Migration and Navigation

Invertebrates

 "Simple" Orientation

Kineses = undirected movement in response to a stimulus.

Taxes = directed (oriented) movement in response to a stimulus. 

Taxes Phototaxis  found in Planaria.  Also show

klinotaxis when respond to intensity of light.  Rate of change of direction increases with light intensity.

Amenotaxis = wind sensing.  Moth finds female by scent.  How?

Can be difficult problem to fly upwind.  Picture yourself in a river.  If standing still can feel direction of current, but if floating at same speed as river current can't tell which direction you are moving unless you look at the shore for orientation.  So how do the moths know which direction is upwind?

Taxes They have to look at the ground and use

information from direction specific movement detectors.  Insects have these.  But there is still a problem. 

Moths (and other animals such as mosquitoes and some migrating birds) fly at night.  No point of reference.

So capable of something more complicated than simple orientation.  

Important definitions Navigation. The process by which cues are

used to determine both the present location of an animal and the location of its target (e.g. home).

Compass. The mechanism by which direction (e.g. North or West) is known.

Map. The ability to know the direction an animal must take to reach its goal.

 Solar orientation Wasps often inhabit a featureless environment and

hunt for over 1/2 hr.  How do they find the way back to their nest?

Tinbergen said they memorized the sun's direction and then performed some sort of constant angle phototaxis to get back home.  Problem because the sun’s azimuth (angle between the sun and the horizon) changes while the wasp is hunting.  The difference can be significant, making any memorized direction invalid later.

A  wasp flying 1 km to hunt near noon could be thrown off, if went home by using the original angle.  Could miss nest by 100m.  Not so much for us, but imagine if we were the size of a wasp.

Solar orientation

 But wasps find their way home.  We now know that virtually all animals that need to can perform the sun compensation necessary for true solar navigation.

Honey bees Forage for food in an irregular

path.  Find it and then go straight home.  Bees which dance in the hive for a long time compensate for sun's changing direction by adjusting the angle of the dance.

Honey bees

If they are messed up with the experiment on the right they can't get back home.  Shows that they don't have a map sense of where home is, but are using the sun and a compensating mechanism to find their way home.

Sun compensation The worker who leaves a dark hive to return

to a food source has to adjust for the movement of the sun while she was in the hive.

Need to stretch our human centered mind to imagine a mechanism and design an experiment to see if it fits.

Three possibilities.

Sun compensation Bees could somehow know the arc of the sun in the

sky and perform spherical geometry to compensate. They could use the average speed of the sun (15o

per hour).  If they use this, the error over the few minutes spent unloading her food and dancing to recruit others would not be very great.

Could remember the rate of movement of the sun when they entered the hive and extrapolate from this.

Which?  How to tell?  Best time to compare the hypotheses is around noon of the summer solstice, June 22, when the sun is moving its fastest.

Experiment.  Train foragers to a feeding station and let them forage for several hours, then close the hive so the bees cannot see the sun.  Mimics conditions on a rainy day.  Foragers can come into the hive, but none can get out. 

Set up a series of feeding stations and move hive to a new location so that the bees can't "cheat" by using landmarks to orient to the food source.  Wait 2 hours, open the hive, and see where the bees go.

Sun compensation Some learning necessary, too, because sun

moves either clockwise or counterclockwise depending on which hemisphere bees are in (and in tropics at different times of the year).  Can't be preprogrammed because bees can't adjust to the change if moved from one hemisphere to another.  Critical period.

Polarized light orientation What do animals do when the sun is not out? Tidal crustaceans.  Run to ocean where

safe when danger threatens.  Placed on opposite side of island and ran onto the land. 

Must be using sky (celestial) cues.  Can test by blocking sun and showing them a mirror image in wrong direction and the animals go wrong.  Shows sun important.

But when sun blocked and no substitute given, animals went in correct direction.  Must have backup, but what is it?

Von Frisch and his bees help to solve the puzzle. When bees dance on a horizontal surface where they can see

the sun or the sky, their dance points directly at the food source. Von Frisch manipulated things.  Found that if the view of the blue

sky was obscured so that only 1% showed, bees oriented correctly.

Used optical filters.  UV blocking filter, ruined dance orientation.  Such a filter is like window glass to us.  If UV transmitted through a filter but light visible to us is blocked (it looks black to us), bees orient correctly.

Polarizing filter used and rotated.  Bees could orient with only a tiny patch of polarized light.

So bees use 2 cues, polarized and UV light, neither of which we can see.

How can polarized light provide a map? Light coming from a blue sky is partially polarized

and there is a pattern that is formed which is symmetrical around the sun. 

Pattern arises because of the scattering of unpolarized light from the sun as it bounces off molecules in the atmosphere.  

Light that is scattered perpendicular to the incoming light is completely polarized while light scattered at other angles is less polarized.  This information can be used to locate the sun even when it is hidden.  Problem is that the information will give 2 locations and the bees have to decide which is correct.  Not clear that they can do this.

Problem is that the information will give 2 locations and the bees have to decide which is correct.  Not clear that they can do this.

Common problem in animal behavior is that information is available to animals, but can't be sure that they are using it.  Can perceive polarized light, but  do they use it?  Every experiment to document how animals use polarized light has been followed by at least one contradictory experiment.

Magnetic field orientation

How to explain the ability of bees and other insects to orient on cloudy days when there is no sun and no polarized light is problem.  How can we show that they use the earth's magnetic field?

See the article on the magnetic sense posted on my web site.

Force bees to dance without providing any view of the sky and after a few days they orient in 8 compass directions (N, NE, E,....). 

If the earth's magnetic field is cancelled out or rotated by magnets orientation is lost or shifted in a predictable way.

There is adaptive value to be able to use minute daily changes in earth's magnetic field as way to calibrate bee's clocks.  There is evidence that bees can do this.  Enables them to go to flowers which are open at specific times of the day.

Migration and Navigation

Vertebrates

If insects can navigate, vertebrates ought to do equally well.  Many examples in text of birds and mammals migrating vast distances.  But some of the systems they use are much simpler than those of bees.

Simplest systems

European warblers.  Long distance flyers from Europe 500km to Central and S. Africa.  Do it with prewired program that says fly SW for 40 days then SE for 20-30 days.  How do we know this?  Orientation experiments in lab.

Small N. A. songbirds are programmed to wait for a cold front to pass over and then to fly SE from the N. American coast out over the Atlantic.  If they are lucky the winds of the cold front will push them to their destination in S. A.  If the winds fail, they die.

Maps and Compasses. Often it gets more complicated.  Albatross

reach small islands in the Pacific to breed and do this after many  months at sea. 

Sea turtles reach reach tiny Asunción Island from as far as 3,000km away 7 years after they hatched there. 

Nesting birds have been displaced 6,000 km and returned to their nests in almost the time it would take if they flew straight home.  Must have map sense (the ability to judge where it is in relation to where home is).  Two kinds.

Graded cues.  Simplest system.  "Hot" and "cold" system.  Animal measures and compares cues such as scent and follows the gradient to "home".  Probably system used by salmon.  Don't need either map or compass for this.

Coordinate system.  This is the system we use when we use longitudes and latitudes or street numbers to find a destination.  There are many possible grid systems available to animals.

Maps and Compasses

There is evidence that some birds have both map and compass ability.  We tinker with the system so that their compass is thrown off 90o.  If the birds always fly 90o to the right of home it shows that they had both a compass and a map.  They knew where home should have been.

Celestial orientation

Many migrating birds use both the sun and stars as compasses.  When the position of either one is shifted, birds shift accordingly. 

Experiments in planetariums show that birds can locate a pole star, identify the direction of celestial rotation and learn constellations.  

Shows they have a compass, how can we show they have a map sense? 

Clock-shifting experiments.  By turning on and off lights it is possible to shift the internal clock of a bird.  If they are clock-shifted 6 hours early, what time will they think it is at actual dawn?

Noon. Their compass will be rotated 90o to the left.  If birds are using the sun for a compass they fly 90o to the left of home from any direction they are taken.

Magnetic compass orientation But need a backup for cloudy days.  Clock-

shifted pigeons fly directly home on cloudy days if they are trained in cloudy conditions.  Need to train them in nasty weather when they are young.

Birds use a magnetic compass under these conditions.  Show with experiments where magnetic field is messed up.

The map sense. So birds can use the sun and magnetic field

for a compass, but bees do this, too.  Birds differ because they also have a map sense.

How do we know?  

The map sense Put contact lenses on homing pigeons which

distorted their sight.  Birds homed normally until they were in the

vicinity of their loft.  Then they either wandered around the vicinity or landed awkwardly and walked home. 

These experiments showed that the birds knew the position of home to within 5km.  At this point they used visual cues to find the loft.

How did we navigate before we had radio, satellites and computers?  Used a clock, sextant, patience and sunny day. 

The sextant measures the elevation of the sun.  The observer can compare the time that the sun is highest in the sky (local noon) with a clock and use this information to find the E/W position.  For example, noon in NY is 3 hours before noon in LA and each 4 minutes of difference represents 1o of longitude. 

For latitude the observer can calculate the highest elevation of the sun at noon.  Sun is lower in the sky in Chicago than in Miami and each degree of elevation corresponds to 1 degree of latitude.

So, with a sextant and an accurate watch a human can construct a map, determine how far he or she has been displaced on the earth's surface and then calculate the direction in which he or she has been moved. 

There is still a problem, because humans still need a compass to get home.  North of the tropics the sun is always in the south at noon so the sun can be the compass if the traveler is lucky enough to have the sun shining at noon.  But with a compass, sextant, charts and a good pocket computer the same calculations can be done for any local time of day.

Pigeons do it differently!   Again we use those birds clock-shifted 6

hours early.  Take them 100km south for release at dawn.  Birds see the sun low on the horizon at what they believe to be noon and should think they are 4,00 miles west of their actual location if they are using the sun for a map. 

In this case the birds should fly east to get home, no matter what they use for a compass.

If they have a true map sense they will know they are 100 km south of home. 

Now they need a compass to tell them where north is.  They must go north to get home.  If they use the sun for a compass, they will fly away from the dawn sun because they think it is the noon sun in the south.  They will mistake east for south and fly west. 

With a true map and a compass other than the sun, the birds would go north.

Result:  On sunny days pigeons go west, on cloudy days they go north.

So what sense do pigeons use for a map? Low frequency sounds.  Pigeons can hear them

and meteorologists have shown that such sounds produced by storm systems travel hundreds or thousands of miles.  Topography of the earth provides a myriad of acoustical signals.  Mountains, forests and rivers hum and sing as the wind passes over them and each feature has its own particular sound. 

We can't hear this, but pigeons could use a comparison of the sound sourced from home (which they had learned) with the sound landscape of the release site and construct a map from this.

Olfactory map Birds learn to associate odors with wind direction. 

An onshore wind may always smell salty or a wind from a nearby forest may smell "piney".  Displaced birds could recognize their local odor as one which always came from the NE at home and fly SW to get there.  There are many questionable experiments to determine this.

One way is to present the pigeons from smelling anything and see how this affects their ability to find home.

Recent studies tend to support a rule for olfaction.

Olfactory map in pigeons One study involved birds that were raised in lofts

where an olive oil odor was always blown in by a fan from the same direction.  Turpentine was blown from a different direction. 

Now a drop of one of the odors was placed on the beak of the bird and the bird released.  The birds always flew off in the direction correlated with the directions in which the odor was blown, not towards home.  Still could be other factors not controlled for.

Magnetic map This is the one given the most attention, but as the

recent article in the news points out, pigeons have so many homing devices, that it is extremely difficult to foul them up permanently. 

We now know that birds have a mechanism to calibrate their compass and their magnetic bearings, so this may be the site of their map sense.  The sense has to be extremely accurate because to use it for a map it is necessary to measure field strength to one part in 5000 or better. 

It is possible, but still not proven.  The map sense is still a mystery. Be sure to read about this in your text.