[Paleopsych] SW: On Animal Navigation
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Animal Behavior: On Animal Navigation
http://scienceweek.com/2005/sw051014-1.htm
The following points are made by James L. Gould (Current Biology 2005
15:R653):
1) Juvenile birds regularly migrate thousands of kilometers. Most fly
at night, without their parents to guide them. The mystery of how
birds manage this apparent impossibility inspires ever-more-heroic
attempts to defeat them at this crucial task. New work [1] reports a
dramatic new way to confound south-bound sparrows: take them along or
above the Arctic Circle aboard an icebreaker; fly them to experimental
sites by helicopter; and see what directions they choose. Only time
will tell whether, once again, the birds have another layer of
yet-to-be-deciphered finesse in their navigation repertoire, or if
they finally have been driven to the computational wall and are
responding in a consistent "does-not-compute" manner.
2) Exactly what does a migrating species require? First, the birds
need a compass to know which direction they are going, and they come
supplied with several: the earth's magnetic field (indicating magnetic
north); the celestial pole (indicating true north, about which the
stars appear to rotate at night); and the Sun's location (as inferred
from patterns of sun-centered polarization which, with a suitable time
sense, also specifies true north). Second, they need to know at least
roughly -- and in some instances quite precisely -- where they are
relative to their goal. In the case of homing pigeons, this ability is
known as a map sense, and has a resolution of a very few kilometers
[2].
3) The map sounds quite mysterious compared to the compass, but they
are both daunting challenges. Consider the problem from the bird's
point of view. First of all, it is cloudy a lot, so much of the time
you can forget about using celestial cues. But then, why not just use
magnetic north? If you are born at a high latitude -- where large
numbers of species breed -- there is often a large discrepancy between
magnetic and true north -- the declination error, which arises in part
from the 1400 kilometer separation of this point from the geographic
pole. Worse, declination generally changes as you fly south. And even
if the evening is clear, the stars and patterns of polarized light
change with both latitude and date.
4) Birds dispose of these problems by periodically calibrating one
compass against the other [3;4]. Recent evidence has shown that when
the sky is clear, the recalibration occurs daily, and takes only about
an hour [5]. The accuracy and sensitivity of this system is
astonishing: in tests performed near the conventional north magnetic
pole -- where the earth's field lines plunge vertically into the
planet, providing no directional information at all -- birds are well
oriented just a few dozen kilometers away, where there is only a 1.1
deg deviation from vertical. (There is another "pole", one of magnetic
intensity, located about 2000 kilometers farther south near the
western edge of Hudson's Bay, which is critically important in
magnetic-map theories.)
5) But how do the birds know which way to fly in the first place. Most
species have an innate starting direction: put them as juveniles or
adults in a cage during migration season, and they will all try to
escape in roughly the direction they ought to fly to get to their
wintering grounds. But there is a danger here in assuming that the
birds know only as much as their behavior suggests: some species
display accurate departure directions in cages, while others select a
consistent but quite incorrect one (west into the sunset). But release
the same "misguided" birds with tracking beacons, and they set off in
the direction they ought to have chosen in the cage. The results of
Åkesson et al. [1] take on more meaning in this light. They found that
sparrows moved gradually east above the Arctic Circle completely
altered their migration strategy after encountering the massive
natural change in declination near the magnetic pole.
References (abridged):
1. Åkesson, S., Morin, J., Muheim, R., and Ottosson, U. (2005).
Dramatic orientation shift of White-crowned Sparrows displaced across
longitudes in the high arctic. Curr. Biol. 15:1591
2. Gould, J.L. (1980). The case for magnetic sensitivity in birds and
bees (such as it is). Am. Sci. 68, 256-267
3. Able, K.P. and Able, M.A. (1990). Calibration of the magnetic
compass of a migratory bird by celestial rotation. Nature 347, 378-380
4. Able, K.P. and Able, M.A. (1995). Interactions in the flexible
orientation system of a migratory bird. Nature 375, 230-232
5. Åkesson, S., Morin, J., Muheim, R., and Ottosson, U. (2002). Avian
orientation: effects of cue-conflict experiments with young migratory
songbirds in the high Arctic. Anim. Behav. 64, 469-475
Current Biology http://www.current-biology.com
--------------------------------
Related Material:
NATURAL HISTORY: ON THE MAGNETIC COMPASS OF SONGBIRDS
The following points are made by W.W. Cochran et al (Science 2004
304:405):
1) Billions of songbirds migrate between continents twice each year,
but their orientation capabilities are almost exclusively studied in
the laboratory. The authors presented birds with experimentally
altered orientation cues and followed their subsequent migratory
flights in the wild. Avian navigation capabilities are very precise
(1), with many individuals returning to the same breeding sites year
after year (1-3) after a voyage of up to 25,000 km (4, ).
2) Migratory songbirds can orient on the basis of compass information
from the sun and its associated polarized light patterns, the stars,
the earth's magnetic field, and the memorization of spatial cues en
route. However, the interactions and relative importance of these cues
remain unclear and a source of much debate. Our knowledge about the
orientation mechanisms of songbirds relies almost exclusively on data
from cue-manipulated captive migrants tested in various orientation
cages, on vanishing bearings based on the first few hundred meters of
flight, and to a much lesser degree on field data (ringing and radar
and visual observations) from unmanipulated natural migrants.
3) On clear evenings, the authors fitted Catharus thrushes with radio
transmitters and placed them in outdoor cages in an artificial
eastward-turned magnetic field from about sunset until the sun was 11
deg or more below the horizon when they were set free. The authors
then radio-tracked the birds in flight to obtain heading data. Because
Catharus thrushes do not compensate for wind drift but individuals
maintain nearly constant preferred headings from night to night, the
authors used measured headings for orientation analyses.
4) In summary: Night migratory songbirds can use stars, sun,
geomagnetic field, and polarized light for orientation when tested in
captivity. The authors studied the interaction of magnetic, stellar,
and twilight orientation cues in free-flying songbirds. The authors
exposed Catharus thrushes to eastward-turned magnetic fields during
the twilight period before takeoff and then followed them for up to
1100 kilometers. Instead of heading north, experimental birds flew
westward. On subsequent nights, the same individuals migrated
northward again. The authors suggest that birds orient with a magnetic
compass calibrated daily from twilight cues, and that this could
explain how birds cross the magnetic equator and deal with
declination.
References (abridged):
1. P. Berthold, E. Gwinner, E. Sonnenschein, Eds., Avian Migration
(Springer, Berlin, 2003)
2. J. P. Hoover, Ecology 84, 416 (2003)
3. P. O. Dunn, D. W. Winkler, Proc. R. Soc. London Ser. B. 266, 2487
(1999)
4. D. C. Outlaw, et al., Auk 120, 299 (2003)
5. W. L. Engels, Biol. Bull. 123, 94 (1962)
Science http://www.sciencemag.org
--------------------------------
Related Material:
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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