Migration

Why do animals migrate? Monarch butterflies go southeast from Canada and the northern US to overwinter, then return northeast to lay their eggs I nmilkweed plants which will feed their larvae when they hatch. Only fly during the day. Is light a cue to migrate?

Is migration a heritable trait? Naïve young black-capped warblers raised indoors in cages exhibit increased nocturnal activity and agitation consistent with preparation to migrate Similar to their parents, who were housed similarly (and whom had presumable migrated previously) Direction is also conserved/innate

Migration timing under selective pressure Two generations of breeding late migrating individuals produces a robust late-leaving population

DQ Why do black redstarts show less restlessness? What does the data on the F1 hybrids tell you about the genetic basis for restlessness preceeding migration?

When the light was turned on earlier in the day (like spring) butterflies wanted to fly east, like they would in spring. Captured in the fall, held under these conditions for a few days before testing.

Circuit for migration UV wavelength is essential for migration. Block UV with a filter and monarchs stop flying. UV detecting cells in the eye project to the clock of the butterfly brain, which communicates to the compass. Disinhibition?

Use of polarized light cues Use of polarized light to maintain heading during migration. Using a filter that changed the orientation of the polarized light, researchers showed that butterflies use the angle of the light relative to the earth to select their flight direction. Compare to using a prism to shift visible light.

Green sea turtles migrate at night

Turtles sense the Earth’s magnetic field Experiment performed in a pool where the turtle was tethered and there were coils surrounding the pool that allowed scientists to change the perceived local magnetic field, which altered their swimming orientation.

Magnetic field detectors in bird beaks and controversy Long thought that birds possessed neurons in the beak that detected magnetic fields “map-sensing” cells Have magnetite, a mineral that orients within a cell according to the earth’s magnetic field The compass and the map work together, with the magnetite cells perhaps detecting not only the magnetic field but metal filled landmarks along the migration route Thought to be conserved from bacteria to sea turtles to pigeons *But the cells in the beak were recently shown to be more consistent with macrophages (high iron, no nucleus) than sensory neurons (Keays D. 2012)

About half of breeding birds in North America migrate annually If sedentary species were ancestral, how did these long migrations evolve? Can go 40,000 kM a year

Short range migrants precede long range migrants evolutionarily Of 9 families related to this bellbird, all migrate, some short and some long range. Suggests that short range migration comes before longer range migration. Dispersal probably precedes short range migration, which would arise if animals dispersed into a land where feeding or breeding was limited by changes in season.

Cost-benefit analysis For migration to incrementally evolve, the benefits must outweigh the costs Costs are substantial Body weight gain prior to migration Easier prey But flight is optimized at migratory weight for some Energy expended in flight, often over water for days at a time

Adaptive value of V-formation Migrating birds must evolve benefits that outweigh costs associated with migration.

The risk of drowning affects migration routes Body condition predicts migratory route. Those with more fat are more likely to cross the gulf while those with less fat reserve hug the coast.

Why take the risky ocean route? 50-90 hours of continuous flight Return flight is inland. Why?

Benefits of migration Food Water Temperature Protein rich insects in northern summers No young to care for in plentiful southern winters Water African herds (zebras, gazelles, wildebeasts) detect the salinity of the water and head north, following rainfall Temperature Monarchs balance cool and moist (small risk of freezing) in the Oyamel forests against warmer and dryer (risk of dehydrating) in areas further south

Monarchs in the fir trees of Oyamel forest Human logging is opening up the forest canopy and exposing butterflies to wet and cold conditions.

When some animals migrate and others don’t: Conditional strategy. Individual blackbirds change their behavior from year to year, suggesting it’s not genetic. If it’s a conditional strategy, all birds should be able to choose the highest fitness alternative given the environmental conditions and their own social status. Maybe dominant males stay resident to take advantage of limited resources, while subordinates migrate to find food to survive, then return to occupy vacated territories or strong enough to compete with former residents.