Animal navigation: the evolution of magnetic orientation

Animals have several types of magnetic organ, often separately specialized for determining direction versus location. Recent results offer hints about how these once-unimaginable detectors may have evolved.     

Altered histology of the thymus and spleen in contaminant-exposed juvenile American alligators

Morphological differences in spleen and thymus are closely related to functional immune differences. Hormonal regulation of the immune system has been demonstrated in reptilian splenic and thymic tissue. Spleens and thymus were obtained from juvenile alligators at two reference sites in Florida, USA: Orange Lake and a National Wildlife Refuge, Lake Woodruff, as well as from a contaminated lake, Lake Apopka. Lake Apopka has been extensively polluted with agricultural pesticides. Tissues were prepared for histological analysis to determine if previously detected endocrine abnormalities associated with contaminant exposure might also be reflected in morphological differences in splenic and thymic structures important for immunological response. Similar tissues were taken from captive-raised juvenile female alligators (3 years old) that were hatched from eggs collected on Lake Woodruff and Lake Apopka. Differences in thymic ratios (medulla/cortex) were found among alligators collected from the two lakes (P = 0.0051). Alligators from Lake Apopka had smaller thymic ratios than animals from either reference lake. Males from Lake Woodruff had significantly smaller lymphocyte sheaths in the spleen than females (P = 0.0009), indicative of a normal sexual dimorphism. Lymphocyte sheath width differed among females obtained from the three lakes, with females from Lake Apopka having the smallest sheath width and those from Orange Lake having the largest. Malpighian body area was largest in alligators from Orange Lake, intermediate in Lake Woodruff, and smallest in Lake Apopka. In contrast to that observed for wild-caught animals, no difference was found in the thymic medulla/cortex ratio of captive-raised female alligators (P = 0.378). Captive-raised female alligators from Lake Apopka and Lake Woodruff displayed lake-associated differences in lymphocyte sheath width as observed in wild animals; Lake Apopka alligators had smaller lymphocyte sheath width compared to Woodruff alligators (P = 0.0396). In contrast to wild-caught animals, area of the Malpighian bodies did not differ by lake in the captive-raised female alligators (P = 0.066). The enlarged thymic cortex suggests a change in T-lymphocyte maturation within the thymus of alligators from a contaminated environment, Lake Apopka. The results point to alterations in the histology of the thymus and spleen. Further studies are required to examine the functional significance of these observations.     

Field evidence of compass orientation in migrating juvenile grunts (Haemulidae)

Juvenile French and white grunts, Haemulon flavolineatum (Desmarest) and H. plumieri (Lacé-pède), were captured during their daily migrations between diurnal resting sites on coral patch reefs and nocturnal feeding grounds in seagrass beds. Grunts captured during morning and evening migrations were released on the route and after displacement up to 100 m or 5 km away. Grunts generally moved in the direction which would have taken them back towards their home reef or to their accustomed feeding sites, indicating that familiar landmarks are not essential for orientation. The spatial precision of migration may serve to partition the feeding area most efficiently. The timing of migrations is also very precise, and appears to be adapted to reduce the vulnerability of grunts to predation near their home reef.     

Plasma catecholamines and plasma corticosterone following restraint stress in juvenile alligators

Ten juvenile alligators, mean body mass 793 g, hatched from artificially incubated eggs and raised under controlled conditions, were held out of water with their jaws held closed for 48 hr. An initial blood sample was taken and further samples collected at 1, 2, 4, 8, 24, and 48 hr. Epinephrine, norepinephrine, and dopamine were measured in plasma aliquots of 1.5 ml using high pressure liquid chromatography with electrochemical detection. Corticosterone was measured by radioimmunoassay. Plasma glucose was measured using the Trinder method and plasma calcium, cholesterol, and triglycerides were measured in an autoanalyzer. Epinephrine was about 4 ng/ml at the initial bleed, but declined steadily to < 0.4 ng/ml by 24 hr. Norepinephrine was also about 4 ng/ml at the initial bleed, but rose to over 8 ng/ml at 1 hr, and then declined to < 0.2 ng/ml at 24 hr. A second, but smaller increase in plasma norepinephrine was seen at 48 hr. Plasma dopamine was low at the initial bleed (< 0.7 ng/ml), rose to over 8 ng/ml at 1 hr, then declined to < 0.2 ng/ml. Plasma corticosterone rose progressively for the first 4 hr, declined at 8 hr and 24 hr, then rose again at 48 hr. Plasma glucose rose significantly by 24 hr and remained elevated for 48 hr. Plasma calcium increased at 1, 2, and 4 hr then returned to levels not significantly different from the initial sample at 24 and 48 hr. The white blood cells showed changes indicating immune system suppression. By the end of the treatment the hetorophil/lymphocyte ratio increased to 4.7. These results suggest that handling alligators, taking multiple blood samples, and keeping them restrained for more than 8 hr is a severe stress to the animals.     

Experimental evidence for geomagnetic orientation in juvenile salmon, Oncorhynchus tschawytscha Walbaum

Juvenile chinook salmon, Oncorhynchus tschawytscha , kept under artificial light in a rectangular holding tank aligned east/west for 18 months, showed a preferred temporal and directional orientation of 270° with respect to water flow and the source of food.
Individual fish transferred from the holding/training tank to an unfamiliar circular test arena in another room devoid of local directional cues showed a mean of means preferred unimodal orientation of 264°.
Controlled re-introduction of individual stimuli revealed a hierarchy of orientation cues; one of these was a response to magnetism. A 90° clockwise shift in the horizontal component of the earth's magnetic field was followed by a significant change in the mean of means axial orientation, for the fish under test, from 258°/78° to 354°/174°. After restoration of the normal magnetic field the mean of means axial orientation reverted to 274°/94°.     

Keynote papers on sandhopper orientation and navigation

This article analyses the relevant studies that have made sandhoppers a model subject for the study of orientation, and traces the development of the paradigm through innovative hypotheses and empirical evidence. Sandhoppers are able to maintain their direction without sensorial contact with the goal, which is their burrowing zone extended along the beach, but very narrow across it. They actively determine the direction of their movements, according to their internal state and the environmental features encountered. Each population shows an 'innate directional tendency' adapted to the shoreline of origin, and the inexpert laboratory-born young behave in a similar way to the adults. Genetic differences have been demonstrated between, as well as within natural populations. The question of the calibration of the sun compass to orientation on a particular shoreline implies a redundancy of mechanisms of orientation. Orientation mechanisms may involve environmental cues perceived through diverse sensory modalities, and range from simple orientation reflexes to sun compass navigational systems. These include scototaxis and geotaxis, and the response to the silhouette of the dune, in addition to sun and moon orientation, which is dependent on the time of the day and orientates daily migrations on the beach. Different modalities of orientation may operate singly, or in conjunction with each other, and their ecological significance may vary according to the habitat and lifestyle of the animals. Taken collectively, the orientation behaviour of the group appears to be a most accommodating phenotype, with considerable adaptive potential. The evidence from comparative studies of different populations promotes consideration of behavioural plasticity as an adaptation to changing coastlines.     

Advances in the neural bases of orientation and navigation

The ability to locomote in one direction (oriented movement),and the ability to navigate toward a distant goal are relatedbehaviors that are phylogenetically widespread. Orientationbehaviors include finding the source of an odor or acousticsignal, using a sun-compass for guidance, and moving relativeto fluid-dynamic cues. Such abilities might require little morethan directionally selective sensors coupled to a turning mechanism.This type of behavior, therefore, can be implemented by relativelysimple circuits. In contrast, navigation involves both the abilityto detect direction, as well as a map-sense that provides position.Navigation is less common and arguably requires greater braincomputation than does simple orientation, but may be presentin arthropods as well as in vertebrates. Great progress hasbeen made in exploring the biophysical and sensory bases forthese behaviors, and in recent years the locations and the identityof the cellular transducers of the sensory stimuli (for example,geomagnetic fields) have been narrowed in some taxa. Similarly,neurons within the central nervous that most likely functionin higher order computational processes have been identified.For example, direction-selective and position-responsive braincells have been located in the brains of mammals and birds,and these cells might contribute to a cognitive map that canenable navigation. One model organism in which orientation andnavigation has been extensively studied is the sea slug Tritoniadiomedea. This animal orients its crawling to chemical, hydrodynamic,and geomagnetic cues. The brain of Tritonia has 7000 relativelylarge neurons that facilitate circuit analysis. Recent workhas characterized both peripheral and central neural correlatesof orientation signals, as well as the control of thrust andturning, and studies of their field behavior have suggestedhow these disparate orientation systems may be integrated. Thesefindings provide the basis for future studies to determine howthe nervous system combines multiple sensory cues into a complexhierarchy of signals that can direct motor output and thereforeguide navigational tasks.     

Plasma steroid concentrations in relation to size and age in juvenile alligators from two Florida lakes

Previous studies have reported a number of physiological differences among juvenile alligators from two well-studied populations (Lake Apopka and Lake Woodruff) in north central Florida. These studies obtained alligators of similar size from each lake under the assumption that the animals were of similar age. Lake Apopka is a hypertrophic lake with a 50-year history of contamination from agricultural and municipal operations, whereas Lake Woodruff is a eutrophic lake and part of a National Wildlife Refuge that receives little point source pollution. If growth rates differ among these areas, it could be argued that differences in endocrine parameters reported previously (e.g. steroid or thyroid hormone concentrations) could be the result of differences in the animals' ages. Using growth annuli in cross-sections of femurs, we estimated the ages of juvenile alligators and compared the relationship of estradiol-17beta (E(2)) and testosterone (T) to size and age within each lake and sex. No differences were detected in the relationship between size and age between the two areas indicating similar growth rates between lakes. Plasma E(2) was positively related to size in females from Lake Apopka, and age in Woodruff females. Males from Lake Apopka had elevated plasma E(2) compared with Lake Woodruff males and did not differ from Woodruff females. No significant relationships were detected for T from either lake, and no differences in plasma T were detected among lakes or sexes. Our data indicate that both size and age can have a significant relationship with steroid concentrations. However, the relationship between steroid concentrations and size or age differed between lakes. We suggest both factors should be considered when conducting physiological studies where there is evidence to suggest growth rates may differ among populations.     

Remote magnetic navigation vs. manual navigation for ablation of ventricular tachycardia: a meta-analysis

Background

The purpose of this study was to prospectively evaluate the efficacy and safety of remote magnetic navigation (RMN) in comparison with manual catheter navigation (MCN) in performing ventricular tachycardia ablation.

Methods

An electronic search was performed using PubMed (1948–2013) and EMBASE (1974–2013) studies comparing RMN with MCN which were published prior to 31 December 2013. Outcomes of interest were as follows: acute success, recurrence rate, complications, total procedure and fluoroscopic times. Standard mean difference (SMD) and its 95 % confidence interval (CI) were used for continuous outcomes; odds ratios (OR) were reported for dichotomous variables.

Results

Four non-randomised studies, including a total of 328 patients, were identified. RMN was deployed in 191 patients. Acute success and long-term freedom from arrhythmias were not significantly different between the RMN and control groups (OR 1.845, 95 % CI 0.731–4.659, p = 0.195 and OR 0.676, 95 % CI 0.383–1.194, p = 0.177, respectively). RMN was associated with less peri-procedural complications (OR 0.279, 95 % CI 0.092–0.843, p = 0.024). Shorter procedural and fluoroscopy times were achieved (95 % CI -0.487 to -0.035, p = 0.024 and 95 % CI -1.467 to -0.984, p<0.001, respectively).

Conclusion

The acute and long-term success rates for VT ablation are equal between RMN and MCN, whereas the RMN-guided procedure can be performed with a lower complication rate and less procedural and fluoroscopic times. More prospective randomised trials will be needed to better evaluate the superior role of RMN for catheter ablation of ventricular tachycardia.     

The magnetic sense and its use in long-distance navigation by animals

True navigation by animals is likely to depend on events occurring in the individual cells that detect magnetic fields. Minimum thresholds of detection, perception and 'interpretation' of magnetic field stimuli must be met if animals are to use a magnetic sense to navigate. Recent technological advances in animal tracking devices now make it possible to test predictions from models of navigation based on the use of variations in magnetic intensity.     

The radial bias: a different slant on visual orientation sensitivity in human and nonhuman primates

It is generally assumed that sensitivity to different stimulus orientations is mapped in a globally equivalent fashion across primate visual cortex, at a spatial scale larger than that of orientation columns. However, some evidence predicts instead that radial orientations should produce higher activity than other orientations, throughout visual cortex. Here, this radial orientation bias was robustly confirmed using (1) human psychophysics, plus fMRI in (2) humans and (3) behaving monkeys. In visual cortex, fMRI activity was at least 20% higher in the retinotopic representations of polar angle which corresponded to the radial stimulus orientations (relative to tangential). In a global demonstration of this, we activated complementary retinotopic quadrants of visual cortex by simply changing stimulus orientation, without changing stimulus location in the visual field. This evidence reveals a neural link between orientation sensitivity and the cortical retinotopy, which have previously been considered independent.     

Mechanisms of magnetic orientation in birds

Behavior and electrophysiological studies have demonstrateda sensitivity to characteristics of the Geomagnetic field thatcan be used for navigation, both for direction finding (compass)and position finding (map). The avian magnetic compass receptorappears to be a light-dependent, wavelength-sensitive systemthat functions as a polarity compass (i.e., it distinguishespoleward from equatorward rather than north from south) andis relatively insensitive to changes in magnetic field intensity.The receptor is within the retina and is based on one or morephotopigments, perhaps cryptochromes. A second receptor systemappears to be based on magnetite and might serve to transducelocation information independent of the compass system. Thisreceptor is associated with the ophthalmic branch of the trigeminalnerve and is sensitive to very small (<50 nanotesla) changesin the intensity of the magnetic field. In neither case hasa neuron that responded to changes in the magnetic field beentraced to a structure that can be identified to be a receptor.Almost nothing is known about how magnetic information is processedwithin the brain or how it is combined with other sensory informationand used for navigation. These remain areas of future research.     

Magnetic orientation and navigation in marine turtles, lobsters, and molluscs: concepts and conundrums

The Earth's magnetic field provides a pervasive source of directionalinformation used by phylogenetically diverse marine animals.Behavioral experiments with sea turtles, spiny lobsters, andsea slugs have revealed that all have a magnetic compass sense,despite vast differences in the environment each inhabits andthe spatial scale over which each moves. For two of these animals,the Earth's field also serves as a source of positional information.Hatchling loggerhead sea turtles from Florida responded to themagnetic fields found in three widely separated regions of theAtlantic Ocean by swimming in directions that would, in eachcase, facilitate movement along the migratory route. Thus, foryoung loggerheads, regional magnetic fields function as navigationalmarkers and elicit changes in swimming direction at crucialgeographic boundaries. Older turtles, as well as spiny lobsters,apparently acquire a "magnetic map" that enables them to usemagnetic topography to determine their position relative tospecific goals. Relatively little is known about the neuralmechanisms that underlie magnetic orientation and navigation.A promising model system is the marine mollusc Tritonia diomedea,which possesses both a magnetic compass and a relatively simplenervous system. Six neurons in the brain of T. diomedea havebeen identified that respond to changes in magnetic fields.At least some of these appear to be ciliary motor neurons thatgenerate or modulate the final behavioral output of the orientationcircuitry. These findings represent an encouraging step towarda holistic understanding of the cells and circuitry that underliemagnetic orientation behavior in one model organism.     

视觉皮层方位敏感性的皮层下起源

自Hubel和Wiesel关于视皮层研究的开拓性工作以来,视觉方位敏感性一直被认为是视皮层细胞独有的功能。本文综述了80年代以来的最新研究成果,表明视觉方位敏感性起源于视网膜、丘脑外膝体等皮层下结构,而在视皮层方位功能柱形成之前,丘脑外膝体已对具有相似最优方位的外膝体神经元作了初步的编组或预安排。     

The visual pigment sensitivity hypothesis: further evidence from fishes of varying habitats

Summary Visual pigments were extracted from the retinas of 8 species of marine teleosts and 4 species of elasmobranchs and a comparison was made of the pigment properties from these fishes, some inhabiting surface waters, others from the mesopelagic zone, and a few migrating vertically between these two environments. An association was found between the spectral position of the absorbance curve and the habitat depth or habitat behavior, with the blue-shifted chrysopsins being the pigments of the twilight zone fishes and the rhodopsins with fishes living near the surface. The retina of the swell shark (Cephaloscyllium ventriosum) yielded extracts with two photopigments; one, a rhodopsin at 498 nm; the second, a chrysopsin at 478 nm. This fish has been reported to practice seasonal vertical migrations between the surface and the mesopelagic waters. In addition to the spectral absorbance, several properties of these visual pigments were examined, including the meta-III product of photic bleaching, regeneration with added 11-cis and 9-cis retinals, and the chromophoric photosensitivity. The chrysopsin properties were found to be fundamentally similar to those of typical vertebrate rhodopsins. Correlating the spectral data with the habitat and habitat behavior of our fishes gives us confidence in the idea that the scotopic pigments have evolved as adaptations to those aspects of their color environment that are critical to the survival of the species.     

The sensitivity of the heart to static magnetic fields

Static magnetic fields induce flow potentials in arterial flows in and around the heart, that have been detected as distortions in the ECG. The resultant currents flowing through the myocardium could alter the rate or rhythm of the heart. No such changes have been seen in animal experiments, or with humans, in static fields up to 8 T. The possible effects of such currents induced by fields larger than 8 T on cardiac pacemaker rate, and arrhythmogenesis are reviewed, using virtual cardiac tissues—computational models of cardiac electrophysiology. Arrhythmogenesis can be by the initiation of ectopic beats, or by re-entry, whose probability of occurrence is increased by any increase in the electrical heterogeneity, in particular, the action potential duration heterogeneity of the ventricle. Focal ectopic activity would be readily detectable, but since re-entrant arrhythmias are very rare events, even a large increase in their probability of occurrence still leaves them unlikely to be observed. Both of these two arrhythmogenic mechanisms would show a steep sigmoidal, or threshold dependence on induced current intensity, with the threshold for increasing the vulnerability to re-entry less than the threshold for initiating activity. Failure to observe them at fields less than 8 T provides only a lower bound for any threshold for arrhythmogenesis.     

Dissociation of exteroceptive and idiothetic orientation cues: effect on hippocampal place cells and place navigation.

Navigation by means of cognitive maps appears to require the hippocampus; hippocampal place cells (PCs) appear to store spatial memories because their discharge is confined to cell-specific places called firing fields (FFs). Experiments with rats manipulated idiothetic and landmark-related information to understand the relationship between PC activity and spatial cognition. Rotating a circular arena in the light caused a discrepancy between these cues. This discrepancy caused most FFs to disappear in both the arena and room reference frames. However, FFs persisted in the rotating arena frame when the discrepancy was reduced by darkness or by a card in the arena. The discrepancy was increased by ''field clamping'' the rat in a room-defined FF location by rotations that countered its locomotion. Most FFs dissipated and reappeared an hour or more after the clamp. Place-avoidance experiments showed that navigation uses independent idiothetic and exteroceptive memories. Rats learned to avoid the unmarked footshock region within a circular arena. When acquired on the stable arena in the light, the location of the punishment was learned by using both room and idiothetic cues; extinction in the dark transferred to the following session in the light. If, however, extinction occurred during rotation, only the arena-frame avoidance was extinguished in darkness; the room-defined location was avoided when the lights were turned back on. Idiothetic memory of room-defined avoidance was not formed during rotation in light; regardless of rotation, there was no avoidance when the lights were turned off, but room-frame avoidance reappeared when the lights were turned back on. The place-preference task rewarded visits to an allocentric target location with a randomly dispersed pellet. The resulting behaviour alternated between random pellet searching and target-directed navigation, making it possible to examine PC correlates of these two classes of spatial behaviour. The independence of idiothetic and exteroceptive spatial memories and the disruption of PC firing during rotation suggest that PCs may not be necessary for spatial cognition; this idea can be tested by recordings during the place-avoidance and preference tasks.