Homing and Migration | Zoology Optional Notes for UPSC PDF Download

Homing and Migration

Homing
Animals "home" when they return to a central place, such as their nest or their territory. Homing is a frequent activity, occurring after foraging bouts or other relatively local movements. This distinguishes homing from migration, which is a long-distance movement between two distinct habitats. The key element to understanding homing behavior is determining which cues provide the directional information that allows animals to move between their home and other locations.

Homing is the return of an individual to its home site.
Animals leave their homes to find food, mates, etc., and return for shelter, feeding of young, etc. Environmental cues such as light, chemicals, magnetism, and visual landmarks are used to find the home base.

A. Honeybees (Apis mellifera)
Like many other animals, honeybees forage from a fixed location, the hive. This type of foraging, often termed "central-place foraging," imposes a requirement for good navigation abilities; the animal must be able to find its way back home after a foraging bout which may take it away from visual or auditory contact with the nest and its residents. Honeybees may forage several kilometers from their hive, making them a good model species for studying navigation in central-place foragers.

Information used in home-based navigation can be divided into two categories:

1. Egocentric information: Generated internally by the animal and is independent of its immediate surroundings. Internal calculations of the distance and direction traveled, which are used in path integration, are examples of egocentric information.
2. Geocentric information: Includes landmarks and any map information available to the animal. Honeybees primarily use path integration in making their way to and from foraging sites. Dance information provides outgoing bees with a distance and direction to be traveled. Flight direction is determined by a sun compass orientation, and the distance of flight by an internal "odometer" that measures the rate at which visual images flow past the eyes. Other inputs, such as odors, provide supplementary information. Once a route is learned, bees incorporate visual landmarks when they make repeat trips to a foraging site. The return trip is governed by path integration as well but also may be informed by landmarks.

How do bees incorporate landmarks into their orientation?
Two basic models, snapshot memory, and cognitive maps, have been proposed. The simplest, and probably correct, model calls for the bee to remember a series of visual images ("snapshots") of the landscape as it passes. The bee also remembers images of particularly prominent landscape features. These images can then be compared with the actual landscape surrounding the bee at any given moment. More complex is the "cognitive map" model, which requires the bee to construct a relatively complete neural representation of the landscape based on its experiences while flying. Tests of the cognitive map model require that displaced bees calculate a novel route home, based on their memory of the landscape map (as humans might).

B. Homing behavior in pigeons (Columba livia)
Pigeons find their way home from unfamiliar sites up to thousands of kilometers from their roost. Pigeon races may feature releases of birds from France, for example, which then find their way home to sites in England or the Netherlands. The extraordinary reliability of homing pigeons makes them excellent subjects for studies of navigation.

How do pigeons find their way home when deposited in an unfamiliar location?
To do this, they must have two kinds of information:

1. The map sense: Their geographic location. In familiar surroundings, landmarks play a predominant role in homing. Pigeons learn visual features of the landscape and use these visual features to determine their current position (map location) relative to their roost. When released in a completely unfamiliar location, they use olfactory cues to assess the odor of their new location and extrapolate the map location from their roost-gained knowledge of winds and odors.
2. The compass sense: The bearing they need to fly from their new location to reach their home. The primary compass information of pigeons comes from the position of the sun in the sky. They compensate for the apparent movement of the sun across the sky by integrating their internal clock with the sun's position. Pigeons whose time sense is shifted display incorrect orientations when released. The pigeon's sun compass interacts with a magnetic compass. Under some conditions, experimental modification of the magnetic field around pigeons causes problems in homing. However, the olfactory and landscape information used in establishing their map sense can be used to correct for compass misinformation.

Migration

Regular, annual, or seasonal mass movements made by animals from their breeding area to another area.
"True migration" refers to the animal making a return trip back to the original breeding grounds, while "one-way migration" often occurs when the habitat deteriorates or the food source is depleted.

Eastern bar-tailed godwits:
One exceptional long-distance migrant is the eastern bar-tailed godwit, which makes a record-breaking non-stop flight over the Pacific Ocean from Alaska to New Zealand. This journey of 11,000 kilometers takes six to eight days and nights of continuous flight, averaging over 60 kilometers per hour. The departure is timed to gain assistance from favorable winds. The birds arrive exhausted and weighing less than half their original weight. They remain for around five months before making the return journey along the western edge of the Pacific.

Why migrate?
Migration enables animals to live in an ideal environment with a good food supply all year. However, there are advantages and disadvantages to migration.

Advantages of migration:

• Animals remain in a favorable temperature
• They grow larger
• Constant food supply
• May lead to the colonization of a new area
• Reduces predation and disease from parasites
• Greater genetic mixing
• Better breeding conditions

Disadvantages of migration:

• May get lost or caught in a storm
• May get eaten
• May use too much energy and become exhausted
• May starve
• Migration is a huge investment of energy

Migrations can be triggered by internal or external factors:

• Internal factors: Maturing of the sex organs or genetic drive.
• External factors: Environmental cues such as shortened day length and a drop in temperature. Migratory birds sense a change in day length through skin and feathers, which is registered by the pineal gland. They become restless and fly for longer periods of time. This behavior is innate and shown by caged birds as well.

Navigation:
Navigation involves an animal finding its way over unknown territory to a known destination.

Methods used by animals to navigate (homing and migration):

1. Visual clues: Animals can use visual landmarks, the shape of coastlines, and topography of the land to navigate.
2. Solar navigation: Many birds and animals, such as honeybees, can use the sun as a compass. They appear to have an internal clock that adjusts for the changing direction of the sun. Bees use the sun to communicate the direction and distance of a food source to other bees in the hive through dances.
3. Magnetic fields: Some animals, like homing pigeons, whales, and dolphins, have magnetic compasses in their heads and navigate using the Earth's magnetic field lines.
4. Navigation by the stars: Birds that migrate at night use a star compass. They orient towards the part of the sky that appears to rotate the least, typically the brightest northern stars.
5. Chemical navigation: Animals like dogs and ants follow scents to find their way. Salmon use the smell of rivers and the surrounding environment to return to their natal stream.
6. Sonar sound navigation: Bats and humpback whales use sonar by emitting high-pitched sounds to navigate and detect objects in their path.