The map sense of pigeons

Bicoordinate navigation based on non-orthogonal gradient fields

The mathematically exact solution for navigating with respect to two non-orthogonal gradient fields requires taking both fields into account conjointly. Such a complex solution may be beyond the brain capability of most animals. Animals may use an approximate, simpler solution by considering each field separately (i.e., the angle between the two gradient directions is ignored). This generates directional biases that may prove useful in determining the nature of the fields actually involved in bicoordinate navigation.

True navigation and magnetic maps in spiny lobsters

Animals are capable of true navigation if, after displacement to a location where they have never been, they can determine their position relative to a goal without relying on familiar surroundings, cues that emanate from the destination, or information collected during the outward journey. So far, only a few animals, all vertebrates, have been shown to possess true navigation. Those few invertebrates that have been carefully studied return to target areas using path integration, landmark recognition, compass orientation and other mechanisms that cannot compensate for displacements into unfamiliar territory. Here we report, however, that the spiny lobster Panulirus argus oriented reliably towards a capture site when displaced 12-37 km to unfamiliar locations, even when deprived of all known orientation cues en route. Little is known about how lobsters and other animals determine position during true navigation. To test the hypothesis that lobsters derive positional information from the Earth's magnetic field, lobsters were exposed to fields replicating those that exist at specific locations in their environment. Lobsters tested in a field north of the capture site oriented themselves southwards, whereas those tested in a field south of the capture site oriented themselves northwards. These results imply that true navigation in spiny lobsters, and perhaps in other animals, is based on a magnetic map sense.

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.

Experimental analysis of behavior of homing pigeons as a result of functional disorders of their lagena

Behavioral experiments concerning the homing abilities of pigeons were done by sectioning their lagenal nerves or interfering with the function of the lagena using a magnet. Twenty-one birds were treated in this way and 30 birds from the same loft of racing pigeons were used as controls. The results of homing tests clearly revealed a magnetic influence on the function of the lagena in terms of the navigation ability of pigeons: the treated birds were either lost or significantly delayed while the controls returned within 30 min of release. The lagena of birds is a unique organ and it is concluded that it is a key element in the magnetic sensory system of birds.

Regional magnetic fields as navigational markers for sea turtles

Young loggerhead sea turtles (Caretta caretta) from eastern Florida undertake a transoceanic migration in which they gradually circle the north Atlantic Ocean before returning to the North American coast. Here we report that hatchling loggerheads, when exposed to magnetic fields replicating those found in three widely separated oceanic regions, responded by swimming in directions that would, in each case, help keep turtles within the currents of the North Atlantic gyre and facilitate movement along the migratory pathway. These results imply that young loggerheads have a guidance system in which regional magnetic fields function as navigational markers and elicit changes in swimming direction at crucial geographic boundaries.

The neurobiology of magnetoreception in vertebrate animals

Diverse vertebrate animals can sense the earth's magnetic field, but little is known about the physiological mechanisms that underlie this sensory ability. Three major hypotheses of magnetic-field detection have been proposed. Electrosensitive marine fish might sense the geomagnetic field through electromagnetic induction, although definitive evidence that such fish actually do so has not yet been obtained. Studies with other vertebrates have provided evidence consistent with two different mechanisms: biogenic magnetite and chemical reactions that are modulated by magnetic fields. Despite recent progress, however, primary magnetoreceptors have not yet been identified unambiguously in any animal.

Infrasound and the avian navigational map

Birds can navigate accurately over hundreds to thousands of kilometres, and this ability of homing pigeons is the basis for a worldwide sport. Compass senses orient avian flight, but how birds determine their location in order to select the correct homeward bearing (map sense) remains a mystery. Also mysterious are rare disruptions of pigeon races in which most birds are substantially delayed and large numbers are lost. Here, it is shown that in four recent pigeon races in Europe and the northeastern USA the birds encountered infrasonic (low-frequency acoustic) shock waves from the Concorde supersonic transport. An acoustic avian map is proposed that consists of infrasonic cues radiated from steep-sided topographic features; the source of these signals is microseisms continuously generated by interfering oceanic waves. Atmospheric processes affecting these infrasonic map cues can explain perplexing experimental results from pigeon releases.

Sensory bases of navigation

Navigating animals need to know both the bearing of their goal (the 'map' step), and how to determine that direction (the 'compass' step). Compasses are typically arranged in hierarchies, with magnetic backup as a last resort when celestial information is unavailable. Magnetic information is often essential to calibrating celestial cues, though, and repeated recalibration between celestial and magnetic compasses is important in many species. Most magnetic compasses are based on magnetite crystals, but others make use of induction or paramagnetic interactions between short-wavelength light and visual pigments. Though odors may be used in some cases, most if not all long-range maps probably depend on magnetite. Magnetitebased map senses are used to measure only latitude in some species, but provide the distance and direction of the goal in others.

Magnetic orientation and the magnetic sense in arthropods

The physical properties of the earth's magnetic field are summarized with the aim of emphasizing their significance as cues that can be exploited in orientational tasks. Past work has revealed magnetic orientation in vertebrates as well as invertebrates, including arthropods. The key finding to date has been that, as opposed to many vertebrates, the magnetic compass of arthropods responds to the polarity, rather than to the inclination of the earth's magnetic field. As in the case of vertebrates, the debate over how arthropods detect magnetic fields has yet to be resolved. Currently, evidence has been reported in support of a detection system based on magnetite crystals together with a variety of detection systems based on events occurring at the molecular level. Interactions between the magnetic and other compasses in orientation experiments suggest the existence of an area in the brain where spatial orientation information from magnetic and other stimuli converges. The slow advance of our knowledge on magnetic orientation in arthropods, as opposed to the much better understanding of magnetic orientation in vertebrates, arises from difficulties in identifying the appropriate behavioural contexts in which arthropods respond to magnetic fields in both laboratory and field situations. Arthropods thus present challenges not only in demonstrating magnetic orientation, but also in elucidating the sensory mechanisms involved in the perception of magnetic fields.

Olfaction and the homing ability of pigeons in the southeastern United States

The importance of atmospheric odors for homing pigeon navigation was tested using birds from a loft located in Savannah, GA, in the southeastern United States. When released from a familiar training site, control pigeons and pigeons given intranasal injections of zinc sulfate to produce anosmia both displayed good homeward orientation and homed quickly. When released from three unfamiliar release sites, in contrast, control birds tended to orient southeast, while zinc sulfate-treated birds were more likely to fly northwest. More importantly, while the majority of control pigeons returned to the home loft, few of the zinc sulfate-treated birds returned. The good performance of both groups from the familiar site indicates that zinc sulfate treatment does not impair the general motor ability or motivation of homing pigeons. Therefore, the observed impairment in homing success of the zinc sulfate-treated pigeons from the unfamiliar locations presumably reflects an impaired ability to use atmospheric odors to navigate home. As such, the data support the hypothesis that successful homing pigeon navigation is based on the perception of atmospheric odors and that olfactory navigation is the primary mechanism used by pigeons over a broad range of geographic areas to approximate their relative position with respect to home from unfamiliar locations.

Decreased latencies for limbic seizures induced in rats by lithium-pilocarpine occur when daily average geomagnetic activity exceeds 20 nanoTesla

A decrease in the latency for the overt display of limbic seizures following the systemic injection of lithium and pilocarpine is weakly associated with enhanced global geomagnetic activity (in nanoTesla; nT). To determine the optimal threshold in global geomagnetic activity that is required for this effect, the seizure onset times for over 300 rats were dichotomized according to successive 5 nT increments. The results suggested that the seizure process occurred about 12% more quickly when the average daily global geomagnetic activity exceeded 20-25 nT and is commensurate with the observations by other researchers.

Biophysical estimation of the environmental importance of electromagnetic fields

The rapid development of science and technology exposes living organisms to a wide range of electromagnetic fields. Some life-style and occupational conditions are associated with a level of electromagnetic fields higher than average. A series of epidemiological studies has raised concern about possible cancer risk of electromagnetic fields generated by power lines and electrical appliances. In contrast, hundreds of thousands of patients world-wide have been cured by use of electromagnetic fields. Biological effects of low-level electromagnetic radiation have become the focus of a number of studies. However, there are not enough basic scientific data related to mechanisms of action of electromagnetic fields. This paper proposes a biophysical approach to the estimation of the environmental importance of electromagnetic fields. The methods of collecting data, dosimetry, possible mechanisms of action and open problems are discussed in this review.

Conceptual approaches to avian navigation systems

The general basis of migratory orientation in birds is most probably an endogenous time-and-direction programme. Directions are selected with respect to celestial as well as geomagnetic clues. These clues appear to be integrated within a system that profits from the special advantages of either kind of environmental signal, and thereby can cope with their limitations. Using these clues, and following a genetically determined intended direction (or sequence of directions) over a genetically determined period of time, a bird may reach a larger population-specific area. However, it will hardly be able to find a particular location, such as, for instance, its previous breeding site. Homing to a familiar site over several hundred kilometers of unfamiliar terrain is substantially based on the smelling of atmospheric trace compounds. At shorter distances from home, orientation by means of--presumably visual--familiar landmarks completes the repertoire of mechanisms guiding a bird back home. These mechanisms are considered to be based on different kinds of 'maps' and 'compasses'. Conceptual approaches to the properties of an 'olfactory map' have as yet only reached an early state of speculation.

Magnetic maps in pigeons

The homing of racing pigeons is often slowed down on days of high sun-spot activity or variability in the earth's magnetic field. The orientation of Ring-billed Gull chicks as well as the vanishing bearings of homing pigeons also seem to change in response to variability in the earth's magnetic field. Furthermore, homing pigeons released under sunny skies at locations where the earth's magnetic field is distributed are disoriented, yet pigeons equipped with devices that change the magnetic field around their head home normally. Weighing all this evidence, it is hard to believe that pigeons use a magnetic map.

Neuroethological aspects of avian orientation

Sensory information, which may be essential for the complex process of orientation of birds, is described in this article. The use of vibrational, visual, chemical, olfactory, magnetic cues and their receptive mechanisms, as far as they are known, are explained. Special reference is given to the behavioral and physiological aspects of magnetic sensitivity.

Electroreception and magnetoreception in simple and complex organisms

A considerable body of evidence now indicates that electromagnetic and geomagnetic detection systems exist in both simple, unicellular organisms and in more complex species such as avians, bees, and marine animals. A major challenge that faces researchers in this field is the identification of physiological mechanisms through which the detection of weak fields provides significant somatosensory cues for direction finding, foraging, and predation. Many of the anatomical, physiological, and biophysical approaches that are being taken in studies of this nature are described in the series of review articles that appear in this issue of Bioelectromagnetics.

[Navigation by means of an olfactory map and a sun compass: the homing ability of pigeons]

Pigeon homing, investigated as a paradigmatic example of bird navigation, appears to be based on two mechanisms of orientation whose functions correspond to those of map and compass. Tasks of the latter are usually accomplished by a sun compass, taking into account the sun's movement and time of day. Under overcast skies, the magnetic field of the earth may be used for compass orientation. The "map" part of the system, responsible for site localization, makes use of olfactory perception of atmospheric trace compounds, which must be concluded to contain positional information in unfamiliar areas up to several hundreds of kilometers from home.