Migration Migration = a form of dispersal which involves movement away from and subsequent return to the same location, typically on an annual basis. To migrate long distance animals must navigate through the environment and use cues to assess where they are in relation to where the are heading.
Who migrates? Amphibians Mammals Fishes Sea Turtles Why do they migrate? Mostly for food
In N. America there are more hours of daylight that allow birds to forage on insects. longer days (summer) more plants more insects = more food for migratory birds than in regions with shorter days In addition to food Wildebeest, Zebras and Gazelles migrate to track water. Monarch butterfly migrates to Mexico for temperature benefit -- less chance of freezing during winter months than if in N. America.
Of course there are costs in addition to the benefits Birds need to double in weight prior to migrating. Extra weight slows down take-offs (and changes take-off angle) and makes birds more easily captured by predators. Some minimization of costs during migratory flights Using a V-formation
Evolution of migration Assume sedentary species were ancestral to migratory ones. How can the ability to travel long distances each year to specific destinations have evolved? Moreover, in some cases, migration seems to have evolved multiple times in a particular taxon.
The migratory pattern for thrushes (Catharus) 12 species: 7 which are resident to Mexico and S. America. 5 are migratory species breed in N. America and winter in Mexico and S. America Migratory behavior evolved 3 times.
Best idea: evolutionary precursor model of migration migration is associated with species that live in open edge habitat rather than species that live in forest.
Navigation mechanisms associated with migration Monarch butterflies use the position of the sun in relation to the time of day for navigation UV light help monarchs get started in the right direction Once in flight, they maintain their compass orientation by polarized skylight pattern In this case only a compass sense is needed. Which way is north or south?
How do you know the compass direction? 1. Use position of sun, and have an idea of what time of day it is. 2. Use earth s magnetic field.
Experimental manipulation of the butterfly biological clock changes their orientation.
Since you can t always see the sun position, as on a cloudy day, butterflies detect the plane of polarization of the sun s e-vector. The e-vector orientation shifts systematically throughout the day, and the U.V. (short wavelengths of light) penetrate cloud cover. Many flying insects have U.V. sensitive photodetectors.
To navigate over unfamiliar territory (not just head off in a particular compass direction) requires BOTH a compass sense and a map sense.
Turtle migration mechanisms
Loggerhead sea turtles Diagram from http:// www.unc.edu/depts/oceanweb/ turtles/ 16
Turtles use 3 separate mechanisms for navigation: 1. Find ocean use visual system 2. Swim in correct initial direction in ocean use vestibular system 3. Swim in correct direction in open ocean use magnetoreception 17
Hatchlings Turtles use light contrast to find the ocean; typically the ocean is brighter than the hills/coast behind. Although this picture shows otherwise, turtles usually hatch at night. Eggs moved from the east to the west coast of Costa Rica: Turtles were still able to go in the direction of the water, implying that turtles do not inherit a specific direction (Carr and Ogren, 1960). 18
Finding Open Ocean The hatchlings use wave direction to navigate out into pelagic waters. 19 http://www.unc.edu/depts/oceanweb/turtles/
Open ocean migration Deep open water - cannot use waves as a navigation tool. First orientate towards light, then use magnetic fields to keep going in that direction. Reversing the magnetic field after the initial orientation causes the turtles to turn around. Latitude recognized by detecting differences in magnetic field inclination and intensity. (Lohmann and Lohmann, 1994) 20
Move the turtles and they know which way to orient to get back to their original location.
Migration as a conditional tactic Migration can be a choice: Individuals of the same species can choose to migrate one year and not the next. Those that migrated the preceding winter often switched to resident option the following winter.
Leap Frog Migratory Pattern Fox Sparrows eliminate the need for competition by migrating to different spots The Alaskan populations migrate straight to California The Canadian population migrates to S. Canada and NW US. Essentially leap frogging eliminates the need for competition and return for breeding is not delayed as ample weight can be gained in a timely manner
Turtle references Carr, A. and Ogren, L. 1960. The ecology and migrations of sea turtles, 4: The green turtle in the Caribbean Sea. Bulletin of the American Museum of Natural History, Volume 121:Article 1, pp. 1-48. Available at http:// www.sefsc.noaa.gov/seaturtlepeerpublications.jsp Lohmann, K. J., Lohmann, C. M. F., Ehrhart, L. M., Bagley, D. A., and T. Swing. 2004. Geomagnetic map used in sea turtle navigation. Nature. 428: 909-910. J. and C. M. F. Lohmann. 2006. Sea turtles, lobsters, and oceanic magnetic maps. Marine and Freshwater Behaviour and Physiology. 39(1): 49-64. Lohmann Lab Website http://www.unc.edu/depts/oceanweb/turtles/ Caribbean Conservaion Corporation and Sea Turtle Survival League http://www.cccturtle.org/ 24