Reasons to Move: Why do Animals Migrate?

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Few sights in the world are more mesmerizing and humbling than the mass migration of animals. Every year millions of animals, ranging from mammals, birds, reptiles and fishes to insects and crustaceans, leave their habitats and migrate to another location for a limited period. But what is migration?

Migration can be defined as ‘persistent and directional movement over a large geographical area and involves voluntary locomotion or effort on the part of the migrating animal.’

Migration is energetically very expensive for animals. That is why only a small fraction of all animal species migrate. But the interesting question is, why do animals migrate if it requires a lot of energy? There can be multiple reasons for animal migration, all of which ensure the survival of the species. Most migrations are triggered when the food resources in an area are not enough to support the entire population. Migration allows these populations of animals to move from an area where food resources are scarce to an area of more abundance. This behavior also prevents the long-term depletion of food resources in that area. The second most important reason for migration is reproduction. However, there can be other reasons such as local climate, season or temperature which influence animal migration by affecting food and reproduction.

A few famous examples are as follows:

1. Birds: Approximately 20% of all bird species migrate annually, most of which are seasonal in nature. The most famous example is the arctic tern which migrates annually from the arctic breeding grounds to the Antarctic region during winter, seeking fresh feeding grounds and escape the cold weather, and back in the summer, covering approximately 19000 km every year.

2. Mammals: Every year during the dry season millions of zebra, wildebeest, and gazelles in and around the Serengeti National park in Africa, begin their annual migration to find drinking water and fresh pastures. It is the largest terrestrial mammal migration on earth. These herds can change the direction of their migration, to adapt to environmental conditions, such as the noise of thunder which is an indicator of rain. Migration is also crucial for the survival of many species of bats and marine mammals like whales and dolphins.

3. Insects: One of the most famous migrations in the world is that of the monarch butterflies. It is a remarkable feat of evolution, because the migration takes 3 generations of butterflies to travel from Mexico to Canada. This migration is necessary because the plants on which these butterflies feed don’t grow in southern regions.  The fourth generation (sometimes called super-generation) travels back from Canada to Mexico to escape the cold temperatures and over-winter.

4. Fish: Every year millions of salmon from different species migrate thousands of kilometers from the oceans to the rivers where they were born to breed and lay eggs because the larva can only grow in fresh water.

5. Other animals: The Christmas Island red crabs undertake majestic mass migrations, which looks like a moving red carpet from the air, every year to breed at the coastal regions. This is because, unlike salmon eggs, the crab eggs and larvae can only grow in ocean water.

Animals don’t use calendars of course. Then how do they know when to migrate? Many species of animals rely on photoperiod i.e. they track the duration of the daylight. As the length of the day becomes shorter or longer, animals instinctively know which season it is and which season is approaching. They can then decide whether to migrate or stay. Other species rely on sensing temperature or food availability which also varies in a cyclical pattern along with seasons. While food driven migrations are seasonal and very regular, there are many factors which affect the availability of food. For this reason, many species use irregular migration patterns. For example, species which reproduce very fast such as locusts and some rodents, can cross the carrying capacity of their environment under favorable conditions. This leads to famine like situation for the next generation, which then undertakes a mass migration of epic proportions. This type of migration is known as irruption. Swarms of migrating locusts are waking nightmares for many people.

Migration can be unidirectional i.e. the animal never comes back to its original habitat (example: human ancestors migrated out of Africa and settled in Europe and Asia when conditions were not hospitable in Africa), or circular i.e. they come back when the environmental conditions are favorable again (example: bird migration).

 Author: Rahul Sureka, CSIR – Centre for Cellular and Molecular Biology, Hyderabad.

(Image Source: http://www.earthporm.com/26-incredible-photos-animal-migration/)

References

http://animals.howstuffworks.com/animal-facts/animal-migration.htm

https://www.nationalgeographic.org/activity/why-animals-migrate/

Attenborough, David (1990). The Trials of Life. London: Collins/BBCBooks. p. 123. ISBN 0-00-219940-8

http://www.stuffyoushouldknow.com/podcasts/animal-migration-wheres-that-gnu-gnoing.htm

https://en.wikipedia.org/wiki/Animal_migration#cite_note-2

http://www.monarch-butterfly.com/monarch-migration.html

https://en.wikipedia.org/wiki/Christmas_Island_red_crab#cite_note-Christmas-5