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ScienceWeek
ANIMAL BEHAVIOR: ON ANT NAVIGATION
The following points are made by Francis Ratnieks (Nature 2005 436:465):
1) There are probably 20,000 ant species and they do not all use the same navigational methods or have equal navigational abilities. Many can reorient on trails by using external cues, including landmarks and the position of the Sun. Leafcutter ants are even thought to use the Earth's magnetic field. But recent research has shown that one common ant, the pharaoh's ant, Monomorium pharaonis, has a sense of geometry, and other species probably do as well.
2) A pharaoh's ant colony forms a foraging-trail network leading from the nest entrance into the surrounding environment. These trails form Y-shaped branches with an internal angle of approximately 60 degrees as they lead away from the entrance. Ants walking the wrong way along a trail are unable to reorient at a trail bifurcation if the angle is 120 degrees. But if the angle is less, then they can. Angles less than 120 degrees give the "Y" bifurcation a nest-environment polarity, whereas at 120 degrees there is only symmetry. The ability to reorient is maximized at the natural bifurcation angle of 60 degrees.
3) Natural selection has made insect societies good at solving a problem that is simple to state but hard to solve -- to send foragers to where the food is. Because social insects have been solving this complex dynamic problem for millions of years, they have probably evolved some simple and elegant solutions. We should care about these solutions because human life depends more and more on engineering systems that must solve similar problems to function efficiently -- electronic messaging, grid computing, transmitting electricity and traffic regulation to name a few. One obvious lesson we might learn is how to make our systems more reliable and robust. If there is one thing that natural selection should be good at, it is eliminating solutions that are not robust. The colony or organism that "crashes" will soon be a dead one.
4) When a physicist plays ants at their own game -- trying to understand a problem fundamental to colony survival that ants have been working on, by means of natural selection, for millions of years -- ants can come out ahead. It is no disgrace to be outsmarted by ants. But are we smart enough to learn from them?[1-3]
References:
1. Feynman, R. P. Don't You Have Time To Think? (ed. Feynman, M.) (Allen Lane, 2005).
2. Feynman, R. P. Surely you're joking, Mr. Feynman! (Norton, New York, 1985).
3. Jackson, D. E., Holcombe, M., Ratnieks, F. L. W. Nature 432, 907-909; 2004.
Nature http://www.nature.com/nature
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ZOOLOGY: ON ANIMAL NAVIGATION
The following points are made by James L. Gould (Current Biology 2004 14:R221):
1) Nearly all animals move in an oriented way, but navigation is something more: the directed movement toward a goal, as opposed to steering toward or away from, say, light or gravity. Navigation involves the neural processing of sensory inputs to determine a direction and perhaps distance. For instance, if a honey bee were to seek food south of its hive, it would depart from home with the sun to its left in the morning, but to its right in the afternoon.
2) Several trends reflecting favorably on natural selection and poorly on human imagination characterized early studies of navigation. One tendency was the assumption that animals sense at most the same cues as we do. Thus, being blind to our own blindness, it came as a total surprise when honey bees and many other species were found to be able to see UV light. As navigation depends on the processing of such cues, the number of "new" senses uncovered in the past fifty years has greatly expanded our thinking about what may be going on in the minds of animals -- and there is no reason to assume the list is complete. To UV must be added polarized light, infra-red light, special odors (pheromones), magnetic fields, electric fields, ultrasonic sounds and infrasonic sounds.
3) The second crippling propensity is what navigation pioneer Donald Griffin called our innate "simplicity filter": the desire to believe that animals do things in the least complex way possible. Experience, however, tells us that animals whose lives depend on accurate navigation are uniformly overengineered. Not only do they frequently wring more information out of the cues that surround them than we can, or use more exotic or weaker cues than we find conceivable, they usually come equipped with alternative strategies -- a series of backups between which they switch depending on which is providing the most reliable information.
4) A honey bee, for instance, may set off for a goal using its time-compensated sun compass. When a cloud covers the sun, it may change to inferring the sun's position from UV patterns in the sky and opt a minute later for a map-like strategy when it encounters a distinctive landmark. Lastly, it may ignore all of these cues as it gets close enough to its goal to detect the odors or visual cues provided by the flowers. This is not to say that animals do not often rely on approximations and neural shortcuts to simplify these daunting tasks.
5) A third stumbling block has been our presumption that because the earliest cases studied involved "imprinting" (irreversible one-trial learning), animals must have simple navigation programs, which need merely to be calibrated to the local contingencies. This is just what at least some relatively short-lived animals do -- like honey bees for instance, who rarely forage for more than three weeks. But most animals live longer, and in consequence many need to recalibrate themselves.
6) Finally, most researchers deliberately ignored or denigrated the evidence for cognitive processing in navigating animals. This legacy of behaviorism (the school of psychology that denied instinct) puts a ceiling on the maximum level of mental activity subject to legitimate investigation. There are many navigating animals whose behavior lacks any hint of cognitive intervention. However, the obvious abilities of hunting spiders and honey bees to plan novel routes make it equally clear that phylogenetic distance to humans is no sure guide to the sophistication of a species' orientation strategies.(1-3)
References:
Able, K.P. and Able, M.A. (1995). Interactions in the flexible orientation system of a migratory bird. Nature 375, 230-232
Gould, J.L. (1980). The case for magnetic-field sensitivity in birds and bees. Am. Sci. 68, 256-267
Walker, M.M. (1998). On a wing and a vector. J. Theor. Biol. 192, 341-349
Current Biology http://www.current-biology.com
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ENTOMOLOGY: ON HONEYBEE NAVIGATION
The following points are made by M.V. Srinivasan (Current Biology 2003 13:R894):
1) Many of us have marvelled at the ability of honeybees to find an attractive flower patch, miles away from their hive, and to return to it repeatedly with unerring accuracy. How do they do this with a brain smaller than a sesame seed? We don't know all the answers yet, but bees seem to be able to estimate the distance to a food source, gauge the direction in which to fly to reach it, and convey this information to their nestmates, so that they can forage from it, too, and thus collectively build up the colony's food reserves.
2) It appears that bees use one or both of the following cues. The first comes from measuring the amount of energy consumed when they fly to the destination by the reduction in the volume of their nectar-laden stomachs; the greater the energy consumed, the further the distance. And the second comes from measuring how much the image of the world appears to move in the eye as they fly to the destination; the greater the extent of image motion, the further the distance.
3) Bees use the Sun as a compass. Even if the Sun is hidden by a cloud, bees can continue to steer correctly by inferring the Sun's position from the pattern of polarized light that it creates in a patch of clear sky. This polarized-light pattern is visible to them but not to us, unless we view the sky through polaroid sunglasses. Bees are also able to compensate for the daily movement of the Sun across the sky by using information from an internal biological clock.
4) A "scout" bee recruits her nestmates to visit a newly discovered flower patch. Upon returning to the hive, the scout performs a "waggle" dance on the vertical surface of the honeycomb. This dance consists of a series of alternating left-hand and right-hand loops, interspersed by a segment in which she waggles her abdomen from side to side. The duration of this waggle phase is a measure of the distance to the food, and the angle between the axis of this waggle segment and the vertical represents the azimuthal angle between the sun and the direction in which the recruit should fly to find the target. Thus, the dance conveys the position of the food source relative to the hive in polar coordinates.
5) A bee that has visited a promising flower patch a few times will rapidly learn the color, shape and fragrance of the flowers, and use these cues to home in on them on return visits. Bees have excellent trichromatic color vision, featuring three types of photoreceptor which are maximally sensitive to the ultraviolet, blue and green wavelengths of light, respectively. Honeybees rival humans in their ability to discriminate subtle color differences. Bees that repeatedly visit a given flower patch also learn to use prominent landmarks, if there are any present, as navigational aids. For example, a landmark could be used as a beacon that guides that bee toward her destination. Landmarks can also function as checkpoints along the route. Thus a bee can learn, for example, that the shortest route to the goal is to fly to the left of landmark A, and to the right of landmark B. There is also some evidence that bees can "count" prominent landmarks that they encounter en route to the food. Finally, honeybees probably use memorized visual "snapshots" of the surrounding environment at the destination to help them to return to the same spot repeatedly and reliably.
References:
1. Collett, T.S. and Zeil, J. (1998). In Spatial representation in animals. In Healy, S. ed. (Oxford University Press), pp. 18-53
2. Srinivasan, M.V., Zhang, S.W., Altwein, M., and Tautz, J. (2000). Honeybee navigation: nature and calibration of the "odometer". Science 287, 851-853
3. von Frisch, K. (1993). The Dance Language and Orientation of Bees. (London: Harvard Univ. Press)
4. Wehner, R., Michel, B., and Antonsen, P. (1996). Visual navigation in insects: Coupling of egocentric and geocentric information. J. Exp. Biol 199, 129-140
Current Biology http://www.current-biology.com
ScienceWeek http://scienceweek.com
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