Experimental studies of the development of migratory orientation mechanisms

Summary Recent experimental studies (since ca 1985) on the ontogeny of orientation mechanisms in migratory birds are reviewed. The processes and interactions are synthesized into a framework that may help identify critical research questions. Birds that grow up in the earth's magnetic field develop the ability to perform appropriate migratory orientation even in the absence of any experience with relevant visual cues. In two species, large changes in direction during the course of migration seem to be controlled by an endogenous time program. In one of these, the pied flycatcher (Ficedula hypoleuca), the correct magnetic orientation seems to occur only when the magnetic fields appropriate to the latitudes encountered en route were experienced at the proper seasonal time. The magnetic compass may be modified by visual experience with either the daytime or night sky. Celestial rotation may be the calibrating reference in this case, as it is in the development of the star compass. Young Savannah sparrows (Passerculus sandwichensis) learn to perform compass orientation at sunset based on polarized skylight. This compass capability seems to be calibrated by magnetic directions. Some problems of experimental design and the interpretation of results from experiments on development are discussed.     

A magnetic pulse leads to a temporary deflection in the orientation of migratory birds

Migratory Australian Silvereyes were treated with a strong magnetic pulse designed to alter the magnetization of the small magnetite particles that are found in birds' heads. Prior to the treatment, the birds preferred the northeasterly migratory direction. The pulse initially resulted in a 90° clockwise shift of orientation; however, within about a week, the birds seemed to return to their original headings. These findings, which seem to suggest an involvement of magnetite in migratory orientation, are in contrast with previous findings which indicated that it is a light-dependent process. They are discussed in view of the current concepts on magnetoreception and on the role of magnetic information in avian orientation.     

Overcast tests with clock-shifted pigeons

In order to investigate the pigeon's compass mechanism, a series of overcast tests with clock-shifted birds were run at two familiar release sites. While controls were able to assume a correct homeward direction, the experimental birds' initial orientation cannot be explained either on the basis of a time-compensated sun compass or of a time-independent magnetic compass. Speculative explanations of our paradoxical results are attempted.     

Magnetic orientation and celestial cues in migratory orientation

Summary Young birds on their first migration possess innate information on the direction of their migration route. It is present in two forms, using celestial rotation and the geomagnetic field as references. These two systems, together with information provided by factors associated with sunset, interact in a complex way to establish the migratory direction. During ontogeny, celestial rotation appears to be dominant; during migration, however, celestial cues appear to be controlled by the magnetic field. The factors associated with sunset — the view of the setting sun, the characteristic pattern of polarized light — are important secondary cues which seem to derive their directional significance from the magnetic field. Their role appears to be more variable, with possible species-specific differences. During spring migration and later autumn migrations, flying in the migratory direction is complemented by navigational processes which enable the birds to return to a specific home site known from previous stays.     

Magnetic compass orientation in the subterranean rodentCryptomys hottentotus (Bathyergidae)

Summary To test whether mole-ratsCryptomys hottentotus were able to use the magnetic field for orientation, laboratory experiments were conducted which were based on the animals' spontaneous tendency to build their nests at the same position in a circular arena. In the local geomagnetic field, the animals preferred the SE-sector. When magnetic north was turned by 120^o or by 180^o, the mole-rats changed their nest position accordingly. This clearly shows that they can use the magnetic field for direction finding.     

The sun compass

Summary The sun compass was discovered by G. Kramer in caged birds showing migratory restlessness. Subsequent experiments with caged brids employing directional training and clock shifts, carried out by Hoffmann and by Schmidt-Koenig, showed that sun azimuth is used and sun altitude ignored. McDonald found the accuracy to be ±3^o–±5^o. According to Hoffmann and to Schmidt-Koenig, caged birds trained at medium northern latitudes were able to allow for the sun's apparent movement north of the arctic circle but not in equatorial and trans-equatorial latitudes.In homing experiments, and employing clock shifts, Schmidt-Koenig demonstrated that the sun compass is used by homing pigeons during initial orientation. This finding supports the existence of a map and compass navigational system. Pigeons living in equatorial latitudes utilize the sun compass even under the extreme solar conditions of equinox (Ranvaud, Schmidt-Koenig, Ganzhorn et al.). The use of the sun compass during zenith passage of the sun is being investigated.     

Ecological causes and consequences of bird orientation

Summary An advanced orientation capability offers possibilities for birds to optimize movement patterns in a wide variety of ecological situations. The adaptive significance of different patterns of angular dispersion and of orientation responses to topography and sociality are elucidated. The orientation capacity is characterized by flexibility, exemplified by reorientation promoting safety and restoration of fat reserves during migration. There are also limitations to the orientation process, leading to costs of migration through mis- or disorientation, and to constraints on the evolution of routes and timing of migratory flights. Young migrants may acquire an erroneous compass sense, and misorient several thousands of kilometers off their normal course. Widespread and dense fog of long duration causes disorientation and mortality among land birds migrating over the sea. Orientational constraints in the evolution of migration routes may be most easily disclosed at high geographic and magnetic latitudes. Here the birds are faced with special difficulties in using their celestial as well as their magnetic compasses. The sun compass could be used for great circle orientation, but observed spring flight trajectories of high arctic shorebirds and geese seem to conform with rhumbline routes.     

The early history of ship magnetism: The Airy-Scoresby controversy

With the advent of iron-built ships in the early nineteenth century the problem of managing a magnetic compass on board presented considerable difficulty. Prominent among the early scientists who tackled the problem were George Biddell Airy and William Scoresby. Airy had provided a mechanical system, employing correctors in the form of steel magnets and wrought iron masses, by which the ship's magnetic field at the compass position is neutralized. He based his system on the concept that the magnetism acquired by an iron ship during her construction remained with the ship throughout her life. Scoresby, on the other hand, thought differently. After having conducted a thorough and systematic series of experiments on iron plates and bars, he concluded that the magnetic character of an iron ship is liable to sudden and unexpected change. Scoresby argued, therefore, that Airy's system was defective and, indeed, dangerous. The aim of this paper is to discuss the two views which brought Airy and Scoresby into a conflict which is documented in a series of letters to the editor of The Athenaeum.     

New theories for new instruments: Fabrizio Mordente's proportional compass and the genesis of Giordano Bruno's atomist geometry

The aim of this article is to shed light on an understudied aspect of Giordano Bruno's intellectual biography, namely, his career as a mathematical practitioner. Early interpreters, especially, have criticized Bruno's mathematics for being “outdated” or too “concrete”. However, thanks to developments in the study of early modern mathematics and the rediscovery of Bruno's first mathematical writings (four dialogues on Fabrizio's Mordente proportional compass), we are in a position to better understand Bruno's mathematics. In particular, this article aims to reopen the question of whether Bruno anticipated the concept of infinitesimal quantity. It does so by providing an analysis of the dialogues on Mordente's compass and of the historical circumstances under which those dialogues were written.     

Depolarization of natural skylight disrupts orientation of an avian nocturnal migrant

Summary Given a view of clear evening skies, migratory blackcaps (Aves:Sylvia atricapilla) orient appropriately in the absence of meaningful information from the geomagnetic field. When the intensity of natural skylight polarization patterns was reduced with pseudodepolarizers by over 90%, test birds were disoriented, and their migratory restlessness in autumn was drastically reduced.     

Genetic and photoperiodic control of an avian reproductive cycle

European blackcaps,Sylvia atricapilla, with one breeding season per year, have a single-peaked annual testes cycle. However, African conspecifics from the Cape Verde Islands with two breeding seasons per annum demonstrate a two-peaked cycle. Both population-specific cycles reflect differences in the respective endogenous circannual rhythms. Experimental hybridization of birds of the two populations resulted in an intermediate pattern of testes cycle, thus demonstrating that there are genetic components for some temporal aspects in an avian reproductive cycle. Another characteristic of the African birds, their extremely rapid juvenile development and early sexual maturity (at an age of 5–6 months) proved largely to be a photoperiodic (short-day) effect in birds hatched in autumn. The same effect could also be induced in European conspecifics exposed to correspondingly short day-lengths.     

Weak static magnetic fields increase the speed of circumnutation in cucumber (Cucumis sativus L.) tendrils

Tendrils are thread-like organs whose function is to support the stems of many species of climbing plants. Tendrils naturally move (circumnutate) in space. Individual tendrils of cucumber (Cucumis sativus L.) Poinset had the vertical component of their mean velocity vector of circumnutation changed when exposed to a range of weak static magnetic fields between 1 and 16 mT. The speed (modulus) of the velocity vector was significantly increased (p=0.016) in the vicinity of a magnet, but its direction did not show a definite trend with respect to the magnet. Although cucumber tendrils bear static positive charges, they did not behave as charged bodies do in a magnetic field, neither did they show a magnetotropic response. In fact, tendrils showed a nastic response to magnetism. Magnetic fields affected some processes underlying the movement of circumnutation, but no clear interpretation of them can be given presently on the basis of the known effects of magnetism on plants. It is clear that cucumber tendrils, because of some inherent susceptibility to magnetism or their particular size and shape, are very sensitive to relatively low static magnetic field strengths.     

Phylogeny and rapid Northern and Southern Hemisphere speciation of goldfinches during the Miocene and Pliocene Epochs

Mitochondrial cytochrome b (cyt b) from 25 out of 31 extant goldfinches, siskins, greenfinches and redpolls (genus Carduelis) has been sequenced from living samples taken around the world, specimens have also been photographed. Phylogenetic analysis consistently gave the same groups of birds, and this grouping was generally related to geographical proximity. It has been supposed that Pleistocene glaciations played a crucial role in the origin of extant diversity and distribution of Northern Hemisphere vertebrates. Molecular comparison of most extant songbird species belonging to the genus Carduelis does not support this assertion. The fossil record of chicken and pheasant divergence time has been used to calibrate the molecular clock; cyt b DNA dendrograms suggest that speciation in Carduelinae birds occurred during the Miocene and Pliocene Epochs (9 – 2 million years ago) in both the Northern and Southern Hemispheres. Only about 4% average amount of nucleotide substitution per lineage is found between the most distant Carduelis species; this suggests a remarkably rapid radiation when compared with the radiation of other passerine songbird genera. In addition, a continuum of small songbird speciation may be found during the Miocene Epoch in parallel with speciation of other orders (i.e. Galliformes, chicken/pheasant). Pleistocene glaciations may have been important in subspeciation (i.e. Eastern European grey-headed goldfinches/Western European black-headed goldfinches) and also in ice-induced vicariance (isolation) (i.e. siskin in Western Europe vs. siskin in Far East Asia) around the world. European isolated Serinus citrinella (citril finch) is not a canary, but a true goldfinch. South American siskins have quickly radiated in the last 4 million years coinciding with the emergence of the Isthmus of Panama; probably, a North American siskin related to C. notata invaded a suitable and varied biotope (the South American island) for Carduelis birds. North American goldfinches may be renamed as siskins, because they have a distant genetic relationship with European goldfinches. Genus Acanthis could be dropped, and thus redpolls should be separated from twite and linnet, the latter (Europeans) probably being related to American goldfinches. Also, reproductive barriers are observed between closely related species and not between other more distant ones. Finally, a tentative classification for genus Carduelis species is suggested. Received 6 March 1998; received after revision 3 July 1998; accepted 7 July 1998     

Sex determination in mammals — Before and after the evolution of SRY

Therian mammals (marsupials and placentals) have an XX female: XY male sex chromosome system, which is homologous to autosomes in other vertebrates. The testis-determining gene, SRY, is conserved on the Y throughout therians, but is absent in other vertebrates, suggesting that the mammal system evolved about 310 million years ago (MYA). However, recent work on the basal monotreme mammals has completely changed our conception of how and when this change occurred. Platypus and echidna lack SRY, and the therian X and Y are represented by autosomes, implying that SRY evolved in therians after their divergence from monotremes only 166 MYA. Clues to the ancestral mechanism usurped by SRY in therians are provided by the monotremes, whose sex chromosomes are homologous to the ZW of birds. This suggests that the therian X and Y, and the SRY gene, evolved from an ancient bird-like sex chromosome system which predates the divergence of mammals and reptiles 310 MYA. Received 4 March 2008; received after revision 22 April 2008; accepted 3 June 2008     

Origin and identification of bacteria which produce kairomones in the frass ofAcrolepiopsis assectella (Lep., Hyponomeutoidea)

The volatiles used by the parasitoidDiadromus pulchellus to find its host, the leek moth, are produced by the bacteria developing in the frass of the host larvae. The origin and the nature of these bacteria were investigated. Samples were taken from healthy leeks and from infested leeks in the field, as well as from the frass of larvae reared in the laboratory either on the host plant or on an artificial diet. The various species of bacteria identified were cultured in the presence of precursors of leek sulphur volatiles and their volatile emissions were analysed.Klebsiella oxytoca and variousBacillus, common decomposers of plant matter, were the principal species producing active volatiles which were alkyl disulphides.     

Response of the starling circadian system to transitions between light and darkness

Summary The effects of exposure to sudden transitions from dark to light (D/L) and light to dark (L/D) were determined on the free running circadian feding rhythm of European stalings (Sturnus vulgaris). D/L transitions (step-up) produced phase advances throughout the circadian cycle. In contrast L/D transitions (step-down) produced both advances and delays. The L/D transition phase-response curve has a contour and shape similar to the phase-response curves previously obtained in birds with light pulses.     

Biological responses inCaenorhabditis elegans to high magnetic fields

Here we describe a device for testing possible influences of high magnetic fields on biological processes, by which alternating-current magnetic stimuli as high as 1.7 T can be administered. Experiments with a simple multicellular organism, the nematodeCaenorhabditis elegans, revealed that intermittent exposure to the magnetic fields modestly inhibited the animal's reproduction as well as its post-embryonic development, and caused a marked but transient derangement in its locomotory behavior. Available evidence indicates that alternating high magnetic fields can elicit both chronic and acute biological effects, but that the effects may be well tolerated or compensated for by the living organism.     

A blue light-reversible reaction in an animal system (Daphnia pulex)

Summary A photoreversible reaction, previously found only in plants and fungi, has now been found in an animal system. Activation of development in diapausing embryos ofDaphnia pulex, induced with white light, was suppressed with subsequent exposure to narrow-band (470 nm) or wide-band (410–525 nm) blue light. Pulses of wide-band blue light repeatedly reversed white light activation.     

Conceptual approaches to avian navigation systems

Summary The general basis of migratory orientation in birds is most probably an endogenous time-and-direction program. Directions are selected with respect to celestial and geomagnetic clues. Using these clues, a bird may reach a large population-specific area; however, it will hardly be able to find a particular location, for instance its previous breeding site. Homing to a familiar site over several hundred kilometres of unfamiliar terrain appears to be based on the smelling of atmospheric trace compounds. Conceptual approaches to the mechanism of olfactory navigation have as yet only reached an early state of speculation.     

Regulation of carbohydrate metabolism by the optic tentacles of the garden snailCryptozona ligulata (Gastropoda-Stylommatophora)

Summary The presence of a hyperglycemic factor in the optic tentacles of the snailC. ligulata is reported here. A preliminary characterization based on crude extracts indicates the factor to be water-soluble, heat labile and to be an albumin. The ablation of optic tentacles and injection of optic tentacle extract into operated and normal snails caused a rise in blood sugar, total carbohydrate and glycogen in the foot muscle and mantle and a decrease in hepatopancreatic glycogen. The ablation also caused a fall in blood free amino acids and a rise in the tissues, which was reversed in the blood and foot muscle by injection of the extract. Possible conversion of amino acids to total carbohydrates and glycogen by gluconeogenesis is suggested.