Migration and Navigation

I. Examples of migration and homing

A. Many animals move to follow food supplies, to avoid bad weather, or to avoid predators even though travel is dangerous.

B. Examples

1. Large mammals, such as wildebeest in the Serengeti, or bison in the American plains, move with the seasons to follow grass growing periods. Elk and caribou move to avoid cold weather and maintain access to feed.

2. Gray whales migrate from Alaska around the tip of Baja California to give birth and mate in the Sea of Cortez. They avoid cold water and predators. Some incidences of whale strandings, such as this pod of sperm whales, occur near magnetic anomolies, suggesting that whales may be using magnetic cues for navigation.

3. Most temperate bats hibernate, but utilize large communal hibernacula which they return to every year. Some bats also migrate such as the Mexican free-tailed bat which feeds on high flying moths and the Lesser Long-tongued bat which is the primary pollinator of Agave and columnar cacti in the Sonoran desert. These bats migrations match when these desert plants are in flower.

4. In this country 245 species of birds migrate to Central or South America including herons, swallows, flycatchers, hawks, falcons, owls and warblers. In Europe similar trips take storks, warblers and other birds to Africa.

5. The premier migrators are seabirds. The longest migration route is that of the Artic Tern which flies from Canada to Antartica and back each year. The slender-billed shearwater flies from S. Australia to Alaska and back.

6. Some insects migrate, also. Monarch butterflies - eastern US populations converge on a few sites in the hills outside Mexico City. They overwinter where the climate is cool and damp. Other overwintering sites occur in California.

7. Some insects, such as the desert ant, (and many other animals) also show phenomenal ability to return directly to their nest after circuitous foraging.

What must the animal possess to be able to perform these navigational feats?

Answer: a compass and a watch

II. Biological clocks

A. Evidence

1. Some animals possess an endogenous circannual (yearly) clock

a. hibernating mammals

b. seabirds which cross the equator

2. Most animals (vertebrates and invertebrates) show a circadian (daily) cycle of activity

a. Circadian rhythyms can be entrained to photoperiod, even flashes of light, although most animals show some delay, i.e. jet lag
b. Circadian rhythyms will persist in darkness

B. Physiological and genetic basis of the clock

1. Mammals - suprachiasmatic nucleus
2. Birds - pineal gland - which is directly sensitive to light
3. Melatonin release (a neurotransmitter) entrains to photoperiod. Can be used to overcome jet lag
4. The clock also has a genetic basis, the Period (Per) gene

III. Mechanisms of navigation

A. Evidence for a compass

1. Starlings transplanted from the Netherlands to Switzerland migrated along the correct heading, but ended up in Spain instead of France

B. Source of the compass

1. Sun

a. Orientation experiment
b. Clock-shift experiments - indicate animals compensate for time

2. Stars

a. Nocturnal birds learn the north star during first summer
b. Planetarium expts show that the star is arbitrary

3. Polarized light can be used to determine the direction of the sun on cloudy days

4. Geomagnetic field

a. Pigeon homing on cloudy day with magnets

b. Pigeon homing in areas with magnetic anomolies
c. Turtle swimming in a Helmholtz coil
d. Earth's field has polarity, inclination and intensity. Birds sense inclination.
e. Some evidence indicates that humans orient to magnetic north

C. Landmarks

1. Gray whales probably use coastline
2. Experiments with insects show they rely on landmarks near their nest