A Putative Mechanism for Magnetoreception by Electromagnetic Induction in the Pigeon Inner Ear

Magnetoreception and the ruling hypothesis

Whereas science is written by humans and cannot escape emotions intervening with scientific thought, the scientific community should be on guard against unnoticeably adopting a favorite hypothesis. When adopting a favorite hypothesis, scientists tend to review their work in favor of this hypothesis and reject contradictory data. In 1890, Thomas Chrowder Chamberlin first described this phenomenon as when 'the search for facts, and their interpretation are dominated by affection for the favored theory until it appears to its advocate to have been overwhelmingly established'. The favorite hypothesis can then quickly transition into a ruling hypothesis, leading to an unconscious bias in favor of supporting evidence and neglect of contradictory observations. This is especially problematic when a scientific field adopts a favorite hypothesis. In this Commentary, we suggest that the field of animal magnetoreception - in particular mechanisms based on radical-pair chemistry and cryptochrome proteins - may be under the reign of a ruling hypothesis. We argue that repeatedly, conclusions are unfounded or otherwise not consistent with the results presented. We use the case of magnetoreception - the only sense that remains without a clearly described receptor - to raise general awareness of the phenomenon of a ruling hypothesis in the scientific community. We emphasize the distinction between the scientist and the scientific community suffering from a hypothesis regime, and further highlight suggestions to mitigate the risk of working under a ruling hypothesis.

Extremely low frequency magnetic field distracts zebrafish from a visual cognitive task

Electromagnetic fields emitted from overhead power lines and subsea cables are widely regarded to be a disruptive factor for animals using the natural magnetic field as orientation cue for guiding their directed movements. However, it is not known if anthropogenic electromagnetic fields also have the potential to disturb animals attending to information from other sensory modalities. To find out, we trained adult zebrafish (Danio rerio) individually to perform avoidance behavior in response to a visual signal (green LED light spot), which in the exposure group was presented simultaneously with a sinusoidally changing magnetic field (0.3 Hz, group A: 0.015 mT, group B: 0.06 mT). Despite the salience of the visual signal, which was both sufficient and necessary to elicit conditioned avoidance responses, the 0.06 mT magnetic condition had a negative impact on learning performance and response behavior. This suggests that extremely low frequency technical magnetic fields of Earth strength amplitude can act as cross-modal distractor that diverts the attention of animals away from environmentally relevant cues based on nonmagnetic sensory modalities. Our research highlights the need to study the role of anthropogenic magnetic fields as sensory pollutant beyond the scope of magnetic orientation behavior.

A new data-driven paradigm for the study of avian migratory navigation

Avian navigation has fascinated researchers for many years. Yet, despite a vast amount of literature on the topic it remains a mystery how birds are able to find their way across long distances while relying only on cues available locally and reacting to those cues on the fly. Navigation is multi-modal, in that birds may use different cues at different times as a response to environmental conditions they find themselves in. It also operates at different spatial and temporal scales, where different strategies may be used at different parts of the journey. This multi-modal and multi-scale nature of navigation has however been challenging to study, since it would require long-term tracking data along with contemporaneous and co-located information on environmental cues. In this paper we propose a new alternative data-driven paradigm to the study of avian navigation. That is, instead of taking a traditional theory-based approach based on posing a research question and then collecting data to study navigation, we propose a data-driven approach, where large amounts of data, not purposedly collected for a specific question, are analysed to identify as-yet-unknown patterns in behaviour. Current technological developments have led to large data collections of both animal tracking data and environmental data, which are openly available to scientists. These open data, combined with a data-driven exploratory approach using data mining, machine learning and artificial intelligence methods, can support identification of unexpected patterns during migration, and lead to a better understanding of multi-modal navigational decision-making across different spatial and temporal scales.

Learned magnetic map cues and two mechanisms of magnetoreception in turtles

Growing evidence indicates that migratory animals exploit the magnetic field of the Earth for navigation, both as a compass to determine direction and as a map to determine geographical position1. It has long been proposed that, to navigate using a magnetic map, animals must learn the magnetic coordinates of the destination2,3, yet the pivotal hypothesis that animals can learn magnetic signatures of geographical areas has, to our knowledge, yet to be tested. Here we report that an iconic navigating species, the loggerhead turtle (Caretta caretta), can learn such information. When fed repeatedly in magnetic fields replicating those that exist in particular oceanic locations, juvenile turtles learned to distinguish magnetic fields in which they encountered food from magnetic fields that exist elsewhere, an ability that might underlie foraging site fidelity. Conditioned responses in this new magnetic map assay were unaffected by radiofrequency oscillating magnetic fields, a treatment expected to disrupt radical-pair-based chemical magnetoreception4-6, suggesting that the magnetic map sense of the turtle does not rely on this mechanism. By contrast, orientation behaviour that required use of the magnetic compass was disrupted by radiofrequency oscillating magnetic fields. The findings provide evidence that two different mechanisms of magnetoreception underlie the magnetic map and magnetic compass in sea turtles.

Cataglyphis ants have a polarity-sensitive magnetic compass

Spatial orientation based on the geomagnetic field (GMF) is a widespread phenomenon in the animal kingdom, predominantly observed in long-distance migrating birds,1 sea turtles,2 lobsters,3 and Lepidoptera.4,5 Although magnetoreception has been studied intensively, the mechanism remains elusive. A crucial question for a mechanistic understanding of magnetoreception is whether animals rely on inclination or polarity-based magnetic information. Inclination-based magnetic orientation utilizes the angle between the magnetic field lines and gravity, indicating poleward and equatorward. In contrast, polarity-based magnetic orientation allows animals to detect the polarity of the GMF, the north and south direction of the field vector. Cataglyphis desert ants are excellent experimental models for testing whether magnetic inclination or polarity of the magnetic field is used for navigation. Desert ants are solitary foragers with exceptional navigational skills.6 When the ants leave their underground nest for the first time to become foragers, they perform learning walks for up to three days to learn the visual panorama and calibrate their compass systems.7,8 The ants repeatedly stop their forward movement during learning walks for performing turns (pirouettes), interrupted by stopping phases. Gaze directions during the longest stopping phases are directed toward the nest entrance.9 We experimentally manipulated look-back behavior systematically by altering polarity or inclination of the GMF. We demonstrate that Cataglyphis ants, contrary to most other insects studied,10 possess a polarity-sensitive magnetic compass, making them ideal experimental models for narrowing down the evidence for particle-based mechanisms underlying magnetosensation in this insect.

Biophysical mechanism of animal magnetoreception, orientation and navigation

We describe a biophysical mechanism for animal magnetoreception, orientation and navigation in the geomagnetic field (GMF), based on the ion forced oscillation (IFO) mechanism in animal cell membrane voltage-gated ion channels (VGICs) (IFO-VGIC mechanism). We review previously suggested hypotheses. We describe the structure and function of VGICs and argue that they are the most sensitive electromagnetic sensors in all animals. We consider the magnetic force exerted by the GMF on a mobile ion within a VGIC of an animal with periodic velocity variation. We apply this force in the IFO equation resulting in solution connecting the GMF intensity with the velocity variation rate. We show that animals with periodic velocity variations, receive oscillating forces on their mobile ions within VGICs, which are forced to oscillate exerting forces on the voltage sensors of the channels, similar or greater to the forces from membrane voltage changes that normally induce gating. Thus, the GMF in combination with the varying animal velocity can gate VGICs and alter cell homeostasis in a degree depending, for a given velocity and velocity variation rate, on GMF intensity (unique in each latitude) and the angle between velocity and GMF axis, which determine animal position and orientation.

The origins of light-independent magnetoreception in humans

The Earth's abundance of iron has played a crucial role in both generating its geomagnetic field and contributing to the development of early life. In ancient oceans, iron ions, particularly around deep-sea hydrothermal vents, might have catalyzed the formation of macromolecules, leading to the emergence of life and the Last Universal Common Ancestor. Iron continued to influence catalysis, metabolism, and molecular evolution, resulting in the creation of magnetosome gene clusters in magnetotactic bacteria, which enabled these unicellular organisms to detect geomagnetic field. Although humans lack a clearly identified organ for geomagnetic sensing, many life forms have adapted to geomagnetic field-even in deep-sea environments-through mechanisms beyond the conventional five senses. Research indicates that zebrafish hindbrains are sensitive to magnetic fields, the semicircular canals of pigeons respond to weak potential changes through electromagnetic induction, and human brainwaves respond to magnetic fields in darkness. This suggests that the trigeminal brainstem nucleus and vestibular nuclei, which integrate multimodal magnetic information, might play a role in geomagnetic processing. From iron-based metabolic systems to magnetic sensing in neurons, the evolution of life reflects ongoing adaptation to geomagnetic field. However, since magnetite-activated, torque-based ion channels within cell membranes have not yet been identified, specialized sensory structures like the semicircular canals might still be necessary for detecting geomagnetic orientation. This mini-review explores the evolution of life from Earth's formation to light-independent human magnetoreception, examining both the magnetite hypothesis and the electromagnetic induction hypothesis as potential mechanisms for human geomagnetic detection.

A conceptual framework on the role of magnetic cues in songbird migration ecology

Migrating animals perform astonishing seasonal movements by orienting and navigating over thousands of kilometres with great precision. Many migratory species use cues from the sun, stars, landmarks, olfaction and the Earth's magnetic field for this task. Among vertebrates, songbirds are the most studied taxon in magnetic-cue-related research. Despite multiple studies, we still lack a clear understanding of when, where and how magnetic cues affect the decision-making process of birds and hence, their realised migratory behaviour in the wild. This understanding is especially important to interpret the results of laboratory experiments in an ecologically appropriate way. In this review, we summarise the current findings about the role of magnetic cues for migratory decisions in songbirds. First, we review the methodological principles for orientation and navigation research, specifically by comparing experiments on caged birds with experiments on free-flying birds. While cage experiments can show the sensory abilities of birds, studies with free-flying birds can characterise the ecological roles of magnetic cues. Second, we review the migratory stages, from stopover to endurance flight, in which songbirds use magnetic cues for their migratory decisions and incorporate this into a novel conceptual framework. While we lack studies examining whether and when magnetic cues affect orientation or navigation decisions during flight, the role of magnetic cues during stopover is relatively well studied, but mostly in the laboratory. Notably, many such studies have produced contradictory results so that understanding the biological importance of magnetic cues for decisions in free-flying songbirds is not straightforward. One potential explanation is that reproducibility of magnetic-cue experiments is low, probably because variability in the behavioural responses of birds among experiments is high. We are convinced that parts of this variability can be explained by species-specific and context-dependent reactions of birds to the study conditions and by the bird's high flexibility in whether they include magnetic cues in a decision or not. Ultimately, this review should help researchers in the challenging field of magnetoreception to design experiments meticulously and interpret results of such studies carefully by considering the migration ecology of their focal species.

Magnetite in the abdomen and antennae of Apis mellifera honeybees

The detection of magnetic fields by animals is known as magnetoreception. The ferromagnetic hypothesis explains magnetoreception assuming that magnetic nanoparticles are used as magnetic field transducers. Magnetite nanoparticles in the abdomen of Apis mellifera honeybees have been proposed in the literature as the magnetic field transducer. However, studies with ants and stingless bees have shown that the whole body of the insect contain magnetic material, and that the largest magnetization is in the antennae. The aim of the present study is to investigate the magnetization of all the body parts of honeybees as has been done with ants and stingless bees. To do that, the head without antennae, antennae, thorax, and abdomen obtained from Apis mellifera honeybees were analyzed using magnetometry and Ferromagnetic Resonance (FMR) techniques. The magnetometry and FMR measurements show the presence of magnetic material in all honeybee body parts. Our results present evidence of the presence of biomineralized magnetite nanoparticles in the honeybee abdomen and, for the first time, magnetite in the antennae. FMR measurements permit to identify the magnetite in the abdomen as biomineralized. As behavioral experiments reported in the literature have shown that the abdomen is involved in magnetoreception, new experimental approaches must be done to confirm or discard the involvement of the antennae in magnetoreception.

Cognitive maps and the magnetic sense in vertebrates

Navigation requires a network of neurons processing inputs from internally generated cues and external landmarks. Most studies on the neuronal basis of navigation in vertebrates have focused on rats and mice and the canonical senses vision, hearing, olfaction, and somatosensation. Some animals have evolved the ability to sense the Earth's magnetic field and use it for orientation. It can be expected that in these animals magnetic cues are integrated with other sensory cues in the cognitive map. We provide an overview of the behavioral evidence and brain regions involved in magnetic sensing in support of this idea, hoping that this will guide future experiments.

Neuromorphic antennal sensory system

Insect antennae facilitate the nuanced detection of vibrations and deflections, and the non-contact perception of magnetic or chemical stimuli, capabilities not found in mammalian skin. Here, we report a neuromorphic antennal sensory system that emulates the structural, functional, and neuronal characteristics of ant antennae. Our system comprises electronic antennae sensor with three-dimensional flexible structures that detects tactile and magnetic stimuli. The integration of artificial synaptic devices adsorbed with solution-processable MoS2 nanoflakes enables synaptic processing of sensory information. By emulating the architecture of receptor-neuron pathway, our system realizes hardware-level, spatiotemporal perception of tactile contact, surface pattern, and magnetic field (detection limits: 1.3 mN, 50 μm, 9.4 mT). Vibrotactile-perception tasks involving profile and texture classifications were accomplished with high accuracy (> 90%), surpassing human performance in "blind" tactile explorations. Magneto-perception tasks including magnetic navigation and touchless interaction were successfully completed. Our work represents a milestone for neuromorphic sensory systems and biomimetic perceptual intelligence.

Assembly and annotation of 2 high-quality columbid reference genomes from sequencing of a Columba livia × Columba guinea F1 hybrid

Pigeons and doves (family Columbidae) are one of the most diverse extant avian lineages, and many species have served as key models for evolutionary genomics, developmental biology, physiology, and behavioral studies. Building genomic resources for columbids is essential to further many of these studies. Here, we present high-quality genome assemblies and annotations for 2 columbid species, Columba livia and Columba guinea. We simultaneously assembled C. livia and C. guinea genomes from long-read sequencing of a single F1 hybrid individual. The new C. livia genome assembly (Cliv_3) shows improved completeness and contiguity relative to Cliv_2.1, with an annotation incorporating long-read IsoSeq data for more accurate gene models. Intensive selective breeding of C. livia has given rise to hundreds of breeds with diverse morphological and behavioral characteristics, and Cliv_3 offers improved tools for mapping the genomic architecture of interesting traits. The C. guinea genome assembly is the first for this species and is a new resource for avian comparative genomics. Together, these assemblies and annotations provide improved resources for functional studies of columbids and avian comparative genomics in general.

Assembly and annotation of two high-quality columbid reference genomes from sequencing of a Columba livia x Columba guinea F1 hybrid

Pigeons and doves (family Columbidae) are one of the most diverse extant avian lineages, and many species have served as key models for evolutionary genomics, developmental biology, physiology, and behavioral studies. Building genomic resources for colubids is essential to further many of these studies. Here, we present high-quality genome assemblies and annotations for two columbid species, Columba livia and C. guinea. We simultaneously assembled C. livia and C. guinea genomes from long-read sequencing of a single F1 hybrid individual. The new C. livia genome assembly (Cliv_3) shows improved completeness and contiguity relative to Cliv_2.1, with an annotation incorporating long-read IsoSeq data for more accurate gene models. Intensive selective breeding of C. livia has given rise to hundreds of breeds with diverse morphological and behavioral characteristics, and Cliv_3 offers improved tools for mapping the genomic architecture of interesting traits. The C. guinea genome assembly is the first for this species and is a new resource for avian comparative genomics. Together, these assemblies and annotations provide improved resources for functional studies of columbids and avian comparative genomics in general.

Insect magnetoreception: a Cry for mechanistic insights

Migratory animals can detect and use the Earth's magnetic field for orientation and navigation, sometimes over distances spanning thousands of kilometers. How they do so remains, however, one of the greatest mysteries in all sensory biology. Here, the author reviews the progress made to understand the molecular bases of the animal magnetic sense focusing on insect species, the only species in which genetic studies have so far been possible. The central hypothesis in the field posits that magnetically sensitive radical pairs formed by photoexcitation of cryptochrome proteins are key to animal magnetoreception. The author provides an overview of our current state of knowledge for the involvement of insect light-sensitive type I and light-insensitive type II cryptochromes in this enigmatic sense, and highlights some of the unanswered questions to gain a comprehensive understanding of magnetoreception at the organismal level.

Calcium/Calmodulin-Dependent Serine Protein Kinase (CASK) Gene Polymorphisms in Pigeons

Calcium/calmodulin-dependent serine protein kinase (CASK) is an multidomain protein involved in tissue development and cell signalling. In skeletal muscle, it is involved in the development of neuromuscular junctions. The participation of a pigeon in racing is a great physical effort that causes many changes in the skeletal muscles. Thus, the purpose of the study was to detect the nucleotide sequence variability in the calcium/calmodulin-dependent serine kinase (CASK) gene in domestic pigeons (Columba livia domestica) and assess the potential impact of DNA polymorphisms on the flight performance of pigeons. The research included a total of 517 individuals. DNA was extracted from the blood. A DNA fragment from nucleotides 8689 to 9049 of the CASK (NW_004973256.1 sequence) of six unrelated pigeons were sequenced. One of the detected polymorphic sites (g.8893G > A), located a very close to the start codon, was selected for genotyping in all individuals. The association studies included a total of 311 young homing pigeons that participated in racing competitions. The homing pigeons showed higher frequencies of the AA genotype than non-homing ones (p < 0.05). In rock pigeons only the GG genotype was found. Further research could confirm the functionality of the CASK g.8893G > A SNP in shaping the racing phenotype of pigeons, and the AA genotype could be useful as a selection criterion in pigeon breeding.

Magnetic actuation of otoliths allows behavioral and brain-wide neuronal exploration of vestibulo-motor processing in larval zebrafish

The vestibular system in the inner ear plays a central role in sensorimotor control by informing the brain about the orientation and acceleration of the head. However, most experiments in neurophysiology are performed using head-fixed configurations, depriving animals of vestibular inputs. To overcome this limitation, we decorated the utricular otolith of the vestibular system in larval zebrafish with paramagnetic nanoparticles. This procedure effectively endowed the animal with magneto-sensitive capacities: applied magnetic field gradients induced forces on the otoliths, resulting in robust behavioral responses comparable to those evoked by rotating the animal by up to 25°. We recorded the whole-brain neuronal response to this fictive motion stimulation using light-sheet functional imaging. Experiments performed in unilaterally injected fish revealed the activation of a commissural inhibition between the brain hemispheres. This magnetic-based stimulation technique for larval zebrafish opens new perspectives to functionally dissect the neural circuits underlying vestibular processing and to develop multisensory virtual environments, including vestibular feedback.

Towards magnetism in pigeon MagR: Iron- and iron-sulfur binding work indispensably and synergistically

The ability to navigate long distances is essential for many animals to locate shelter, food, and breeding grounds. Magnetic sense has evolved in various migratory and homing species to orient them based on the geomagnetic field. A highly conserved iron-sulfur cluster assembly protein IscA is proposed as an animal magnetoreceptor (MagR). Iron-sulfur cluster binding is also suggested to play an essential role in MagR magnetism and is thus critical in animal magnetoreception. In the current study, we provide evidence for distinct iron binding and iron-sulfur cluster binding in MagR in pigeons, an avian species that relies on the geomagnetic field for navigation and homing. Pigeon MagR showed significantly higher total iron content from both iron- and iron-sulfur binding. Y65 in pigeon MagR was shown to directly mediate mononuclear iron binding, and its mutation abolished iron-binding capacity of the protein. Surprisingly, both iron binding and iron-sulfur binding demonstrated synergistic effects, and thus appear to be integral and indispensable to pigeon MagR magnetism. These results not only extend our current understanding of the origin and complexity of MagR magnetism, but also imply a possible molecular explanation for the huge diversity in animal magnetoreception.

Biological Effects of Electric, Magnetic, and Electromagnetic Fields from 0 to 100 MHz on Fauna and Flora: Workshop Report

ABSTRACT

This report summarizes effects of anthropogenic electric, magnetic, and electromagnetic fields in the frequency range from 0 to 100 MHz on flora and fauna, as presented at an international workshop held on 5-7 November in 2019 in Munich, Germany. Such fields may originate from overhead powerlines, earth or sea cables, and from wireless charging systems. Animals and plants react differentially to anthropogenic fields; the mechanisms underlying these responses are still researched actively. Radical pairs and magnetite are discussed mechanisms of magnetoreception in insects, birds, and mammals. Moreover, several insects as well as marine species possess specialized electroreceptors, and behavioral reactions to anthropogenic fields have been reported. Plants react to experimental modifications of their magnetic environment by growth changes. Strong adverse effects of anthropogenic fields have not been described, but knowledge gaps were identified; further studies, aiming at the identification of the interaction mechanisms and the ecological consequences, are recommended.

Morphology of the "prefrontal" nidopallium caudolaterale in the long-distance night-migratory Eurasian blackcap (Sylvia atricapilla)

Migrating birds have developed remarkable navigational capabilities to successfully master biannual journeys between their breeding and wintering grounds. To reach their intended destination, they need to calculate navigational goals from a large variety of natural directional and positional cues to set a meaningful motor output command. One brain area, which has been associated with such executive functions, is the nidopallium caudolaterale (NCL), which, due to its striking similarities in terms of neurochemistry, connectivity and function, is considered analogous to the mammalian prefrontal cortex. To establish a baseline for further analyses elucidating the neuronal correlates underlying avian navigation, we performed quantitative and qualitative analyses of dopaminergic fibres in the brains of long-distance night-migratory Eurasian blackcaps (Sylvia atricapilla). We identified four regions in the caudal telencephalon, each of which was characterized by its specific dopaminergic innervation pattern. At least three of them presumably constitute subareas of the NCL in Eurasian blackcaps and could thus be involved in integrating navigational input from different sensory systems. The observed heterogeneity and parcellation of the NCL subcompartments in this migratory species could be a consequence of the special demands related to navigation.

Integration and evaluation of magnetic stimulation in physiology setups

A large number of behavioral experiments have demonstrated the existence of a magnetic sense in many animal species. Further, studies with immediate gene expression markers have identified putative brain regions involved in magnetic information processing. In contrast, very little is known about the physiology of the magnetic sense and how the magnetic field is neuronally encoded. In vivo electrophysiological studies reporting neuronal correlates of the magnetic sense either have turned out to be irreproducible for lack of appropriate artifact controls or still await independent replication. Thus far, the research field of magnetoreception has little exploited the power of ex vivo physiological studies, which hold great promise for enabling stringent controls. However, tight space constraints in a recording setup and the presence of magnetizable materials in setup components and microscope objectives make it demanding to generate well-defined magnetic stimuli at the location of the biological specimen. Here, we present a solution based on a miniature vector magnetometer, a coil driver, and a calibration routine for the coil system to compensate for magnetic distortions in the setup. The magnetometer fits in common physiology recording chambers and has a sufficiently small spatial integration area to allow for probing spatial inhomogeneities. The coil-driver allows for the generation of defined non-stationary fast changing magnetic stimuli. Our ex vivo multielectrode array recordings from avian retinal ganglion cells show that artifacts induced by rapid magnetic stimulus changes can mimic the waveform of biological spikes on single electrodes. However, induction artifacts can be separated clearly from biological responses if the spatio-temporal characteristics of the artifact on multiple electrodes is taken into account. We provide the complete hardware design data and software resources for the integrated magnetic stimulation system.

Myths in magnetosensation

The ability to detect magnetic fields is a sensory modality that is used by many animals to navigate. While first postulated in the 1800s, for decades, it was considered a biological myth. A series of elegant behavioral experiments in the 1960s and 1970s showed conclusively that the sense is real; however, the underlying mechanism(s) remained unresolved. Consequently, this has given rise to a series of beliefs that are critically analyzed in this manuscript. We address six assertions: (1) Magnetoreception does not exist; (2) It has to be magnetite; (3) Birds have a conserved six loci magnetic sense system in their upper beak; (4) It has to be cryptochrome; (5) MagR is a protein biocompass; and (6) The electromagnetic induction hypothesis is dead. In advancing counter-arguments for these beliefs, we hope to stimulate debate, new ideas, and the design of well-controlled experiments that can aid our understanding of this fascinating biological phenomenon.

Insights into the behavioural responses of juvenile thornback ray Raja clavata to alternating and direct current magnetic fields

As part of energy transition, marine renewable energy devices (MRED) are currently expanding in developed countries inducing the deployment of dense networks of submarine power cables. Concern has thus raised about the cable magnetic emissions (direct or alternating current) because of potential interference with the sensorial environment of magneto-sensitive species, such as sharks and rays. This study sought to assess the short-term behavioural responses of juvenile thornback rays (Raja clavata) (n = 15) to direct and alternating (50 Hz) uniform 450-μT artificial magnetic fields using 1 h focal-sampling design based on a detailed ethogram. Careful control of magnetic fields' temporal and spatial scales was obtained in laboratory conditions through a custom-made Helmholtz coil device. Overall, qualitative or quantitative behavioural responses of juvenile rays did not significantly vary between control vs. exposed individuals over the morning period. Nonetheless, rays under direct current magnetic field increased their activity over the midday period. Synchronisation patterns were also observed for individuals receiving alternating current exposure (chronologic and qualitative similarities) coupled with a high inter-individual variance. Further studies should consider larger batches of juveniles to address the effect of long-term exposure and explore the sensitivity range of rays with dose-response designs.

The biological effects of geomagnetic field exhibit an important potential for cancer and vascular diseases

Xu et al. recently demonstrated that cryptochrome 4 (CRY4) protein, as a light-dependent magnetic receptor, can sense geomagnetic fields to guide night-migratory songbirds' navigation and evolution by the formation of composite radical pairs and electron transport. We aim to comment on CRY4 through radical pairs and electron transport for magnetic sensitive in night-migratory songbirds' migration and evolution. Additionally, we find that the role of magnetic fields is deeply concerning to the scientific community and very enlightening for the diagnosis and treatment of cancer and vascular disease. We believe that this commentary makes a significant contribution to the literature because it elaborates on the importance of the geomagnetic field to night-migratory songbirds and extends the diagnostic and therapeutic value to cancer and vascular disease. This article is protected by copyright. All rights reserved.

No apparent effect of a magnetic pulse on free-flight behaviour in northern wheatears (Oenanthe oenanthe) at a stopover site

Naïve migrants reach their wintering grounds following a clock-and-compass strategy. During these inaugural migrations, birds internalise, among others, cues from the Earth's magnetic field to create a geomagnetic map, with which they navigate to destinations familiar to them on subsequent migrations. Geomagnetic map cues are thought to be sensed by a magnetic-particle-based receptor, which can be specifically affected by a magnetic pulse. Indeed, the orientation of experienced but not naïve birds was compromised after magnetic pulsing, indicating geomagnetic map use. Little is known about the importance of this putative magnetoreceptor for navigation and decision-making in free-flying migrants. Therefore, we studied in unprecedented detail how a magnetic pulse would affect departure probability, nocturnal departure timing, departure direction and consistency in flight direction over 50–100 km in experienced and naïve long-distant migrant songbirds using a large-scale radio-tracking system. Contrary to our expectations and despite a high sample size (n_total = 137) for a free-flight study, we found no significant after-effect of the magnetic pulse on the migratory traits, suggesting the geomagnetic map is not essential for the intermediate autumn migration phase. These findings warrant re-thinking about perception and use of geomagnetic maps for migratory decisions within a sensory and ecological context.

Magnetoreception and magnetic navigation in fishes: a half century of discovery

As the largest and most diverse vertebrate group on the planet, fishes have evolved an impressive array of sensory abilities to overcome the challenges associated with navigating the aquatic realm. Among these, the ability to detect Earth's magnetic field, or magnetoreception, is phylogenetically widespread and used by fish to guide movements over a wide range of spatial scales ranging from local movements to transoceanic migrations. A proliferation of recent studies, particularly in salmonids, has revealed that fish can exploit Earth's magnetic field not only as a source of directional information for maintaining consistent headings, but also as a kind of map for determining location at sea and for returning to natal areas. Despite significant advances, much about magnetoreception in fishes remains enigmatic. How fish detect magnetic fields remains unknown and our understanding of the evolutionary origins of vertebrate magnetoreception would benefit greatly from studies that include a wider array of fish taxa. The rich diversity of life-history characteristics that fishes exhibit, the wide variety of environments they inhabit, and their suitability for manipulative studies, make fishes promising subjects for magnetoreception studies.

The importance of time of day for magnetic body alignment in songbirds

Spontaneous magnetic alignment is the simplest known directional response to the geomagnetic field that animals perform. Magnetic alignment is not a goal directed response and its relevance in the context of orientation and navigation has received little attention. Migratory songbirds, long-standing model organisms for studying magnetosensation, have recently been reported to align their body with the geomagnetic field. To explore whether the magnetic alignment behaviour in songbirds is involved in the underlying mechanism for compass calibration, which have been suggested to occur near to sunset, we studied juvenile Eurasian reed warblers (Acrocephalus scirpaceus) captured at stopover during their first autumn migration. We kept one group of birds in local daylight conditions and an experimental group under a 2 h delayed sunset. We used an ad hoc machine learning algorithm to track the birds' body alignment over a 2-week period. Our results show that magnetic body alignment occurs prior to sunset, but shifts to a more northeast-southwest alignment afterwards. Our findings support the hypothesis that body alignment could be associated with how directional celestial and magnetic cues are integrated in the compass of migratory birds.

Investigating the impact of weak geomagnetic fluctuations on pigeon races

The existence of avian magnetic orientation has been proved by many experimental studies, however, evidence for the use of magnetic cues by homing pigeons remains controversial. To investigate magnetic orientation by homing pigeons, we analyzed the results of pigeon races relative to weak fluctuations in the geomagnetic field, assuming that such disturbances could impact navigational efficiency if based on magnetoreception. Whereas most of the previous studies recorded and analyzed vanishing bearing of individually released pigeons, we evaluated relative duration of the homeward flight (homing speed, as a proxy of navigational efficiency) and its dependence on specific geomagnetic indices in racing pigeons released collectively. Our analysis of orientation efficiency of about 289 pigeon races over 15 years suggested slight negative correlations between geomagnetic fluctuations and homing time. Although the interpretation of this finding is manifold and not clear, it suggests that natural magnetic variations or disturbances can affect the homing orientation performance of pigeons. We suggest that studying pigeon races may have a heuristic potential and since these races are regularly and frequently organized in many countries all over the globe, examining homing performance relative to a suite of environmental variables may be useful for exploring hypotheses about pigeon navigation.

Bird-Inspired Self-Navigating Artificial Synaptic Compass

Extrasensory neuromorphic devices that can recognize, memorize, and learn stimuli imperceptible to human beings are of considerable interest in interactive intelligent electronics research. This study presents an artificially intelligent magnetoreceptive synapse inspired by the magnetocognitive ability used by birds for navigation and orientation. The proposed synaptic platform is based on arrays of ferroelectric field-effect transistors with air-suspended magneto-interactive top-gates. A suspended gate of an elastomeric composite with superparamagnetic particles laminated with an electrically conductive polymer is mechanically deformed under a magnetic field, facilitating control of the magnetic-field-dependent contact area of the suspended gate with an underlying ferroelectric layer. The remanent polarization of the ferroelectric layer is electrically programmed with the deformed suspended gate, resulting in analog conductance modulation as a function of the magnitude, number, and time interval of the input magnetic pulses. The proposed extrasensory magnetoreceptive synapse may be used as an artificially intelligent synaptic compass that facilitates barrier-adaptable navigation and mapping of a moving object.

Modulation of MagR magnetic properties via iron-sulfur cluster binding

Iron-sulfur clusters are essential cofactors found in all kingdoms of life and play essential roles in fundamental processes, including but not limited to respiration, photosynthesis, and nitrogen fixation. The chemistry of iron-sulfur clusters makes them ideal for sensing various redox environmental signals, while the physics of iron-sulfur clusters and its host proteins have been long overlooked. One such protein, MagR, has been proposed as a putative animal magnetoreceptor. It forms a rod-like complex with cryptochromes (Cry) and possesses intrinsic magnetic moment. However, the magnetism modulation of MagR remains unknown. Here in this study, iron-sulfur cluster binding in MagR has been characterized. Three conserved cysteines of MagR play different roles in iron-sulfur cluster binding. Two forms of iron-sulfur clusters binding have been identified in pigeon MagR and showed different magnetic properties: [3Fe-4S]-MagR appears to be superparamagnetic and has saturation magnetization at 5 K but [2Fe-2S]-MagR is paramagnetic. While at 300 K, [2Fe-2S]-MagR is diamagnetic but [3Fe-4S]-MagR is paramagnetic. Together, the different types of iron-sulfur cluster binding in MagR attribute distinguished magnetic properties, which may provide a fascinating mechanism for animals to modulate the sensitivity in magnetic sensing.

Quantum magnetic imaging of iron organelles within the pigeon cochlea

The ability of pigeons to sense geomagnetic fields has been conclusively established despite a notable lack of determination of the underlying biophysical mechanisms. Quasi-spherical iron organelles previously termed "cuticulosomes" in the cochlea of pigeons have potential relevance to magnetoreception due to their location and iron composition; however, data regarding the magnetic susceptibility of these structures are currently limited. Here quantum magnetic imaging techniques are applied to characterize the magnetic properties of individual iron cuticulosomes in situ. The stray magnetic fields emanating from cuticulosomes are mapped and compared to a detailed analytical model to provide an estimate of the magnetic susceptibility of the individual particles. The images reveal the presence of superparamagnetic and ferrimagnetic domains within individual cuticulosomes and magnetic susceptibilities within the range 0.029 to 0.22. These results provide insights into the elusive physiological roles of cuticulosomes. The susceptibilities measured are not consistent with a torque-based model of magnetoreception, placing iron storage and stereocilia stabilization as the two leading putative cuticulosome functions. This work establishes quantum magnetic imaging as an important tool to complement the existing array of techniques used to screen for potential magnetic particle-based magnetoreceptor candidates.

A magnet attached to the forehead disrupts magnetic compass orientation in a migratory songbird

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, attaching small magnets to birds, with the intention of depriving 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 tested 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.

Electrophysiology and the magnetic sense: a guide to best practice

Magnetoreception, sensing the Earth’s magnetic field, is used by many species in orientation and navigation. While this is established on the behavioural level, there is a severe lack in knowledge on the underlying neuronal mechanisms of this sense. A powerful technique to study the neuronal processing of magnetic cues is electrophysiology but, thus far, few studies have adopted this technique. Why is this the case? A fundamental problem is the introduction of electromagnetic noise (induction) caused by the magnetic stimuli, within electrophysiological recordings which, if too large, prevents feasible separation of neuronal signals from the induction artefacts. Here, we address the concerns surrounding the use of electromagnetic coils within electrophysiology experiments and assess whether these would prevent viable electrophysiological recordings within a generated magnetic field. We present calculations of the induced voltages in typical experimental situations and compare them against the neuronal signals measured with different electrophysiological techniques. Finally, we provide guidelines that should help limit and account for possible induction artefacts. In conclusion, if great care is taken, viable electrophysiological recordings from magnetoreceptive cells are achievable and promise to provide new insights on the neuronal basis of the magnetic sense.

Evolution of sensory systems

Sensory systems evolve and enable organisms to perceive their sensory Umwelt, the unique set of cues relevant for their survival. The multiple components that comprise sensory systems - the receptors, cells, organs, and dedicated high-order circuits - can vary greatly across species. Sensory receptor gene families can expand and contract across lineages, resulting in enormous sensory diversity. Comparative studies of sensory receptor function have uncovered the molecular basis of receptor properties and identified novel sensory receptor classes and noncanonical sensory strategies. Phylogenetically informed comparisons of sensory systems across multiple species can pinpoint when sensory changes evolve and highlight the role of contingency in sensory system evolution.

Cryptochromes in Mammals and Birds: Clock or Magnetic Compass?

Species throughout the animal kingdom use the Earth's magnetic field (MF) to navigate using either or both of two mechanisms. The first relies on magnetite crystals in tissue where their magnetic moments align with the MF to transduce a signal transmitted to the central nervous system. The second and the subject of this paper involves cryptochrome (CRY) proteins located in cone photoreceptors distributed across the retina, studied most extensively in birds. According to the "Radical Pair Mechanism" (RPM), blue/UV light excites CRY's flavin cofactor (FAD) to generate radical pairs whose singlet-to-triplet interconversion rate is modulated by an external MF. The signaling product of the RPM produces an impression of the field across the retinal surface. In birds, the resulting signal on the optic nerve is transmitted along the thalamofugal pathway to the primary visual cortex, which projects to brain regions concerned with image processing, memory, and executive function. The net result is a bird's orientation to the MF's inclination: its vector angle relative to the Earth's surface. The quality of ambient light (e.g., polarization) provides additional input to the compass. In birds, the Type IV CRY isoform appears pivotal to the compass, given its positioning within retinal cones; a cytosolic location therein indicating no role in the circadian clock; relatively steady diurnal levels (unlike Type II CRY's cycling); and a full complement of FAD (essential for photosensitivity). The evidence indicates that mammalian Type II CRY isoforms play a light-independent role in the cellular molecular clock without a photoreceptive function.

Magnetosensitivity in the Stingless Bee Tetragonisca angustula: Magnetic Inclination Can Alter the Choice of the Flying Departure Angle From the Nest

It is known that animals are sensitive to the geomagnetic field. In the case of insects, magnetoreception has been reported in several ant species and in some bees and wasps. One study showed that the stingless bee Tetragonisca angustula is able to sense the modification of the magnetic field inclination. The aim of the present manuscript is to continue that study in T. angustula, analyzing the nest arrival and departure angles in the presence of magnetic fields generated by magnets. The bees flying to and from the nest were recorded and the flying trajectories were obtained by analyzing the video frame by frame. The magnetic field was generated by 6, 9, or 12 magnets contained inside an Eppendorf tube and fixed near the nest. Our results show that T. angustula bees are sensitive to magnetic fields because the departure angles are influenced by the magnets. It was observed that these bees are sensitive to the polarization of the magnetic field vector that influences the choice of flying up or down, and this sensitivity has a window until about 80 μT (about four times the local geomagnetic field), with the magnetic field information for higher magnetic field intensities being ignored by the bees. Bioelectromagnetics. 2021;42:51-59. © 2020 Bioelectromagnetics Society.

Neuronal circuits and the magnetic sense: central questions

Magnetoreception is the ability to sense the Earth's magnetic field, which is used for orientation and navigation. Behavioural experiments have shown that it is employed by many species across all vertebrate classes; however, our understanding of how magnetic information is processed and integrated within the central nervous system is limited. In this Commentary, we review the progress in birds and rodents, highlighting the role of the vestibular and trigeminal systems as well as that of the hippocampus. We reflect on the strengths and weaknesses of the methodologies currently at our disposal, the utility of emerging technologies and identify questions that we feel are critical for the advancement of the field. We expect that magnetic circuits are likely to share anatomical motifs with other senses, which culminates in the formation of spatial maps in telencephalic areas of the brain. Specifically, we predict the existence of spatial cells that encode defined components of the Earth's magnetic field.

Animal navigation: a noisy magnetic sense?

Diverse organisms use Earth's magnetic field as a cue in orientation and navigation. Nevertheless, eliciting magnetic orientation responses reliably, either in laboratory or natural settings, is often difficult. Many species appear to preferentially exploit non-magnetic cues if they are available, suggesting that the magnetic sense often serves as a redundant or 'backup' source of information. This raises an interesting paradox: Earth's magnetic field appears to be more pervasive and reliable than almost any other navigational cue. Why then do animals not rely almost exclusively on the geomagnetic field, while ignoring or downplaying other cues? Here, we explore a possible explanation: that the magnetic sense of animals is 'noisy', in that the magnetic signal is small relative to thermal and receptor noise. Magnetic receptors are thus unable to instantaneously acquire magnetic information that is highly precise or accurate. We speculate that extensive time-averaging and/or other higher-order neural processing of magnetic information is required, rendering the magnetic sense inefficient relative to alternative cues that can be detected faster and with less effort. This interpretation is consistent with experimental results suggesting a long time course for magnetic compass and map responses in some animals. Despite possible limitations, magnetoreception may be maintained by natural selection because the geomagnetic field is sometimes the only source of directional and/or positional information available.

Orientation and navigation in Bufo bufo: a quest for repeatability of arena experiments

Research on navigation in animals is hampered by conflicting results and failed replications. In order to assess the generality of previous results, male Bufo bufo were collected during their breeding migration and translocated to two testing sites, 2.4 and 2.9 km away, respectively, from their breeding pond in the north of Vienna (Austria). There each toad was tested twice for orientation responses in a circular arena, on the night of collection and four days later. On the first test day, the toads showed significant axial orientation along their individual former migration direction. On the second test day, no significant homeward orientation was detected. Both results accord with findings of previous experiments with toads from another population. We analysed the potential influence of environmental factors (temperature, cloud cover and lunar cycle) on toad orientations using a MANOVA approach. Although cloud cover and lunar cycle had small effects on the second test day, they could not explain the absence of homeward orientation. The absence of homing responses in these tests may be either caused by the absence of navigational capabilities of toads beyond their home ranges, or by inadequacies of the applied method. To resolve this question, tracking of freely moving toads should have greater potential than the use of arena experiments.

Functional evolution of vertebrate sensory receptors

Sensory receptors enable animals to perceive their external world, and functional properties of receptors evolve to detect the specific cues relevant for an organism's survival. Changes in sensory receptor function or tuning can directly impact an organism's behavior. Functional tests of receptors from multiple species and the generation of chimeric receptors between orthologs with different properties allow for the dissection of the molecular basis of receptor function and identification of the key residues that impart functional changes in different species. Knowledge of these functionally important sites facilitates investigation into questions regarding the role of epistasis and the extent of convergence, as well as the timing of sensory shifts relative to other phenotypic changes. However, as receptors can also play roles in non-sensory tissues, and receptor responses can be modulated by numerous other factors including varying expression levels, alternative splicing, and morphological features of the sensory cell, behavioral validation can be instrumental in confirming that responses observed in heterologous systems play a sensory role. Expression profiling of sensory cells and comparative genomics approaches can shed light on cell-type specific modifications and identify other proteins that may affect receptor function and can provide insight into the correlated evolution of complex suites of traits. Here we review the evolutionary history and diversity of functional responses of the major classes of sensory receptors in vertebrates, including opsins, chemosensory receptors, and ion channels involved in temperature-sensing, mechanosensation and electroreception.

The biophysical, molecular, and anatomical landscape of pigeon CRY4: A candidate light-based quantal magnetosensor

The biophysical and molecular mechanisms that enable animals to detect magnetic fields are unknown. It has been proposed that birds have a light-dependent magnetic compass that relies on the formation of radical pairs within cryptochrome molecules. Using spectroscopic methods, we show that pigeon cryptochrome clCRY4 is photoreduced efficiently and forms long-lived spin-correlated radical pairs via a tetrad of tryptophan residues. We report that clCRY4 is broadly and stably expressed within the retina but enriched at synapses in the outer plexiform layer in a repetitive manner. A proteomic survey for retinal-specific clCRY4 interactors identified molecules that are involved in receptor signaling, including glutamate receptor-interacting protein 2, which colocalizes with clCRY4. Our data support a model whereby clCRY4 acts as an ultraviolet-blue photoreceptor and/or a light-dependent magnetosensor by modulating glutamatergic synapses between horizontal cells and cones.

A current synthesis on the effects of electric and magnetic fields emitted by submarine power cables on invertebrates

The goal of clean renewable energy production has promoted the large-scale deployment of marine renewable energy devices, and their associated submarine cable network. Power cables produce both electric and magnetic fields that raise environmental concerns as many marine organisms have magneto and electroreception abilities used for vital purposes. Magnetic and electric fields' intensities decrease with distance away from the cable. Accordingly, the benthic and the sedimentary compartments are exposed to the highest field values. Although marine invertebrate species are the major fauna of these potentially exposed areas, they have so far received little attention. We provide extensive background knowledge on natural and anthropogenic marine sources of magnetic and electric fields. We then compile evidence for magneto- and electro-sensitivity in marine invertebrates and further highlight what is currently known about their interactions with artificial sources of magnetic and electric fields. Finally we discuss the main gaps and future challenges that require further investigation.

Magnetoreception: A Dynamo in the Inner Ear of Pigeons

A phylogenetically diverse set of animals are able to orient by the Earth's magnetic field, but how they do so is an open problem. A new study identifies ion channels in the avian inner ear that could detect magnetic fields via induced electric fields.

On a Search for a Neurogenomics of Cognitive Processes Supporting Avian Migration and Navigation

The migratory behavioral profile of birds is characterized by considerable variation in migratory phenotype, and a number of distinct orientation and navigational mechanisms supports avian migration and homing. As such, bird navigation potentially offers a unique opportunity to investigate the neurogenomics of an often spectacular, naturally occurring spatial cognition. However, a number of factors may impede realization of this potential. First, aspects of the migratory behavior displayed by birds, including some navigational-support mechanisms, are under innate/genetic influence as, for example, young birds on their first migration display appropriate migratory orientation and timing without any prior experience and even when held in captivity from the time of birth. Second, many of the genes with an allelic variation that co-varies with migratory phenotype are genes that regulate processes unrelated to cognition. Where cognition and navigation clearly converge is in the familiar landmark/landscape navigation best studied in homing pigeons and known to be dependent on the hippocampus. Encouraging here are differences in the hippocampal organization among different breeds of domestic pigeons and a different allelic profile in the LRP8 gene of homing pigeons. A focus on the hippocampus also suggests that differences in developmentally active genes that promote hippocampal differentiation might also be genes where allelic or epigenetic variation could explain the control of or comparison-group differences in a cognition of navigation. Sobering, however, is just how little has been learned about the neurogenomics of cognition (“intelligence”) in humans despite the vast resources and research activity invested; resources that would be unimaginable for any avian study investigating bird navigation.

Magnetoreception in fishes: the effect of magnetic pulses on orientation of juvenile Pacific salmon

A variety of animals sense Earth's magnetic field and use it to guide movements over a wide range of spatial scales. Little is known, however, about the mechanisms that underlie magnetic field detection. Among teleost fish, growing evidence suggests that crystals of the mineral magnetite provide the physical basis of the magnetic sense. In this study, juvenile Chinook salmon (Oncorhynchus tshawytscha) were exposed to a brief but strong magnetic pulse capable of altering the magnetic dipole moment of biogenic magnetite. Orientation behaviour of pulsed fish and untreated control fish was then compared in a magnetic coil system under two conditions: (1) the local magnetic field and (2) a magnetic field that exists near the southern boundary of the natural oceanic range of Chinook salmon. In the local field, no significant difference existed between the orientation of the control and pulsed groups. By contrast, orientation of the two groups was significantly different in the magnetic field from the distant site. These results demonstrate that a magnetic pulse can alter the magnetic orientation behaviour of a fish and are consistent with the hypothesis that salmon have magnetite-based magnetoreception.

Infrasonic hearing in birds: a review of audiometry and hypothesized structure-function relationships

The perception of airborne infrasound (sounds below 20 Hz, inaudible to humans except at very high levels) has been documented in a handful of mammals and birds. While animals that produce vocalizations with infrasonic components (e.g. elephants) present conspicuous examples of potential use of infrasound in the context of communication, the extent to which airborne infrasound perception exists among terrestrial animals is unclear. Given that most infrasound in the environment arises from geophysical sources, many of which could be ecologically relevant, communication might not be the only use of infrasound by animals. Therefore, infrasound perception could be more common than currently realized. At least three bird species, each of which do not communicate using infrasound, are capable of detecting infrasound, but the associated auditory mechanisms are not well understood. Here we combine an evaluation of hearing measurements with anatomical observations to propose and evaluate hypotheses supporting avian infrasound detection. Environmental infrasound is mixed with non-acoustic pressure fluctuations that also occur at infrasonic frequencies. The ear can detect such non-acoustic pressure perturbations and therefore, distinguishing responses to infrasound from responses to non-acoustic perturbations presents a great challenge. Our review shows that infrasound could stimulate the ear through the middle ear (tympanic) route and by extratympanic routes bypassing the middle ear. While vibration velocities of the middle ear decline towards infrasonic frequencies, whole-body vibrations - which are normally much lower amplitude than that those of the middle ear in the 'audible' range (i.e. >20 Hz) - do not exhibit a similar decline and therefore may reach vibration magnitudes comparable to the middle ear at infrasonic frequencies. Low stiffness in the middle and inner ear is expected to aid infrasound transmission. In the middle ear, this could be achieved by large air cavities in the skull connected to the middle ear and low stiffness of middle ear structures; in the inner ear, the stiffness of round windows and cochlear partitions are key factors. Within the inner ear, the sizes of the helicotrema and cochlear aqueduct are expected to play important roles in shunting low-frequency vibrations away from low-frequency hair-cell sensors in the cochlea. The basilar papilla, the auditory organ in birds, responds to infrasound in some species, and in pigeons, infrasonic-sensitive neurons were traced back to the apical, abneural end of the basilar papilla. Vestibular organs and the paratympanic organ, a hair cell organ outside of the inner ear, are additional untested candidates for infrasound detection in birds. In summary, this review brings together evidence to create a hypothetical framework for infrasonic hearing mechanisms in birds and other animals.