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ScienceWeek
ANIMAL COGNITION: PIGEON HOMING AND HIGHWAYS
The following points are made by H-P. Lipp et al (Current Biology 2004 14:1239):
1) The most widely accepted explanation for pigeon homing over distances of 20 km and more is that they rely on a "map-and-compass" strategy. It has remained undisputed that pigeons have an internal clock and an internal sense of compass direction home and that this latter sense depends on the position of the sun, if visible. Yet directional knowledge alone is not sufficient for successful homing, and so pigeons must also have a large-scale mental map containing information about their current position with regard to their loft [1].
2) Mechanisms of position determination and the nature of the mental map used by homing pigeons have remained controversial for decades. Supporters of the magnetic theory of pigeon homing claim a predominant role of the earth's magnetic field for both compass and map mechanisms [2]. Others propose a major role of the olfactory system and atmospheric gradients [3,4]. Although vision is helpful yet not mandatory for successful long-distance homing [5], there is general agreement that pigeons rely at least partially on visual cues for flights within their familiar home range, 2-4 km around the loft. Whether the local visual information is used by the birds for homing from distant release sites -- a strategy coined "pilotage" -- has been equally controversial [2].
3) Likewise, the nature of the objects used by pigeons for pilotage has been debated. Breeders of racing pigeons have often observed that large flocks of homing pigeons fly along major highways, and it is a familiar observation for most pigeon breeders that the birds often do not approach the home loft according to a straight compass direction from the release site. Early attempts to identify topographic guide-rails used by homing pigeons (e.g., roads, railways, powerlines) by means of airplane tracking have yielded equivocal results. Some studies reported positive evidence; helicopter tracking studies even found that pigeons were circling over road crossings. However, even in these positive cases, observations were rare and anecdotal.
4) The authors present an analysis of 216 GPS-recorded pigeon tracks over distances up to 50 km. Experienced pigeons released from familiar sites during 3 years around Rome, Italy, were significantly attracted to highways and a railway track running toward home, in many cases without anything forcing them to follow such guide-rails. Birds often broke off from the highways when these veered away from home, but many continued their flight along the highway until a major junction, even when the detour added substantially to their journey. The degree of road following increased with repeated releases but not flight length. Significant road following (in 40%-50% of the tracks) was mainly observed from release sites along northwest-southeast axis.
5) The authors suggest their data demonstrate the existence of a learned road-following homing strategy of pigeons and the use of particular topographical points for final navigation to the loft. Apparently, the better-directed early stages of the flight compensated the added final detour. During early and middle stages of the flight, following large and distinct roads is likely to reflect stabilization of a compass course rather than the presence of a mental roadmap. A cognitive (roadmap) component manifested by repeated crossing of preferred topographical points, including highway exits, is more likely when pigeons approach the loft area. However, it might only be expected in pigeons raised in an area characterized by navigationally relevant highway systems.
References (abridged):
1. Gould, J.L. (2004). Animal navigation. Curr. Biol. 14, R221-R224
2. Wiltschko, R. and Wiltschko, W. (2003). Avian navigation: from historical to modern concepts. Anim. Behav. 65, 257-272
3. Papi, F. (1990). Olfactory navigation in birds. Experientia 46, 352-363
4. Wallraff, H.G. (2004). Avian olfactory navigation: its empirical foundation and conceptual state. Anim. Behav. 67, 189-204
5. Schmidt-Koenig, K. and Schlichte, H.-J. (1972). Homing in pigeon with impaired vision. Proc. Natl. Acad. Sci. USA 69, 2446-2447
Current Biology http://www.current-biology.com
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Related Material:
ANIMAL BEHAVIOR: ON MAGNETORECEPTION IN THE HOMING PIGEON
The following points are made by C.V. Mora et al (Nature 2004 432:508):
1) Two conflicting hypotheses compete to explain how a homing pigeon can return to its loft over great distances. One proposes the use of atmospheric odors[1] and the other the Earth's magnetic field[2-4] in the "map" step of the "map and compass" hypothesis of pigeon homing[5]. Although magnetic effects on pigeon orientation provide indirect evidence for a magnetic "map", numerous conditioning experiments have failed to demonstrate reproducible responses to magnetic fields by pigeons. This has led to suggestions that homing pigeons and other birds have no useful sensitivity to the Earth's magnetic field.
2) The authors made a series of modifications to an existing operant conditioning procedure to fulfill two conditions that seem to be vital for magnetic discrimination learning in non-avian species. These are that (1) the magnetic stimulus discriminated is a localized, non-uniform magnetic anomaly superimposed on the uniform background field of the Earth, and (2) movement by the experimental subjects is necessary to produce the behavioral response measured in the experiments. Although this combination of experimental parameters mitigates against rapid achievement of powerful discrimination by separating the stimulus, response and reinforcement in both space and time --compared with standard key-pecking experiments -- failure to fulfill either or both of the above conditions has characterized all the unsuccessful or irreproducible attempts to condition pigeons and many other species to magnetic fields.
3) Using a Yes-No signal-detection procedure, four individually trained pigeons were required to discriminate between the presence and absence of an induced magnetic field anomaly while freely walking in a wooden tunnel. The intensity profile of the anomaly was "wave-shaped" and peaked in the center of the tunnel at 189 micro tesla (microT) (background level of 44 microT) with an inclination of -80 deg (background level of -64 deg). The birds were conditioned to jump onto a platform at one end of the tunnel when the anomaly was present and onto an identical platform at the other end of the tunnel when the anomaly was absent. Choice of the correct platform was rewarded with food whereas incorrect choices were punished with a time penalty.
4) In summary: The authors demonstrate that homing pigeons (Columba livia) can discriminate between the presence and absence of a magnetic anomaly in a conditioned choice experiment. This discrimination is impaired by attachment of a magnet to the cere (part of the beak), local anaesthesia of the upper beak area, and bilateral section of the ophthalmic branch of the trigeminal nerve, but not of the olfactory nerve. These results suggest that magnetoreception (probably magnetite-based) occurs in the upper beak area of the pigeon. Traditional methods of rendering pigeons anosmic might therefore cause simultaneous impairment of magnetoreception so that future orientation experiments will require independent evaluation of the pigeon's magnetic and olfactory systems.
References (abridged):
1. Papi, F., Fiore, L., Fiaschi, V. & Benvenuti, S. Olfaction and homing in pigeons. Monit. Zool. Ital. (N.S.) 6, 85-95 (1972)
2. Gould, J. L. The case for magnetic sensitivity in birds and bees (such as it is). Am. Sci. 68, 256-267 (1980)
3. Moore, B. R. Is the homing pigeon's map geomagnetic? Nature 285, 69-70 (1980)
4. Walcott, C. Magnetic orientation in homing pigeons. IEEE Trans. Magnet. 16, 1008-1013 (1980)
5. Kramer, G. Wird die Sonnenhöhe bei der Heimfindeorientierung verwertet? J. Ornithol. 94, 201-219 (1953)
Nature http://www.nature.com/nature
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Related Material:
MAGNETITE IN A VERTEBRATE MAGNETORECEPTOR
Notes by ScienceWeek:
An enormous variety of what are essentially experiments in viability has occurred during the more than 3.5 billion years of biological evolution on Earth, and among these experiments is a striking diversity of biological devices that function to sense changes in the environment of the organism. Consider, for example, magnetic field detection:
The following points are made by C.E. Diebel et al (Nature 2000 406:299):
1) The key behavioral, physiological, and anatomical components of a magnetite-based magnetic sense have been previously demonstrated in rainbow trout (Oncorynchus mykiss), with candidate receptor cells located within a discrete sub-layer of the olfactory tissues (olfactory lamellae) in the nose of the trout. These receptor cells were shown to contain iron-rich crystals similar in size and shape to magnetite crystals extracted from salmon.
2) The authors now demonstrate that these crystals, mapped to individual receptors by *confocal and atomic force microscopy, are magnetic: the crystals are uniquely associated with dipoles detected by *magnetic force microscopy. Analysis of their magnetic properties identifies the crystals as *single-domain magnetite particles. In addition, 3-dimensional reconstruction of the candidate receptors using confocal and atomic microscopy imaging confirm that several magnetite crystals are arranged in a chain of approximately 1 micron length within the receptor, and that the receptor is a multi-lobed single cell.
3) The authors suggest these results are consistent with a magnetite-based detection mechanism, since 1-micron chains of single-domain magnetite crystals are highly suitable for the behavioral and physiological responses to magnetic intensity previously reported for the trout.
4) The authors conclude that "understanding should now be sought of how the chains of crystals could transduce a magnetic field into an electrical signal in the nervous system."
Nature http://www.nature.com/nature
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Notes by ScienceWeek:
confocal and atomic force microscopy: In general, a "confocal microscope" is a microscope in which an aperture in the illuminating system confines the illumination to a small spot on the specimen, and a corresponding aperture in the imaging system (which may be the same aperture in reflecting and fluorescence devices) allows only light transmitted, reflected, or emitted by the same spot to contribute to the image. By suitable mechanical or optical means, the spots are made to scan the specimen as in a television raster. Compared to conventional microscopy, confocal techniques offer improved resolution and improved rejection of out-of-focus noise. In atomic force microscopy, a tip is fixed to a cantilever whose position is monitored while the tip scans the surface. The force between the tip and the surface determines the position of the cantilever. When recorded in atomic resolution, the image represents a map of atomic forces at the surface. The advantage of atomic force microscopy is that the probed surface does not need to be electrically conducting.
magnetic force microscopy: This technique is capable of determining magnetic domain structure in a variety of magnetic materials, including small particles with a spatial resolution of less than 100 nanometers. Because it is sensitive to magnetic forces, the technique can also image magnetic structures that are covered by a layer of non-magnetic material.
single-domain magnetite particles: An oxide of iron, magnetite (magnetic iron ore) is attracted by a magnet but does not attract particles of iron to itself. In this context, the term "domain" refers to a region in which magnetic moments are uniformly arrayed.
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