Theoretical analysis of flux amplification by soft magnetic material in a putative biological magnetic-field receptor

Birds are endowed with a magnetic sense that allows them to detectEarth’s magnetic field and to use it for orientation. Physiological andbehavioral experiments have shown the upper beak to host amagnetoreceptor. Putative magnetoreceptive structures in the beak arenerve terminals that each contain a dozen or so of micrometer-sizedclusters of superparamagnetic nanocrystals made of magnetite/maghemiteand numerous electron-opaque platelets filled with a so farunidentified, amorphous ferric iron compound. The platelets typicallyform chainlike structures, which have been proposed to function asmagnetic flux focusers for detecting the intensity of the geomagneticfield. Here, we test that proposition from first principles and developan unconstrained model to determine the equilibrium distribution ofmagnetization along a linear chain of platelets which we assume tobehave magnetically soft and to have no magnetic remanence. Ouranalysis, which is valid for arbitrary values of the intrinsic magneticsusceptibility chi, shows that chi needs to be much greater than unityto amplify the external field by two orders of magnitude in a chain ofplatelets. However, the high amplification is confined to the centralregion of the chain and subsides quadratically toward the ends of thechain. For large values of chi, the possibility opens up of realizingmagnetoreceptor mechanisms on the basis of attraction forces betweenadjacent platelets in a linear chain. The force in the central region ofthe chain may amount to several pN, which would be sufficient to convertmagnetic input energy into mechanical output energy. The strikingfeature of an ensemble of platelets is its ability to organize intotightly spaced chains under the action of an external field of givenstrength. We discuss how this property can be exploited for amagnetoreception mechanism

Involvement of the avian dorsal thalamic nuclei in homing pigeon navigation

The navigational ability of birds has been a focus of popular and scientific interest for centuries, but relatively little is known about the neuronal networks that support avian navigation. In the brain, regions like the piriform cortex, olfactory bulbs, hippocampal formation, vestibular nuclei, and the wulst, are among the brain regions often discussed as involved in avian navigation. However, despite large literature showing a prominent role of some anterior and dorsal thalamic nuclei in mammalian spatial navigation, little is known about the role of the thalamus in avian navigation. Here, we analyzed a possible role of the dorsal anterior thalamic nuclei in avian navigation by combining olfactory manipulations during the transport of young homing pigeons to a release site and c-Fos immunohistochemistry for the mapping brain activity. The results reveal that odor modulated neurons in the avian dorsolateral lateral (DLL) subdivision of the anterior thalamic nuclei are actively involved in processing outward journey, navigational information. Outward journey information is used by pigeons to correctly determine the homeward direction. DLL participation in acquiring path-based information, and its modulation by olfactory exposure, broadens our understanding of the neural pathways underlying avian navigation.Fundação para a Ciência e a Tecnologia (FCT); National Science Foundationinfo:eu-repo/semantics/publishedVersio

Brain Geometry and its Relation to Migratory Behavior in Birds

A central concern in neuroscience can simply be brought down to the question of how a brains organization relates to its great diversity of functions. It is generally agreed that this relation must be based on multiscale organizational principles, ranging from the macroscopic level of the entire organ down to the cellular and molecular level. The functional correlates may also be seen as hierarchical constructs ranging from phylogenetic constraints and selectable life history traits down to perception, action and cognition. Here we focus on the relationship between macroscopic brain measures and a conspicuous life history variable in many animal species, migration. Migratory songbirds tend to have smaller brains than resident species. However, in the absence of data providing a detailed mapping of variation in brain subdivisions onto variation in migratory behaviour, offering a causal interpretation of the observed difference in brain size is difficult. Here we describe a set of large scale, geometric measures, which, despite different phylogenetic affiliations, discriminate migratory status across multiple avian lineages and eco-geographical regions. We build our investigation on complete, serial-section based, 3-D volumetric reconstructions of telencephalic subdivisions involving four song bird genera, which differ in their migratory status: long distance (more than 3000 km) and modest or no (0-3000 km) migratory behaviour. Our findings suggest that migratory behaviour as a population level trait can be discriminated at the level of geometrical forebrain measures. We finally discuss the results with respect to the developmental patterns that are largely responsible for the observed differences in brain geometries

Distortion of the Local Magnetic Field Appears to Neither Disrupt Nocturnal Navigation nor Cue Shelter Recognition in the Amblypygid \u3ci\u3eParaphrynus laevifrons\u3c/i\u3e

Many arthropods are known to be sensitive to the geomagnetic field and exploit the field to solve spatial problems. The polarity of the geomagnetic field is used, for instance, as an orientation cue by leafcutter ants as they travel on engineered trails in a rainforest and by Drosophila larvae as they move short distances in search of food. A ubiquitous orientation cue like the geomagnetic field may be especially useful in complex, cluttered environments like rainforests, where the reliability of celestial cues used to navigate in more open environments may be poor. The neotropical amblypygid Paraphrynus laevifrons is a nocturnal arachnid that wanders nightly in the vicinity of its shelter and occasionally travels 30 m or more in the rainforest understory before it returns to its shelter. Here, we conducted a field experiment to determine whether navigation by P. laevifrons is guided by the ambient magnetic field and a complementary laboratory experiment to assess whether a magnetic anomaly could be used to pinpoint the entrance of a shelter. In the field experiment, subjects were fitted with a radio transmitter and a small, powerful magnet or a similar-sized brass disk and displaced 10 m from their shelter. The return rate of magnet-fitted subjects was similar to that of brass-fitted subjects and to that of subjects in an earlier study fitted with only a radio transmitter. In the laboratory experiment, we trained P. laevifrons with a protocol under which the amblypygid Phrynus marginemaculatus rapidly learns—in 1–14 trials over two daily sessions—to associate an olfactory stimulus with access to a shelter. The conditioned stimulus here was a magnetic anomaly characterized by a high total field intensity and a 180° reversal of the polarity of the ambient magnetic field. The magnetic anomaly–shelter contingency was not learned in 50 trials conducted over 10 daily sessions. These results imply prima facie that P. laevifrons does not rely on a magnetic compass to locate or recognize a shelter and, perhaps, that the magnetic field cannot be detected, but alternative explanations are discussed

Plasticity in D1-Like Receptor Expression Is Associated with Different Components of Cognitive Processes

Dopamine D1-like receptors consist of D1 (D1A) and D5 (D1B) receptors and play a key role in working memory. However, their possibly differential contribution to working memory is unclear. We combined a working memory training protocol with a stepwise increase of cognitive subcomponents and real-time RT-PCR analysis of dopamine receptor expression in pigeons to identify molecular changes that accompany training of isolated cognitive subfunctions. In birds, the D1-like receptor family is extended and consists of the D1A, D1B, and D1D receptors. Our data show that D1B receptor plasticity follows a training that includes active mental maintenance of information, whereas D1A and D1D receptor plasticity in addition accompanies learning of stimulus-response associations. Plasticity of D1-like receptors plays no role for processes like response selection and stimulus discrimination. None of the tasks altered D2 receptor expression. Our study shows that different cognitive components of working memory training have distinguishable effects on D1-like receptor expression

Global navigation Mechanisms of animal global navigation: comparative perspectives and enduring challenges

Animals navigate over a range of distances, but it has been the global navigation of species migrating among spatially restricted, seasonal homes separated by thousands of kilometers that continues to defy a thorough mechanistic explanation. We survey the navigational behavior of migratory salmon, whales, sea turtles, and birds, as well as dispersing monarch butterflies, to promote the idea that an explicitly comparative approach to global navigation can provide insight into the evolution and properties of navigational mechanisms. The navigational abilities of migrant birds and sea turtles are used to illustrate the concepts of true navigation and vector navigation, leading us to consider the selective forces that might shape the evolution of navigational mechanisms. We propose that different navigational mechanisms, with different scales of accuracy, are likely employed during the course of migration. Furthermore, superficially similar global migratory behavior in different taxonomic groups is likely characterized by different sensory, representational and neural mechanisms reflective of groupspecific adaptation to the physical properties of a migratory environment. key words: grid-based navigation, map-based navigation, true navigation, vector navigation, migrant birds, migrant sea turtles