mouritsen, 2013). The magnetic field lines leave the Magnetic South Pole and re-enter the Magnetic North Pole. The polarity of the magnetic field lines always points toward Magnetic North; therefore, they can provide a highly reliable directional reference that can be used as the basis for a magnetic compass anywhere on planet Earth except at the magnetic poles. At the magnetic poles, the field lines point directly into the sky (at the Magnetic South Pole) or directly into the Earth (at the Magnetic North Pole). At the magnetic equator, the magnetic field lines are parallel to the Earth’s surface. The angle between the magnetic field lines and the Earth’s surface is called “magnetic inclination.” Thus, magnetic inclination changes gradually from −90° at the Magnetic South Pole to 0° at the magnetic equator to +90° at the Magnetic North Pole (see Figure 8.1). The Earth’s magnetic field intensity ranges from c. 30,000 nT (nano Tesla = 10 −9 T; 1 T = 1Vs m −2 ; 1 nT = 10 −5 Gauss) near the magnetic equator to c. 60,000 nT at the magnetic poles. Earth-strength magnetic fields are usually measured with a calibrated three-axial flux-gate magnetometer. In theory, magnetic inclination and magnetic intensity can be useful for determining one’s position, but, on most parts of the Earth, magnetic inclination and intensity changes predominantly from North to South but not much from East to West; therefore, it seems easier to determine latitude than longitude from geomagnetic field information (Mouritsen, 2013). The Magnetic North Pole is currently located in northern Canada, and the Magnetic South Pole is currently located south of Australia. Consequently, the geographic and magnetic poles do not coincide (see Figure 8.1). The devia- tion between geographic and Magnetic North is called the “magnetic declination.” Magnetic declination is the angle between Magnetic North (i.e., the direction in which the north end of a compass needle points in) and Geographic North. The declination is positive when Magnetic North is east of Geographic North and negative when Magnetic North is west of Geographic North. Declination is mostly small, but near the magnetic poles declination can pose a serious problem for navigating birds using a magnetic compass unless they find a way to compensate for it. On the other hand, magnetic declination could, in theory, be a useful parameter to determine, for example, East-West position if it would be combined with other map cues (Mouritsen, 2013). The direction of the magnetic field around a wire can be determined by the “right hand rule”: If you grasp around the wire with your right hand so that your thumb is pointing in the direction of the current, then the magnetic field around the wire runs in the direction in which your fingers are pointing. The magnetic field decreases with distance as you move away from the wire. If you create a coil of wire, then the magnetic field created is much stronger inside of the coil than on the outside of the coil because many parallel magnetic field lines created by different parts of the wire coincide and thus add up in the center of the coil. This is the reason why coil constructions are typically used to produce and alter magnetic fields (Mouritsen, 2013). The typical coil constructions, which are used to produce Earth-strength magnetic fields for scientific experiments, are so-called “Helmholtz coils”—a pair of parallel coils placed one radius apart from each other (Kirschvink, 1991). In a pair of Helmholtz coils, the magnetic field is very homogeneous within a central space of c. 60% of the radius of the coils (Kirschvink, 1991). The magnetic field generated in the center of a pair of Helmholtz coils is B = (0.9*10 −6 T m/A* n * I )/ R , where T is the unit Tesla, n is the number of turns in each coil, I is the current flow- ing through the coils measured in ampere (A), and R is the radius of the coils measured in meters (m) (Kirschvink, 1991). One pair of Helmholtz coils can only alter the magnetic field along one axis. To make any desired 3D magnetic field, three pairs of Helmholtz coils oriented perpendicular to each other are ideally needed. If one adds an artificially created field to an existing field (such as that of the Earth), then the resultant field is calculated by simple vector addition of the two fields (see Figure 8.2; ...
Context 2
... Earth’s magnetic field provides potentially useful information, which birds could use for directional and/ or positional information. It has been clearly demon- strated that birds are able to sense the compass direction of the Earth’s magnetic field and that they can use this information as part of a compass sense. Magnetic information could also be useful as part of a map sense, and there is a growing body of evidence that birds are able to determine their approximate position on the Earth on the basis of geomagnetic cues. In addition to direct uses for orientation and navigation, magnetic information also seems to be able to influence other physiological processes, such as fattening and migratory motivation, as a trigger for changes in behavior. Although the behavioral responses to geomagnetic cues are relatively well understood, the physiological mechanisms enabling birds to sense the Earth’s magnetic field are only starting to be understood, and understanding the magnetic sense(s) of animals, including birds, remains one of the most significant unsolved problems in biology. It is very challenging to sense magnetic fields as weak as that of the Earth using only biologically available materials. Only two basic mechanisms are considered theoretically viable in terrestrial animals: iron-mineral-based magnetoreception and radical-pair based magnetoreception. On the basis of current scientific evidence, iron-mineral-based magnetoreception and radical-pair-based magnetoreception mechanisms seem to exist in birds, but they seem to be used for different purposes. Plausible primary sensory molecules and a few brain areas involved in processing magnetic information have been identified in birds for each of these two types of magnetic senses. Nevertheless, we are still far away from understanding the detailed function of any of the at least two different magnetic senses existing in some if not all bird species, and, at present, no primary sensory structure has been identified beyond reasonable doubt to be the source of avian magnetoreception. This is an exciting but challenging field in which several major discoveries are likely to be made in the next 1–2 decades. Moving electric charges such as electrons produce magnetic fields. On the microscopic scale, electron (and nuclear) spins can generate magnetic fields. On the mac- roscopic scale, a magnetic field, B , is, for instance, generated around a wire when current runs through it. The magnetic field at a given location can be described as a three-dimensional (3D) vector for which the strength, B , is measured as magnetic flux density using the unit “Tesla” (T), 1 T = 1 (V*s)/m 2 = 1 (N*s)/(C*m) = 10,000 Gauss (V =Volt, s = second, m = meter, N = Newton, C = Cou- lomb). Some materials, which are called “ferromagnetic,” can be permanently magnetized by a magnetic field, and this magnetization remains after the magnetizing field has been removed. Magnetite (Fe 3 O 4 ), an iron oxide, is a well- known example of a ferromagnetic mineral (Mouritsen, 2013). The Earth generates its own magnetic field (the geomagnetic field), which is mostly caused by electric currents in the liquid outer core of the Earth (the “dynamo effect”). The magnetic field measured at the Earth’s surface is similar to the magnetic field one would expect to see if a large dipole magnet was placed in the center of the Earth (see Figure 8.1). The Earth’s magnetic field currently has a magnetic field South Pole near the Earth’s geographic North Pole (referred to as “Magnetic North” or “Magnetic North Pole” in biology). Throughout this chapter, I will follow the convention used in the bird orientation research literature and use the term “Magnetic North” or “Magnetic North Pole” to refer, not to the physical magnetic North Pole, but to the magnetic pole located closest to the geographic North Pole. Likewise, the magnetic field North Pole near the Earth’s geographic South Pole will be referred to as “Magnetic South” or “Magnetic South Pole” (Mouritsen, 2013). The magnetic field lines leave the Magnetic South Pole and re-enter the Magnetic North Pole. The polarity of the magnetic field lines always points toward Magnetic North; therefore, they can provide a highly reliable directional reference that can be used as the basis for a magnetic compass anywhere on planet Earth except at the magnetic poles. At the magnetic poles, the field lines point directly into the sky (at the Magnetic South Pole) or directly into the Earth (at the Magnetic North Pole). At the magnetic equator, the magnetic field lines are parallel to the Earth’s surface. The angle between the magnetic field lines and the Earth’s surface is called “magnetic inclination.” Thus, magnetic inclination changes gradually from −90° at the Magnetic South Pole to 0° at the magnetic equator to +90° at the Magnetic North Pole (see Figure 8.1). The Earth’s magnetic field intensity ranges from c. 30,000 nT (nano Tesla = 10 −9 T; 1 T = 1Vs m −2 ; 1 nT = 10 −5 Gauss) near the magnetic equator to c. 60,000 nT at the magnetic poles. Earth-strength magnetic fields are usually measured with a calibrated three-axial flux-gate magnetometer. In theory, magnetic inclination and magnetic intensity can be useful for determining one’s position, but, on most parts of the Earth, magnetic inclination and intensity changes predominantly from North to South but not much from East to West; therefore, it seems easier to determine latitude than longitude from geomagnetic field information (Mouritsen, 2013). The Magnetic North Pole is currently located in northern Canada, and the Magnetic South Pole is currently located south of Australia. Consequently, the geographic and magnetic poles do not coincide (see Figure 8.1). The devia- tion between geographic and Magnetic North is called the “magnetic declination.” Magnetic declination is the angle between Magnetic North (i.e., the direction in which the north end of a compass needle points in) and Geographic North. The declination is positive when Magnetic North is east of Geographic North and negative when Magnetic North is west of Geographic North. Declination is mostly small, but near the magnetic poles declination can pose a serious problem for navigating birds using a magnetic compass unless they find a way to compensate for it. On the other hand, magnetic declination could, in theory, be a useful parameter to determine, for example, East-West position if it would be combined with other map cues (Mouritsen, 2013). The direction of the magnetic field around a wire can be determined by the “right hand rule”: If you grasp around the wire with your right hand so that your thumb is pointing in the direction of the current, then the magnetic field around the wire runs in the direction in which your fingers are pointing. The magnetic field decreases with distance as you move away from the wire. If you create a coil of wire, then the magnetic field created is much stronger inside of the coil than on the outside of the coil because many parallel magnetic field lines created by different parts of the wire coincide and thus add up in the center of the coil. This is the reason why coil constructions are typically used to produce and alter magnetic fields (Mouritsen, 2013). The typical coil constructions, which are used to produce Earth-strength magnetic fields for scientific experiments, are so-called “Helmholtz coils”—a pair of parallel coils placed one radius apart from each other (Kirschvink, 1991). In a pair of Helmholtz coils, the magnetic field is very homogeneous within a central space of c. 60% of the radius of the coils (Kirschvink, 1991). The magnetic field generated in the center of a pair of Helmholtz coils is B = (0.9*10 −6 T m/A* n * I )/ R , where T is the unit Tesla, n is the number of turns in each coil, I is the current flow- ing through the coils measured in ampere (A), and R is the radius of the coils measured in meters (m) (Kirschvink, 1991). One pair of Helmholtz coils can only alter the magnetic field along one axis. To make any desired 3D magnetic field, three pairs of Helmholtz coils oriented perpendicular to each other are ideally needed. If one adds an artificially created field to an existing field (such as that of the Earth), then the resultant field is calculated by simple vector addition of the two fields (see Figure 8.2; Kirschvink, 1991). Therefore, it is also possible to use a single pair of Helmholtz coils to make any 3D magnetic field, but in that case, this single pair of coils must be oriented very precisely in 3D space (see Figure 8.2; Mouritsen, 2013). Although the Helmholtz arrangement is easy to calculate and construct, the central homogeneous space can be increased to c. 110% of the radius of the coils by using more elaborate coil designs such as the Merritt-4-coil system (Kirschvink, 1991; Zapka et al., 2009, Figure 20.2 in Mouritsen, 2013). To control for artefacts, one would— independent of the coil design chosen—expect the coils to be “double wrapped” (Kirschvink, 1991; Kirschvink et al., 2010). This means that during construction of the coils, each coil contains two separate but identically wrapped wires, each with separate connectors, so that one can either run current through both halves of the windings in the same direction (then the magnetic field in the center of the coil will change), or one can run the current through one half of the coils in one direction but in the opposite direction through the second half of the windings. In that case, the current running through one half of the windings will create a magnetic field, which exactly cancels the magnetic field produced by the other half of the windings, and the background field is not changed. By using double-wrapped coils, exactly the same amount of current is sent through the coils whether the magnetic ...
Context 3
... Although the behavioral responses to geomagnetic cues are relatively well understood, the physiological mechanisms enabling birds to sense the Earth’s magnetic field are only starting to be understood, and understanding the magnetic sense(s) of animals, including birds, remains one of the most significant unsolved problems in biology. It is very challenging to sense magnetic fields as weak as that of the Earth using only biologically available materials. Only two basic mechanisms are considered theoretically viable in terrestrial animals: iron-mineral-based magnetoreception and radical-pair based magnetoreception. On the basis of current scientific evidence, iron-mineral-based magnetoreception and radical-pair-based magnetoreception mechanisms seem to exist in birds, but they seem to be used for different purposes. Plausible primary sensory molecules and a few brain areas involved in processing magnetic information have been identified in birds for each of these two types of magnetic senses. Nevertheless, we are still far away from understanding the detailed function of any of the at least two different magnetic senses existing in some if not all bird species, and, at present, no primary sensory structure has been identified beyond reasonable doubt to be the source of avian magnetoreception. This is an exciting but challenging field in which several major discoveries are likely to be made in the next 1–2 decades. Moving electric charges such as electrons produce magnetic fields. On the microscopic scale, electron (and nuclear) spins can generate magnetic fields. On the mac- roscopic scale, a magnetic field, B , is, for instance, generated around a wire when current runs through it. The magnetic field at a given location can be described as a three-dimensional (3D) vector for which the strength, B , is measured as magnetic flux density using the unit “Tesla” (T), 1 T = 1 (V*s)/m 2 = 1 (N*s)/(C*m) = 10,000 Gauss (V =Volt, s = second, m = meter, N = Newton, C = Cou- lomb). Some materials, which are called “ferromagnetic,” can be permanently magnetized by a magnetic field, and this magnetization remains after the magnetizing field has been removed. Magnetite (Fe 3 O 4 ), an iron oxide, is a well- known example of a ferromagnetic mineral (Mouritsen, 2013). The Earth generates its own magnetic field (the geomagnetic field), which is mostly caused by electric currents in the liquid outer core of the Earth (the “dynamo effect”). The magnetic field measured at the Earth’s surface is similar to the magnetic field one would expect to see if a large dipole magnet was placed in the center of the Earth (see Figure 8.1). The Earth’s magnetic field currently has a magnetic field South Pole near the Earth’s geographic North Pole (referred to as “Magnetic North” or “Magnetic North Pole” in biology). Throughout this chapter, I will follow the convention used in the bird orientation research literature and use the term “Magnetic North” or “Magnetic North Pole” to refer, not to the physical magnetic North Pole, but to the magnetic pole located closest to the geographic North Pole. Likewise, the magnetic field North Pole near the Earth’s geographic South Pole will be referred to as “Magnetic South” or “Magnetic South Pole” (Mouritsen, 2013). The magnetic field lines leave the Magnetic South Pole and re-enter the Magnetic North Pole. The polarity of the magnetic field lines always points toward Magnetic North; therefore, they can provide a highly reliable directional reference that can be used as the basis for a magnetic compass anywhere on planet Earth except at the magnetic poles. At the magnetic poles, the field lines point directly into the sky (at the Magnetic South Pole) or directly into the Earth (at the Magnetic North Pole). At the magnetic equator, the magnetic field lines are parallel to the Earth’s surface. The angle between the magnetic field lines and the Earth’s surface is called “magnetic inclination.” Thus, magnetic inclination changes gradually from −90° at the Magnetic South Pole to 0° at the magnetic equator to +90° at the Magnetic North Pole (see Figure 8.1). The Earth’s magnetic field intensity ranges from c. 30,000 nT (nano Tesla = 10 −9 T; 1 T = 1Vs m −2 ; 1 nT = 10 −5 Gauss) near the magnetic equator to c. 60,000 nT at the magnetic poles. Earth-strength magnetic fields are usually measured with a calibrated three-axial flux-gate magnetometer. In theory, magnetic inclination and magnetic intensity can be useful for determining one’s position, but, on most parts of the Earth, magnetic inclination and intensity changes predominantly from North to South but not much from East to West; therefore, it seems easier to determine latitude than longitude from geomagnetic field information (Mouritsen, 2013). The Magnetic North Pole is currently located in northern Canada, and the Magnetic South Pole is currently located south of Australia. Consequently, the geographic and magnetic poles do not coincide (see Figure 8.1). The devia- tion between geographic and Magnetic North is called the “magnetic declination.” Magnetic declination is the angle between Magnetic North (i.e., the direction in which the north end of a compass needle points in) and Geographic North. The declination is positive when Magnetic North is east of Geographic North and negative when Magnetic North is west of Geographic North. Declination is mostly small, but near the magnetic poles declination can pose a serious problem for navigating birds using a magnetic compass unless they find a way to compensate for it. On the other hand, magnetic declination could, in theory, be a useful parameter to determine, for example, East-West position if it would be combined with other map cues (Mouritsen, 2013). The direction of the magnetic field around a wire can be determined by the “right hand rule”: If you grasp around the wire with your right hand so that your thumb is pointing in the direction of the current, then the magnetic field around the wire runs in the direction in which your fingers are pointing. The magnetic field decreases with distance as you move away from the wire. If you create a coil of wire, then the magnetic field created is much stronger inside of the coil than on the outside of the coil because many parallel magnetic field lines created by different parts of the wire coincide and thus add up in the center of the coil. This is the reason why coil constructions are typically used to produce and alter magnetic fields (Mouritsen, 2013). The typical coil constructions, which are used to produce Earth-strength magnetic fields for scientific experiments, are so-called “Helmholtz coils”—a pair of parallel coils placed one radius apart from each other (Kirschvink, 1991). In a pair of Helmholtz coils, the magnetic field is very homogeneous within a central space of c. 60% of the radius of the coils (Kirschvink, 1991). The magnetic field generated in the center of a pair of Helmholtz coils is B = (0.9*10 −6 T m/A* n * I )/ R , where T is the unit Tesla, n is the number of turns in each coil, I is the current flow- ing through the coils measured in ampere (A), and R is the radius of the coils measured in meters (m) (Kirschvink, 1991). One pair of Helmholtz coils can only alter the magnetic field along one axis. To make any desired 3D magnetic field, three pairs of Helmholtz coils oriented perpendicular to each other are ideally needed. If one adds an artificially created field to an existing field (such as that of the Earth), then the resultant field is calculated by simple vector addition of the two fields (see Figure 8.2; Kirschvink, 1991). Therefore, it is also possible to use a single pair of Helmholtz coils to make any 3D magnetic field, but in that case, this single pair of coils must be oriented very precisely in 3D space (see Figure 8.2; Mouritsen, 2013). Although the Helmholtz arrangement is easy to calculate and construct, the central homogeneous space can be increased to c. 110% of the radius of the coils by using more elaborate coil designs such as the Merritt-4-coil system (Kirschvink, 1991; Zapka et al., 2009, Figure 20.2 in Mouritsen, 2013). To control for artefacts, one would— independent of the coil design chosen—expect the coils to be “double wrapped” (Kirschvink, 1991; Kirschvink et al., 2010). This means that during construction of the coils, each coil contains two separate but identically wrapped wires, each with separate connectors, so that one can either run current through both halves of the windings in the same direction (then the magnetic field in the center of the coil will change), or one can run the current through one half of the coils in one direction but in the opposite direction through the second half of the windings. In that case, the current running through one half of the windings will create a magnetic field, which exactly cancels the magnetic field produced by the other half of the windings, and the background field is not changed. By using double-wrapped coils, exactly the same amount of current is sent through the coils whether the magnetic field is being changed or not. Double-wrapped coils also allow for truly double-blinded experiments (Kirschvink, 1991; Zapka et al., 2009; Harris et al., 2009; Hein et al., 2010, 2011; Engels et al., 2012). An excellent presentation of the theoretical background and practical instructions on how to construct various coil designs for changing Earth-strength magnetic fields can be found in Kirschvink (1991). Orientation and navigation skills are essential for the survival of all migratory birds. All first-time migrants are faced with the challenge of finding an unfamiliar wintering area, often thousands of kilometers away (Berthold, 1991; Mouritsen and Mouritsen, 2000; Mouritsen, 2003). Many bigger birds are day migratory and travel in groups, which means that young birds of these species might simply follow experienced birds that know the way. However, most small ...
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Citations
... The sensory basis for magnetic compass orientation, however, is still not fully resolved and is the subject of intensive research (e.g. Mouritsen, 2015Mouritsen, , 2018Nordmann et al., 2017). ...
... The sensory basis for magnetic compass orientation is the subject of active research and intense scientific debate (e.g. Mouritsen, 2015;Nordmann et al., 2017;Mouritsen, 2018). Several different models and hypotheses have been put forward to date, proposing very distinct avian magnetic senses that could be used to acquire directional (i.e. ...
... compass) information from Earth's magnetic field for orientation purposes (e.g. Mouritsen, 2015Mouritsen, , 2018. The radical pair model, a leading concept for the magnetic compass of birds, proposes reversible light-dependent chemical reactions inside the retina of the birds' eyes as the basis for the avian magnetic sense providing directional information, with the yield of these reactions depending on the alignment of a specific type of molecule (cryptochromes) to the magnetic field (e.g. ...
For studies on magnetic compass orientation and navigation performance in small bird species, controlled experiments with orientation cages inside an electromagnetic coil system are the most prominent methodological paradigm. These are, however, not applicable when studying larger bird species and/or orientation behaviour during free flight. For this, researchers have followed a very different approach. By attaching small magnets to birds, they intended to deprive them of access to meaningful magnetic information. Unfortunately, results from studies using this approach appear rather inconsistent. As these are based on experiments with birds under free flight conditions, which usually do not allow exclusion of other potential orientation cues, an assessment of the overall efficacy of this approach is difficult to conduct.
Here, we directly test the efficacy of small magnets for temporarily disrupting magnetic compass orientation in small migratory songbirds using orientation cages under controlled experimental conditions. We found that birds which have access to the Earth's magnetic field as their sole orientation cue show a general orientation towards their seasonally appropriate migratory direction. When carrying magnets on their forehead under these conditions, the same birds become disoriented. However, under changed conditions that allow birds access to other (i.e. celestial) orientation cues, any disruptive effect of the magnets they carry appears obscured.
Our results provide clear evidence for the efficacy of the magnet approach for temporarily disrupting magnetic compass orientation in birds, but also reveal its limitations for application in experiments under free flight conditions.
... Therefore, an accurate and adaptable navigational mechanism robust to environmental change is required. In particular, birds are known to use a multifactorial navigational mechanism, especially during migration [4][5][6] but it is still unclear how different potential methods of navigation interact or function. A highly debated navigational mechanism involves birds using the geomagnetic field and different potential navigational strategies (compass or map) associated with different geomagnetic properties (e.g., intensity, inclination). ...
... Geomagnetic fields may be used for two different types of navigational strategies, compass and map. Birds can use the Earth's magnetic field to derive a constant direction for compass navigation, similar to using a simple magnetic compass to maintain a constant direction [4,[24][25][26]. This is however affected by local and global changes in the magnetic field and needs to be regularly calibrated [7,[27][28][29]. ...
... The Earth's magnetic field is a bipolar magnet and the magnetic poles are at distance to the geographic poles located at the rotational axis at the Earth's surface [4]. The magnetic field is a three-dimensional vector, typically measured in the North-East-Centre (or Down) coordinate system (Fig. 2). ...
Background
Different theories suggest birds may use compass or map navigational systems associated with Earth’s magnetic intensity or inclination, especially during migratory flights. These theories have only been tested by considering properties of the Earth’s magnetic field at coarse temporal scales, typically ignoring the temporal dynamics of geomagnetic values that may affect migratory navigational capacity.
Methods
We designed a simulation experiment to study if and how birds use the geomagnetic field during migration by using both high resolution GPS tracking data and geomagnetic data at relatively fine spatial and temporal resolutions in comparison to previous studies. Our simulations use correlated random walks (CRW) and correlated random bridge (CRB) models to model different navigational strategies based on underlying dynamic geomagnetic data. We translated navigational strategies associated with geomagnetic cues into probability surfaces that are included in the random walk models. Simulated trajectories from these models were compared to the actual GPS trajectories of migratory birds using 3 different similarity measurements to evaluate which of the strategies was most likely to have occurred.
Results and conclusion
We designed a simulation experiment which can be applied to different wildlife species under varying conditions worldwide. In the case of our example species, we found that a compass-type strategy based on taxis, defined as movement towards an extreme value, produced the closest and most similar trajectories when compared to original GPS tracking data in CRW models. Our results indicate less evidence for map navigation (constant heading and bi-gradient taxis navigation). Additionally, our results indicate a multifactorial navigational mechanism necessitating more than one cue for successful navigation to the target. This is apparent from our simulations because the modelled endpoints of the trajectories of the CRW models do not reach close proximity to the target location of the GPS trajectory when simulated with geomagnetic navigational strategies alone. Additionally, the magnitude of the effect of the geomagnetic cues during navigation in our models was low in our CRB models. More research on the scale effects of the geomagnetic field on navigation, along with temporally varying geomagnetic data could be useful for further improving future models.
... The steepness of the magnetic field lines relative to the Earth's surface, called "inclination", gradually changes from vertical at the poles to parallel at the magnetic equator. At the same time, the magnetic field intensity gradually increases from ∼30 µT around the equator to ∼60 μT at the poles (Mouritsen 2015;Wiltschko and Wiltschko 1995). Based on these cues, birds could theoretically derive both directional and positional information from geomagnetic parameters. ...
In addition to other natural orientation cues such as the stars, the sun, landmarks and olfactory cues, migrating birds possess the ability to orient by the Earth’s magnetic field. In recent years, neuroscientific research has pinpointed brain regions and connecting neuronal pathways that seem to be involved in processing magnetic information. To date, the most compelling neuroanatomical and behavioural evidence comes from the visual and trigeminal sensory systems. We expect that navigational information from both systems could be integrated in higher-order brain structures, such as the hippocampus and the “decision-making” caudolateral nidopallium. This review summarizes the current state of research on the neurosensory basis of magnetoreception in birds.
... This angle decreases as it approaches the Equator (inclination=0⁰) and becomes negative (points up) as it reaches the South pole (inclination=-90⁰). Similarly, intensity of the geomagnetic field exists in a gradient, where it is strongest at the poles (~60,000 nT) and weakest at the Equator (~30,000 nT) [2]. Despite some variation due to secular drift or anomalies created by magnetic storms and enriched deposits of iron in the crust of the Earth, the geomagnetic field acts as a reliable, constant spatial grid. ...
... mN= magnetic North, mS= magnetic South. Reused with permission from[2]. ...
A variety of organisms have been shown to use the Earth’s magnetic field to orient in local-spaces and to navigate long-distances. Although behavioural evidence of magnetoreception has been reported in a diverse range of taxa, the proximate mechanisms of this phenomenon have yet to be revealed. Some animals such as birds, appear to use a light-dependent radical-pair-based magnetic compass. Ancient, light-sensitive proteins called Cryptochrome (Cry) are currently the only known molecule found in vertebrates to create radical-pairs, and thus are putative receptors. Cry is associated with the visual system where it is co-localized with both short- and long-wavelength retinal cone photoreceptors in adult birds, and therefore well-suited for light-dependent magnetoreception. Unfortunately, due to the molecular inaccessibility of the avian model, Cry-cone interactions have seldom been manipulated, and the requirement of Cry for magnetoreception has yet to be tested in vertebrates. Additionally, Cry’s location in photoreceptors of other animal’s that display magnetic behaviors is largely unknown. This thesis utilized zebrafish (Danio rerio) to test if cry was associated with cones in developing and adult fish retina. Zebrafish have six paralogs of cry and while most participate in the circadian clock, the function of cry2 and cry4 are unknown. Here, I show that cry4 is expressed in larval and adult zebrafish short-wavelength-sensitive (Ultraviolet-sensitive (UV)) cones. Using nitroreductase (NTR)-mediated cell ablation and reverse transcription quantitative-PCR (RT-qPCR), I found that cry4 expression decreased when UV cones were ablated but was unaffected when neighboring blue cones were ablated in larval and adult retina. cry2 did not appear to be expressed in UV cones and was unchanged after UV or blue cone ablation in both developmental stages. Although zebrafish magnetic behavior has only been reported in adults, this work suggests larval fish may also have the molecular framework for magnetoreception. While zebrafish are non-migratory, they can be used to model other fish that migrate long-distances. Salmonids regenerate UV cones as they prepare to migrate back to their natal streams for spawning. Currently, the functional significance of this process has yet to determined. These findings could provide one explanation for this as UV cones may enable magnetoreception via cry. In summary, I describe the localization of cry4 in the zebrafish retina towards understanding whether fish have the molecular mechanisms for light-dependent magnetoreception
... Pour déterminer leur direction à suivre à partir d'un point sur une carte, les oiseaux peuvent ensuite utiliser un compas interne. Un tel compas interne peut se baser sur la position du soleil calibrée en rapport à une horloge interne (Padget et al. 2018), ou éventuellement sur d'autres indices stellaires du même type (Foster et al. 2018), ou peut être basé sur la perception du champ magnétique terrestre, qui est répandu chez les oiseaux mais ne semble pas strictement nécessaire à la navigation chez les procellariiformes (Bonadonna et al. 2005, Mouritsen 2015, Pollonara et al. 2015. ...
It is essential to understand how animals make foraging decisions to acquire food in order to better anticipate their responses to environmental changes. Breeding seabirds make central-place foraging trips at sea, from their colony. The deployment of small GPS devices on them reveals that they travel for tens to thousands of kilometers, in search of prey for which very little information is known. The behavioural strategies they use to increase their chances to encounter prey, and the implications of these strategies with regards to human fishing activities remain open questions. This thesis offers to examine these questions in three chapters, through theoretical simulations, empirical analyses of foraging trips of various species and populations of seabirds, and the spatiotemporal matching of seabirds and fishing vessels movements. First, our random walk simulations indicate that straight-line phases within path are not sufficient to conclude that seabirds anticipate where to find their prey, contrary to previous conclusions proposed in the literature. However it is possible and easy to analyze biases in the directions individuals follow when they forage, to infer which sources of information they use to decide where to forage. Second, we compare individual fidelity strategies between species, populations and/or ecological contexts through the use of multivariate statistical models (GLMM). Many seabirds display individual fidelity in the direction they forage from the colony, suggesting they rely on memory. Our results show that this is also the case in different species and populations of tropical seabirds, where individuals can remain faithful to a foraging direction for several consecutive days. These results are surprising and difficult to explain as the species we studied are targeting prey whose distribution is supposedly very stochastic and ephemeral. It suggests that the use of memory might be much more widespread in foraging seabirds than anticipated, at least for decisions at large spatial scales. Finally, our analyses on the responses of albatrosses to fishing boats suggest that their responses can be modulated according to species and energetic constraints, and that encounters of fishing boats during a foraging trip have little influence on the strategy used by individuals on their next foraging trip. The attraction of albatrosses to boats might be mainly a local process (at the scale of the perception range) and may be largely opportunistic. Overall, our empirical results anchored in a solid theoretical framework suggest that seabird’s foraging cannot be summarized as encountering rare and unpredictable resources, but might imply resource selection processes after resources are encountered, and/or a decision as to rely either on memory or public information. With that regard, anthropic resources may only be one type of resources among others for seabirds. Many of the analytical tools used here could be transferred to other seabirds and other central place foragers. Indeed, a wider comparative approach is necessary to understand the complex variations in behavioural plasticity observed here, and their consequences regarding future environmental changes.
... The Earth's magnetic field represents an omnipresent navigational cue for birds during migration and could theoretically provide both directional (i.e. a 'compass') and positional (i.e. a 'map') information [1][2][3][4][5]. A large body of evidence suggests that birds possess a magnetic inclination 'compass' [1,2,6], which is embedded in their visual system [7][8][9][10]. ...
Even though previously described iron-containing structures in the upper beak of pigeons were almost certainly macrophages, not magnetosensitive neurons, behavioural and neurobiological evidence still supports the involvement of the ophthalmic branch of the trigeminal nerve (V1) in magnetoreception. In previous behavioural studies, inactivation of putative V1-associated magnetoreceptors involved either application of the surface anaesthetic lidocaine to the upper beak or sectioning of V1. Here, we compared the effects of lidocaine treatment, V1 ablations and sham ablations on magnetic field-driven neuronal activation in V1-recipient brain regions in European robins. V1 sectioning led to significantly fewer Egr-1-expressing neurons in the trigeminal brainstem than in the sham-ablated birds, whereas lidocaine treatment had no effect on neuronal activation. Furthermore, Prussian blue staining showed that nearly all iron-containing cells in the subepidermal layer of the upper beak are nucleated and are thus not part of the trigeminal nerve, and iron-containing cells appeared in highly variable numbers at inconsistent locations between individual robins and showed no systematic colocalization with a neuronal marker. Our data suggest that lidocaine treatment has been a nocebo to the birds and a placebo for the experimenters. Currently, the nature and location of any V1-associated magnetosensor remains elusive.
... Migratin birds, hunting eels and sharks or flowers attracting their insect pollinators: all use their own electric fields. [140][141][142][143][144][145][146][147] Behavioral and cognitive effects in humans have been documented too. Electric stimulations are used for the treatment of depression, bipolar mood disorders and mood elevation. ...
In the legacy of Thomas Henry Huxley, and his ‘epigenetic’ philosophy of biology, cells are proposed to represent a trinity of three memory-storing media: Senome, Epigenome, and Genome that together comprise a cell-wide informational architecture. Our current preferential focus on the Genome needs to be complemented by a similar focus on the Epigenome and a here proposed Senome, representing the sum of all the sensory experiences of the cognitive cell and its sensing apparatus. Only then will biology be in a position to embrace the whole complexity of the eukaryotic cell, understanding its true nature which allows the communicative assembly of cells in the form of sentient multicellular organisms.
... Magnetoreception, the ability to sense the Earth's magnetic field, has been most intensively and in depth studied in birds, notably in migratory birds and homing pigeons (for recent reviews see e.g. Mouritsen 2014, Kishkinev & Chernetsov 2015, Wiltschko & Wiltschko 2015. The established research paradigm is observation of vanishing directions after release in the field, tracking of flight routes using telemetry, or observation of preferred escape direction in the Emlen's funnel. ...
Magnetoreception has been widely studied in birds mainly through the paradigm of homing or seasonally appropriate migratory direction. It was found that in total darkness or under selected light regimes (differing in colour and/or intensities), migratory birds display orientation towards certain “fixed” directions which do not correspond to the migratory or homing direction. This “fixedorientation” might correspond to the so-called magnetic alignment recorded in animals of different non-avian taxa. Here we demonstrate that also “common”, non-migratory birds, pheasants, adopt a preferred position and body orientation when drinking at a circular dish. We recorded these parameters by means of camera traps in a pheasantry under control conditions and under experimental exposure to bright blue light. We identified three types of orientation at the edge of drinking dish: standing radially or tangentially with left or right eye to the dish. The position of tangentially drinking chicks was significantly non-random. While the position of radially drinking chicks was random under control conditions, it became significantly non-random, concentrated at about the north and south pole of the dish, under bright blue light. Our results show that this alignment has some similarities with the “fixed orientation”. We suggest that the preference towards a “fixed” direction serves to calibration, organization and reading of the mental (cognitive) map of the space and as a direction indicator. We discuss heuristic potential of the presented research (experimental and evaluation) design for further study on magnetoreception.
... bacteria relying on simple photoreceptors, H€ ader, 1987; J ekely, 2009) or whether it is capable of navigating successfully halfway around the world during migration (e.g. birds or mammals equipped with an array of complex sensory systems, Alerstam, Gudmundsson, Green, & Hedenstr€ om, 2001;Gagliardo, 2013;Mouritsen, 2015;Wiltschko & Wiltschko, 2009). Besides influencing this basic navigational framework, the sensory input might also affect the navigational strategy within a single individual. ...
Most animals possess multiple sensory systems, which can be used during navigation. Different senses obtain environmental information on different spatial scales and thus provide a different basis for efficient navigation. Here we used the weakly electric fish Gnathonemus petersii to investigate how different sensory inputs influence the navigational strategy and whether landmark information can be transferred flexibly between two sensory systems. Fish were trained to swim through a maze using a certain route indicated by either visual landmarks, electrical landmarks or without any landmarks. In subsequent tests, egocentric (internal cues, such as motion patterns) and allocentric cues (external cues like landmarks) were put in conflict by relocating the local landmarks. We found that all fish, independent of the available sensory input, chose the egocentric over the allocentric route. However, visual landmarks significantly improved the training duration compared to the other groups, suggesting an involvement of allocentric visual cues during route acquisition. In a second experimental series, fish were trained to use either visual or electrical landmarks for navigation and were subsequently tested in sensory transfer tests. Fish trained with visual landmarks were able to learn this allocentric navigation task and were capable of cross-modal landmark recognition, although navigation based on electrical landmarks was less efficient. The fish trained with electrical landmarks did not learn the task at all, suggesting that the short perceptual range of the electric sense prevented learning of allocentric navigation. Together our results show that the type of sensory input influences the efficiency of allocentric navigation in G. petersii and that these fish can use egocentric and allocentric strategies flexibly to navigate successfully under varying environmental conditions.
... Generating a map based on geomagnetic field parameters is not far-fetched. Apart from naturally occuring magnetic anomalies and irregular daily changes, which will set a lower limit of > 10 km for the accuracy of a magnetic map (Mouritsen 2013(Mouritsen , 2015, in many regions on Earth, magnetic parameters (declination, inclination and/or intensity) form a more or less consistent grid of isolines, whose values gradually and predictably change (see e.g. Boström et al. 2012). ...
... In a more detailed study, Holland and Helm (2013) reported that experienced migratory songbirds, which have already acquired a navigational map (Perdeck 1958;Mouritsen 2003;Holland 2014), reacted with a directed, but deflected orientation response (Beason and Semm 1996;Munro et al. 1997;Wiltschko et al. 2009;Holland 2010), whereas juvenile migratory songbirds, which had not yet established a map (Perdeck 1958;Mouritsen 1998Mouritsen , 2003Mouritsen and Mouritsen 2000;Holland 2014) were unaffected by the pulse treatments (Holland and Helm 2013). To sum up, a growing body of evidence supports the existence of a magnetic "map" sense based on magnetic iron particles associated with the trigeminal system (for reviews, see Wiltschko and Wiltschko 2013;Mouritsen 2015;Mouritsen et al. 2016;Hore and Mouritsen 2016). ...
The Earth's magnetic field is one of several natural cues, which migratory birds can use to derive directional ("compass") information for orientation on their biannual migratory journeys. Moreover, magnetic field effects on prominent aspects of the migratory programme of birds, such as migratory restlessness behaviour, fuel deposition and directional orientation, implicate that geomagnetic information can also be used to derive positional ("map") information. While the magnetic "compass" in migratory birds is likely to be based on radical pair-forming molecules embedded in their visual system, the sensory correlates underlying a magnetic "map" sense currently remain elusive. Behavioural, physiological and neurobiological findings indicate that the sensor is most likely innervated by the ophthalmic branch of the trigeminal nerve and based on magnetic iron particles. Information from this unknown sensor is neither necessary nor sufficient for a functional magnetic compass, but instead could contribute important components of a multifactorial "map" for global positioning. Positional information could allow migratory birds to make vitally important dynamic adaptations of their migratory programme at any relevant point during their journeys.
