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Great Circle Hypotheis  

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Magnetic-Latitude Hypothesis

Compass Bearings Hypothesis

Suns' Azimuth Hypothesis

Expansion-Contraction Hypothesis

Always Advance Hypothesis

Never Go Back Hypothesis

 

 

Hypotheses On Navigation By Migrating Monarch Butterflies

globe                                      Sextant

Schmidt-Koenig Great Circle Hypothesis

Basic Idea:

Natural selection has favored migrating monarch butterflies navigating by following local Great Circle routes to the overwintering sites in Mexico.


Assumptions for Eastern Population

  1. The only goal for migrating monarch butterflies is the overwintering sites in Mexico.
  2. Monarch butterflies have some means of determining the true bearings for local Great Circle routes to the overwintering sites in Mexico.

Assumptions for Western Population

There are at least three possibilities for the western population.

  1. The western population is continuous with the eastern population and have not evolved a local migration. The butterflies 'know' their location relative to the overwintering in Mexico and attempt to fly local Great Circle routes to this destination. Although there are no published observations indicating that migrants in the western population consistently attempt to fly SSE towards the overwintering sites in Mexico, this may be the case for the region of Idaho, Montana, and Wyoming.
  2. The western population may or may not be continuous with the eastern population but the butterflies have independently evolved a local migration. Consequently, the assumption that the overwintering sites in Mexico are the ultimate goal does not apply. Instead, the butterflies in the western population migrate towards a narrow band of favourable habitat along the coast of California. Monarch butterflies are believed to migrate to the coastal overwintering sites from the entire region west of the Rocky mountains extending south from about Latitude N32° to the northern limit of milkweed distribution at about Latitude N49°. Only the north-western section of Washington is excluded. The butterflies migrate out of a vast region that extends about 1900 km from north to south and funnel into a narrow band that extends from about 1,200 km from Latitude N32° to Latitude N40°. This convergence suggests that if the butterflies have a evolved a separate Great Circle route, the 'goal' is located far off shore.
  3. The western population may or may not be continuous with the eastern population but the butterflies have not evolved a local migration. The migrants are will outside the region where their navigation system can function and are lost. While they may be following a distorted version of the Great Circle route fly in a directed manner in response to particular stimuli, such as wind direction, this behaviour is not adaptive. Even though some individuals from inland locations manage to reach overwintering sites along the coast, this is largely a matter of chance. Most of the interior population dies off as the habitable range of the western population contracts to the band of favourable locations along the coast and some scattered locations inland (See Wenner's model).


Limitations of the Hypothesis

  1. Hypothesis provides limited insight into navigational mechanisms because it does not explain how migrants are able to determine their location on the Globe relative to the location of the overwintering sites with sufficient precision to calculate Great Circle routes.
  2. Boundaries of geographic region where hypothesis applies are not known, particularly in west.
  3. May not apply to western population.
  4. Additional assumptions are necessary if hypothesis is applied to western population (see Remarks).


Advantages of navigating by following Great Circle routes

  1. Monarch butterflies are always following the shortest distance from their current location to the overwintering grounds.
  2. The migrants can not lose their way as long as they remain within the geographic region where the model applies.
  3. Because of (2), the butterflies can fly high above the ground in a wide range of wind conditions (e.g. right crosswinds, tailwinds and left crosswinds).


Problems of navigating by following Great Circle routes

  1. Great Circle routes to the overwintering grounds in Mexico originating from most of eastern North America cross the Gulf of Mexico.
  2. If Great Circle route to a single goal is followed by western population, this goal is located far off shore.
  3. Because of the curvature of the earth, butterflies flying in any direction except directly N, E, S, or W, must continuously alter their bearing to remain on course.


Problems in testing the Schmidt-Koenig Great Circle Hypothesis

  1. Predicted bearings for Great Circle routes are similar to the predicted bearings for magnetoclinic routes for a large part of the range of the eastern population.
  2. The geographic boundaries of the region in which the hypothesis applies are not clearly defined. Western population may not be included (see Remarks).
  3. Goal not clearly defined for western population.



Tests of the Schmidt-Koenig Great Circle Hypothesis


Prediction for Eastern Population

  1. Preferred directions of migrating monarch butterflies at observation sites are identical to local Great Circle routes to the overwintering sites in Mexico at Latitude N19°30°', Longitude W100°20'.

Predictions for Western Population

  1. Assuming that monarch butterflies have not evolved a local migration, their goal is overwintering sites in Mexico and preferred directions of migrating monarch butterflies at observation sites are identical to local Great Circle routes to overwintering sites in Mexico at Latitude N19°30', Longitude W100°20'.
  2. Assuming that monarch butterflies have evolved a local migration, their goal is a band of favourable habitat along the California Coast extending from about Latitude 32° to Latitude 40°. A line extending from Latitude N49°, Longitude W120° along the Washington, British Columbia border extended SW crosses the N32° Parallel at about Longitude W135°, or 1300 km WSW of San Francisco. As a first approximation, this point in the Pacific Ocean will be considered the 'goal' of the western population. the preferred directions of migrating monarch butterflies are identical to local Great Circle routes to this goal. The butterflies are assumed to stop migrating when they reach the coast of California within the band of favourable habitat.


Methods for Eastern Population

  1. Determine the bearing for the local Great Circle route from the field site to center of the region in southern Mexico where the overwintering sites are located (Latitude N19°30', Longitude W100°20').
  2. Record at least 25, and preferably 50, Category I vanishing bearings, or at least 50, and preferably 100, Category II vanishing bearings of migrating monarch butterflies. If possible, Category II vanishing bearings should be obtained over a range of wind conditions.
  3. Convert magnetic vanishing bearings to true bearings by correcting for the magnetic declination (variation) of the field site.
  4. Plot the data as a Rose Diagram (a circular histogram). See the "Methods of Observing Migrating Butterflies" section of the Red Admiral and Painted Lady Web Site for an example of a Rose diagram. A string of dots, with each dot representing a single observation, can be used instead of bars.
  5. Add the Great Circle direction to the Rose Diagram.
  6. Calculate the mean vector (mean vanishing bearing), as shown in Batschelet (1981) or Zar (1996), and add to Rose Diagram.
  7. Use the Rayleigh test in Batschelet (1981) or Zar (1996) to test if the calculated mean vector is significant. Proceed with analysis only if the Rayleigh test indicates that there is a significant mean direction to the data.
  8. Look up 95% Confidence Interval for mean bearing in Batschelet (1981) or Zar (1996)
  9. Indicate the boundaries for 95% confidence limits on the Rose diagram (i.e. equal to mean bearing ± 95% C.I.).
  10. Examine the Rose Diagram. If the data are obviously skewed to one side, or bimodal (two peaks), you should not proceed with the analysis. Data analysis to this point has been an exercise in descriptive statistics. More complicated procedures are necessary to continue. If the data are not obviously skewed or bimodal, proceed to Decision Rules.

Methods for Western Population

  1. Assuming that the goal of the butterflies is the overwintering sites in Mexico, start at step 1 for eastern population.
  2. Assuming that monarch butterflies in western population have independently evolved a local migration and that the 'goal' is a point in the Pacific Ocean at Latitude N32°, and Longitude W135°:
    • Determine the bearing for the local Great Circle route from the field site to Latitude N32°, Longitude W135°.
    • Proceed to step 2 for eastern population.


Decision Rules

  1. If bearing for the local Great Circle route being tested is not included within the arc delineated by the 95% confidence limits of the true bearing for the preferred direction, than the two bearings are significantly different and the hypothesis is rejected. Preferred directions of Monarch butterflies are not local Great Circle routes to their goal (i.e. goal for eastern population is overwintering sites in Mexico; 'goal' of western population is either the overwintering sites in Mexico (no independently evolved local migration) or a point in the Pacific Ocean located at Latitude N33°, Longitude W135° (an independently evolved local migration).
  2. If bearing for Great Circle route being tested is included within the arc delineated by the 95% confidence limits for mean bearing (true), then the two bearings are not significantly different and the hypothesis cannot be rejected. Local preferred directions of migrating Monarch butterflies may be Great Circle routes to their goal.


Remarks

The Great Circle hypothesis is taken from Schmidt-Koenig's (1979) paper on mean directions of migrating monarch butterflies at sites in the eastern United States. He found that at a number of sites the mean direction of the migrants could not be distinguished from the Great Circle direction to the overwintering sites. Although the hypothesis does not specify the mechanism by which the butterflies navigate, Schmidt-Koenig (1979) suggested that the butterflies may use some type of celestial mechanism other than a sun compass because their mean direction was not affected by overcast skies. Additional field studies and a reanalysis of the data led Schmidt-Koenig (1985) to tentatively conclude that mean directions of the butterflies in the eastern United States fit the predictions of Kiepenheuer (1984) Magnetoclinic model.

globe on standThe western population represents a serious challenge to the model. There are no published observations indicating that migrants from the western population consistently attempt to fly SE towards the overwintering sites in southern Mexico. However, each year concentrations of monarch butterflies occur at overwintering sites along the coast of California in Autumn and Winter. Tagging data by Fred and Norah Urquhart indicates movement from the interior third of the western United States to the coast. There are at least four hypotheses that could explain the data:

  1. Wenner and Harris (1993) hypothesis. Monarch butterflies in the western population are not migratory. The population experiences a range expansion from the coastal regions in the spring and a range contraction bask to the coastal regions in the autumn. These movements are not directed but occur though haphazard dispersal. Most butterflies located outside the coastal regions perish during the Fall.
  2. Inappropriate behaviour hypothesis. Butterflies in the western population are migratory but the observed pattern is an artifact resulting from an inappropriate expression of the eastern population migration pattern. The butterflies are programmed to carry out the migration but their navigation system is 'unaware' of their western location. The butterflies are attempting to navigate by following Great Circle routes, but their navigational mechanism is confused and simply directs them S to W until they reach favourable habitat along the California Coast.
  3. Locally evolved migration hypothesis. Monarch butterflies in the western population are migratory, but have independently evolved a local Great Circle migration suitable for the region. The theoretical goal of the butterflies is located at an appropriate point in the Pacific Ocean to funnel the western population into the narrow strip of favourable habitat along the California coast.
  4. Conditional migration hypothesis. Butterflies in the western population are migratory and are exhibiting one of two distinct migration patterns characteristic of the species. Each butterfly has the capacity to express either the western or the eastern migration pattern, depending upon circumstances. At the end of the summer, individuals that are on the east side of the Rocky Mountains express the eastern migration pattern while those that are sufficiently west of the Rocky Mountains express the western migration pattern.

Because recent genetic testing of monarch butterflies showed no significant differences among the two populations the question of whether the western population is isolated remains open. The first hypothesis could apply whether or not the western population is isolated. The western population has been isolated for a sufficiently long time for the butterflies to lose the capacity for long distance migration. Alternatively, the western population may not be isolated but the butterflies do not receive the appropriate combination of stimuli to trigger migratory behaviour on the west side of the Rocky Mountains. The second hypothesis implies that the western population is either the result of a relatively recent founding event, or that there is sufficient gene flow from the eastern population into the western population to swamp out local adaptation. In contrast, the third hypothesis implies that the western population has been completely, or nearly completely, isolated from the eastern population for a sufficiently period of time for a local migration to evolve. Finally, the forth hypothesis implies that sufficient gene flow occurs in among the two populations to maintain two distinct, facultative, migration patterns. In addition, the butterflies are somehow able to determine at the appropriate time whether they are on the west side or the east side of the Rocky Mountains and respond accordingly.


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