Unraveling navigational strategies in migratory insects

Long-distance migration is a strategy some animals use to survive a seasonally changing environment. To reach favorable grounds, migratory animals have evolved sophisticated navigational mechanisms that rely on a map and compasses. In migratory insects, the existence of a map sense (sense of position) remains poorly understood, but recent work has provided new insights into the mechanisms some compasses use for maintaining a constant bearing during long-distance navigation. The best-studied directional strategy relies on a time-compensated sun compass, used by diurnal insects, for which neural circuits have begun to be delineated. Yet, a growing body of evidence suggests that migratory insects may also rely on other compasses that use night sky cues or the Earth's magnetic field. Those mechanisms are ripe for exploration.     

Sea turtles, lobsters, and oceanic magnetic maps

Numerous marine animals can sense the Earth's magnetic field and use it as a cue in orientation and navigation. Two distinct types of information can potentially be extracted from the Earth's field. Directional or compass information enables animals to maintain a consistent heading in a particular direction such as north or south. In contrast, positional or map information can be used by animals to assess geographic location and, in some cases, to navigate to specific target areas. Marine animals exploit magnetic positional information in at least two different ways. For hatchling loggerhead sea turtles, regional magnetic fields function as open-sea navigational markers, eliciting changes in swimming direction at crucial points in the migratory route. Older sea turtles, as well as spiny lobsters, use magnetic information in a more complex way, exploiting it as a component of a classical navigational map, which permits an assessment of position relative to specific geographic destinations. These “magnetic maps” have not yet been fully characterized. They may be organized in several fundamentally different ways, some of which bear little resemblance to human maps, and they may also be used in conjunction with unconventional navigational strategies. Unraveling the nature of magnetic maps and exploring how they are used represents one of the most exciting frontiers of behavioral and sensory biology.     

Spontaneous preferences for magnetic compass direction in the American red-spotted newt, <Emphasis Type="Italic">Notophthalmus viridescens</Emphasis> (Salamandridae,Urodela)

In addition to other sensory modalities, migratory vertebrates are able to use the earths’ magnetic field for orientation and navigation. The magnetic cue may also serve as a reference for other orientation mechanisms. In this study, significant evidence is shown that, even in darkness, newts (Notophthalmus viridescens, Salamandridae) spontaneously align according to the natural or to the deviated earth’s magnetic field lines, thereby demonstrating a magnetic compass sensitivity. All newts preferred compass directions close to east or west or chose the E/W axially and hence sought to maintain a specific angle or axis relative to the magnetic field vector. Such an active alignment is considered an essential precondition for magnetic orientation. When the horizontal magnetic vector was experimentally compensated, animals became disoriented. We infer that the animals have either learned the preferred magnetic direction/axis individually or that these choices are innate and could even be seasonally different as in migrating birds. It is still an unanswered question as to how and where the physical and physiological mechanisms of magnetic transduction and reception take place. The visual system and other light-dependent (radical pairs) mechanisms alone are often claimed to be in function, but this must now be reconsidered given the results from animals when deprived of light. The results may therefore point to putative receptor mechanisms involving magnetite elements in specialized magneto-receptors.     

Human spatial navigation: cognitive maps, sexual dimorphism, and neural substrates.

Recent research on navigation has been particularly notable for the increased understanding of the factors affecting human navigation and the neural networks supporting it. The use of virtual reality environments has made it possible to explore the effect of environment layout and content on way-finding performance, and it has shown that these effects may interact with the sex and age of subjects. Functional brain imaging, combined with the use of virtual environments, has revealed strong parallels between humans and other animals in the neural basis of navigation.     

Magnetic maps in animals: a theory comes of age?

The magnetic map hypothesis proposes that animals can use spatial gradients in the Earth's magnetic field to help determine geographic location. This ability would permit true navigation--reaching a goal from an entirely unfamiliar site with no goal-emanating cues to assist. It is a highly contentious hypothesis since the geomagnetic field fluctuates in time and spatial gradients may be disturbed by geological anomalies. Nevertheless, a substantial body of evidence offers support for the hypothesis. Much of the evidence has been indirect in nature, such as the identification of avian magnetoreceptor mechanisms with functional properties that are consistent with those of a putative map detector or the patterns of orientation of animals exposed to temporal and/or spatial geomagnetic anomalies. However; the most important advances have been made in conducting direct tests of the magnetic map hypothesis by exposing experienced migrants to specific geomagnetic values representing simulated displacements. Appropriate shifts in the direction of orientation, which compensate for the simulated displacements, have been observed in newts, birds, sea turtles, and lobsters, and provide the strongest evidence to date for magnetic map navigation. Careful experimental design and interpretation of orientation data will be essential in the future to determine which components of the magnetic field are used to derive geographic position.     

Lesions of the Basal Forebrain Cholinergic System in Mice Disrupt Idiothetic Navigation

Loss of integrity of the basal forebrain cholinergic neurons is a consistent feature of Alzheimer’s disease, and measurement of basal forebrain degeneration by magnetic resonance imaging is emerging as a sensitive diagnostic marker for prodromal disease. It is also known that Alzheimer’s disease patients perform poorly on both real space and computerized cued (allothetic) or uncued (idiothetic) recall navigation tasks. Although the hippocampus is required for allothetic navigation, lesions of this region only mildly affect idiothetic navigation. Here we tested the hypothesis that the cholinergic medial septo-hippocampal circuit is important for idiothetic navigation. Basal forebrain cholinergic neurons were selectively lesioned in mice using the toxin saporin conjugated to a basal forebrain cholinergic neuronal marker, the p75 neurotrophin receptor. Control animals were able to learn and remember spatial information when tested on a modified version of the passive place avoidance test where all extramaze cues were removed, and animals had to rely on idiothetic signals. However, the exploratory behaviour of mice with cholinergic basal forebrain lesions was highly disorganized during this test. By contrast, the lesioned animals performed no differently from controls in tasks involving contextual fear conditioning and spatial working memory (Y maze), and displayed no deficits in potentially confounding behaviours such as motor performance, anxiety, or disturbed sleep/wake cycles. These data suggest that the basal forebrain cholinergic system plays a specific role in idiothetic navigation, a modality that is impaired early in Alzheimer’s disease.     

磁导航技术辅助麦芒仿生鼻肠营养管的设计和初步实验研究

目的:为解决现有液囊鼻肠营养管置管过程中漂浮时间长、成功率低的问题,设计了基于磁导航技术辅助的磁性麦芒仿生鼻肠营养管。方法:分析了麦芒在管腔内能够快速单向移动的力学原理,结合磁外科相关技术,提出了鼻肠管头端内嵌磁性麦芒仿生结构设计方案和磁导航技术辅助快速推进的操作方法,并在体内外模拟了其运动情况。结果:体外管路模拟实验提示麦芒仿生结构在外力作用下能够快速移动。家兔肠道实验显示,磁性麦芒仿生结构在导航磁体引导下,能够在肠道内快速单向移动。结论:磁性麦芒仿生结构在导航磁体的引导下能够实现消化道内快速单向移动,该设计进一步优化有望用于临床。     

磁场对蜜蜂行为影响的研究进展

地球内部存在着天然磁性现象,被称为地磁场。当今社会电子通讯发达,电磁波信号频繁,地球表面充满了电磁场。蜜蜂作为重要的授粉昆虫,其活动范围大部分都处于磁场环境中。目前研究者普遍认为蜜蜂能利用磁场进行导航,但环境中异常的磁场同样会影响蜜蜂的定位导航、飞行行为、采集活动和学习认知能力等。本文对前人的研究结果进行了综述,以期系统地了解磁场对蜜蜂行为特性的影响。     

生物磁学在鸟类定向研究中的进展

地球上广泛存在的地磁场能够为导航提供可靠的信息,因此很多鸟类在迁徙和归巢过程中都使用地磁信息来保证航行方向的正确,在迁徙的鸟类中已经发现有18种是利用地磁罗盘进行定向和导航的。本文从鸟类使用的磁罗盘、航行地图以及磁感应机制等几方面阐述了目前在鸟类生物磁学方面的研究进展。     

What scatter-hoarding animals have taught us about small-scale navigation

Many animals use cues for small-scale navigation, including beacons, landmarks, compasses and geometric properties. Scatter-hoarding animals are a unique system to study small-scale navigation. They have to remember and relocate many individual spatial locations, be fairly accurate in their searching and have to remember these locations for long stretches of time. In this article, we review what is known about cue use in both scatter-hoarding birds and rodents. We discuss the importance of local versus global cues, the encoding of bearings and geometric rules, the use of external compasses such as the Sun and the influence of the shape of experimental enclosures in relocating caches or hidden food. Scatter-hoarding animals are highly flexible in how and what they encode. There also appear to be differences in what scatter-hoarding birds and rodents encode, as well as what scatter-hoarding animals in general encode compared with other animals. Areas for future research with scatter-hoarding animals are discussed in light of what is currently known.     

Theoretical analysis of an iron mineral-based magnetoreceptor model in birds

Sensing the magnetic field has been established as an essential part of navigation and orientation of various animals for many years. Only recently has the first detailed receptor concept for magnetoreception been published based on histological and physical results. The considered mechanism involves two types of iron minerals (magnetite and maghemite) that were found in subcellular compartments within sensory dendrites of the upper beak of several bird species. But so far a quantitative evaluation of the proposed receptor is missing. In this article, we develop a theoretical model to quantitatively and qualitatively describe the magnetic field effects among particles containing iron minerals. The analysis of forces acting between these subcellular compartments shows a particular dependence on the orientation of the external magnetic field. The iron minerals in the beak are found in the form of crystalline maghemite platelets and assemblies of magnetite nanoparticles. We demonstrate that the pull or push to the magnetite assemblies, which are connected to the cell membrane, may reach a value of 0.2 pN -- sufficient to excite specific mechanoreceptive membrane channels in the nerve cell. The theoretical analysis of the assumed magnetoreceptor system in the avian beak skin clearly shows that it might indeed be a sensitive biological magnetometer providing an essential part of the magnetic map for navigation.     

A geometric model for initial orientation errors in pigeon navigation

All mobile animals respond to gradients in signals in their environment, such as light, sound, odours and magnetic and electric fields, but it remains controversial how they might use these signals to navigate over long distances. The Earth's surface is essentially two-dimensional, so two stimuli are needed to act as coordinates for navigation. However, no environmental fields are known to be simple enough to act as perpendicular coordinates on a two-dimensional grid. Here, we propose a model for navigation in which we assume that an animal has a simplified ‘cognitive map’ in which environmental stimuli act as perpendicular coordinates. We then investigate how systematic deviation of the contour lines of the environmental signals from a simple orthogonal arrangement can cause errors in position determination and lead to systematic patterns of directional errors in initial homing directions taken by pigeons. The model reproduces patterns of initial orientation errors seen in previously collected data from homing pigeons, predicts that errors should increase with distance from the loft, and provides a basis for efforts to identify further sources of orientation errors made by homing pigeons.     

Hierarchical state-space estimation of leatherback turtle navigation ability

Remotely sensed tracking technology has revealed remarkable migration patterns that were previously unknown; however, models to optimally use such data have developed more slowly. Here, we present a hierarchical Bayes state-space framework that allows us to combine tracking data from a collection of animals and make inferences at both individual and broader levels. We formulate models that allow the navigation ability of animals to be estimated and demonstrate how information can be combined over many animals to allow improved estimation. We also show how formal hypothesis testing regarding navigation ability can easily be accomplished in this framework. Using Argos satellite tracking data from 14 leatherback turtles, 7 males and 7 females, during their southward migration from Nova Scotia, Canada, we find that the circle of confusion (the radius around an animal's location within which it is unable to determine its location precisely) is approximately 96 km. This estimate suggests that the turtles' navigation does not need to be highly accurate, especially if they are able to use more reliable cues as they near their destination. Moreover, for the 14 turtles examined, there is little evidence to suggest that male and female navigation abilities differ. Because of the minimal assumptions made about the movement process, our approach can be used to estimate and compare navigation ability for many migratory species that are able to carry electronic tracking devices.     

Principles of goal-directed spatial robot navigation in biomimetic models

Mobile robots and animals alike must effectively navigate their environments in order to achieve their goals. For animals goal-directed navigation facilitates finding food, seeking shelter or migration; similarly robots perform goal-directed navigation to find a charging station, get out of the rain or guide a person to a destination. This similarity in tasks extends to the environment as well; increasingly, mobile robots are operating in the same underwater, ground and aerial environments that animals do. Yet despite these similarities, goal-directed navigation research in robotics and biology has proceeded largely in parallel, linked only by a small amount of interdisciplinary research spanning both areas. Most state-of-the-art robotic navigation systems employ a range of sensors, world representations and navigation algorithms that seem far removed from what we know of how animals navigate; their navigation systems are shaped by key principles of navigation in ‘real-world’ environments including dealing with uncertainty in sensing, landmark observation and world modelling. By contrast, biomimetic animal navigation models produce plausible animal navigation behaviour in a range of laboratory experimental navigation paradigms, typically without addressing many of these robotic navigation principles. In this paper, we attempt to link robotics and biology by reviewing the current state of the art in conventional and biomimetic goal-directed navigation models, focusing on the key principles of goal-oriented robotic navigation and the extent to which these principles have been adapted by biomimetic navigation models and why.     

Underwater linear polarization: physical limitations to biological functions

Polarization sensitivity is documented in a range of marine animals. The variety of tasks for which animals can use this sensitivity, and the range over which they do so, are confined by the visual systems of these animals and by the propagation of the polarization information in the aquatic environment. We examine the environmental physical constraints in an attempt to reveal the depth, range and other limitations to the use of polarization sensitivity by marine animals. In clear oceanic waters, navigation that is based on the polarization pattern of the sky appears to be limited to shallow waters, while solar-based navigation is possible down to 200-400 m. When combined with intensity difference, polarization sensitivity allows an increase in target detection range by 70-80% with an upper limit of 15 m for large-eyed animals. This distance will be significantly smaller for small animals, such as plankton, and in turbid waters. Polarization-contrast detection, which is relevant to object detection and communication, is strongly affected by water conditions and in clear waters its range limit may reach 15 m as well. We show that polarization sensitivity may also serve for target distance estimation, when examining point source bioluminescent objects in the photic mesopelagic depth range.     

Stochastic dynamics of magnetosomes and a mechanism of biological orientation in the geomagnetic field

The rotations of nanoscopic magnetic particles, magnetosomes, embedded into the cytoskeleton are considered. Under the influence of thermal disturbances, a great number of magnetosomes are shown to move chaotically between two stable equilibrium positions, in which their magnetic moments are neither parallel nor antiparallel to the static Earth's magnetic field (MF). The random rotations attain the value of order of a radian. The rate of the transitions and the probability of magnetosomes to be in the different states depend on the MF direction with respect to an averaged magnetosome's orientation. This effect explains the ability of migratory animals to orient themselves faultlessly in long term passages in the absence of the direct visibility of optical reference points. The sensitivity to deviation from an "ideal" orientation is estimated to be 2-4 degrees. Possible involvement of the stochastic dynamics of magnetosomes in biological magnetic navigation is discussed.     

Resolution of uncertainty in prefrontal cortex

Making optimal decisions in the face of uncertain or incomplete information arises as a common problem in everyday behavior, but the neural processes underlying this ability remain poorly understood. A typical case is navigation, in which a subject has to search for a known goal from an unknown location. Navigating under uncertain conditions requires making decisions on the basis of the current belief about location and updating that belief based on incoming information. Here, we use functional magnetic resonance imaging during a maze navigation task to study neural activity relating to the resolution of uncertainty as subjects make sequential decisions to reach a goal. We show that distinct regions of prefrontal cortex are engaged in specific computational functions that are well described by a Bayesian model of decision making. This permits efficient goal-oriented navigation and provides new insights into decision making by humans.     

Easy Rider: Monkeys Learn to Drive a Wheelchair to Navigate through a Complex Maze

The neurological bases of spatial navigation are mainly investigated in rodents and seldom in primates. The few studies led on spatial navigation in both human and non-human primates are performed in virtual, not in real environments. This is mostly because of methodological difficulties inherent in conducting research on freely-moving monkeys in real world environments. There is some incertitude, however, regarding the extrapolation of rodent spatial navigation strategies to primates. Here we present an entirely new platform for investigating real spatial navigation in rhesus monkeys. We showed that monkeys can learn a pathway by using different strategies. In these experiments three monkeys learned to drive the wheelchair and to follow a specified route through a real maze. After learning the route, probe tests revealed that animals successively use three distinct navigation strategies based on i) the place of the reward, ii) the direction taken to obtain reward or iii) a cue indicating reward location. The strategy used depended of the options proposed and the duration of learning. This study reveals that monkeys, like rodents and humans, switch between different spatial navigation strategies with extended practice, implying well-conserved brain learning systems across different species. This new task with freely driving monkeys provides a good support for the electrophysiological and pharmacological investigation of spatial navigation in the real world by making possible electrophysiological and pharmacological investigations.     

Magnetic orientation and magnetoreception in birds and other animals

Animals use the geomagnetic field in many ways: the magnetic vector provides a compass; magnetic intensity and/or inclination play a role as a component of the navigational map, and magnetic conditions of certain regions act as sign posts or triggers, eliciting specific responses. A magnetic compass is widespread among animals, magnetic navigation is indicated e.g. in birds, marine turtles and spiny lobsters and the use of magnetic sign posts has been described for birds and marine turtles. For magnetoreception, two hypotheses are currently discussed, one proposing a chemical compass based on a radical pair mechanism, the other postulating processes involving magnetite particles. The available evidence suggests that birds use both mechanisms, with the radical pair mechanism in the right eye providing directional information and a magnetite-based mechanism in the upper beak providing information on position as component of the map. Behavioral data from other animals indicate a light-dependent compass probably based on a radical pair mechanism in amphibians and a possibly magnetite-based mechanism in mammals. Histological and electrophysiological data suggest a magnetite-based mechanism in the nasal cavities of salmonid fish. Little is known about the parts of the brain where the respective information is processed.