Migration is common in the animal kingdom, and is undertaken to avoid extreme weather conditions and predators, and find suitable habitats for food and breeding. Migrations are usually horizontal and cover up to thousands of kilometers. They can also be vertical along altitudes or between depths in seas. Migration is still an area of intense research, and much is unknown about this universal phenomenon.
Animal migration is an important biological phenomena that takes place all over the world, and is considered to be universal, as both land and aquatic species migrate (1, 2).
What is Migration?
Migration is the movement of large numbers or all the individuals of a species in search of suitable environmental conditions. The popular well known migrations are horizontal, with the distances covered varying from a few kilometers to thousands of kilometers, along the north-south, as well as east-west axes. Vertical migration along the altitude also occur. On land this can happen up and down mountains or between different depths in oceans (3, 4).
An important feature of migration is that animals travel from one habitat to another (3).
Reasons For Migrations
There are various reasons for animal migrations.
Seasonal Variations in Temperatures
Animals migrate to escape extremes in temperatures- heat or cold. Migrations are common from regions where there are distinct seasons, with cold freezing winters. The places that had ideal climatic conditions change into different landscapes in winter (5).
Search for Food
Accompanying the change in weather, are variations in food availability. In temperate regions where summer is mild, food in terms of seeds or insects are plentiful. However, in winter when the ground is frozen and nothing can grow there is no seeds, and insects which have a short life span die, so food resources also become scarce (5). For example swallows are summer visitors in temperate regions like North America or Europe when the temperatures and food is ideal, and they leave for southern countries in the end of autumn (5).
Sometimes food necessary for different stages of an animal differs so they migrate to places with suitable food resources. For example, after hatching young salmon migrate from small streams to the warm seas thousands of kilometers away abundant in food, so they can survive and grow, and then come back when they are adults to the streams (3).
The breeding grounds and habitats with ideal food are not always the same. For this reason the humpback whales undertake the longest migrations among mammals from the cold poles. For the whales the cold regions have abundant food, where they live in summer and feed. Then they travel to the warm tropical waters which in fact are poor in food, but are ideal for the growth of their young (6). During the whole winter adult whales do not feed at all. They return in summer again to polar regions, by which time they are close to starvation (1). Similarly, the salmons return to the small tributaries of streams which are the best places for breeding and laying eggs, so they travel back against the flow of the stream, to lay eggs and die (1, 3).
Predation is now included as a reason for short migrations among fish. In 2013, scientists established that Danish roaches move from lakes to streams to escape cormorants their predators (7).
Animals across the animal kingdom migrate regardless of size. They include birds, mammals, insects, fish, crustaceans, amphibians, reptiles, and slime moulds (3). They fly, walk, swim or drift carried by ocean currents (1).
The smallest migratory animals are 1-2 mm in size, these are the zooplankton in the ocean, like crab larvae and copepods (3). They live deep in the ocean during the day, but swim up to the surface everyday at dusk (4). They migrate vertically up 900 meters (8). The largest are the baleen whales weighing many tonnes and which are 24-27 meters long (3).
1800 species of birds migrate, with the longest animal migration recorded by Arctic Terns which fly 70,000 kilometers from Arctic to Antarctica (9).
Insects travel great distances covering thousands of kilometers too. However, since their life-span is short to outlast the migration more than one generation is involved. For example, Monarch butterflies from North America fly in autumn to Mexico. In spring they start their journey back north, but they stop in southern USA to lay eggs, and die. The next generation of butterflies covers some distance and lays eggs. These eggs hatch to give the third generation of butterflies that reach the north (1).
Similarly, dragonflies migrate from India, through Maldives, Sychelles, Mozambique to Uganda and back, covering 14,000 to 18,000 kilometers. Four generations are needed to cover one round trip (10).
A single species of migratory animal can show a range of behavior, some migrate, some do not, depending on the environment, as in the European black cap warbler (1). In some birds one gender will migrate while the other remains resident in the temperate areas.
Sometimes migratory animals suspend migration if their numbers dwindle. The pronghorn antelopes stopped migrating for 50 years until their numbers recovered in the USA (1).
Cues Triggering and Guiding Migration
Though migrations have been studied for long, there is still a lot of mystery shrouding this phenomenon (1). There are many possible cues triggering migrations.
Hormonal changes are one, as in Christmas red crab which migrate from land to the sea to lay eggs (11). Other reasons are changing length of the day, and genetic programming of the animals (1).
Birds have developed a complex system to guide them. Eyes detect star and sun patterns, their inner ear navigates using the earth's magnetic forces, and nose follows a map of smells to guide it through different landscapes on its way (12).
The earth's magnetic poles is used to detect north-south direction by 50 animals including birds, mammals, insects and reptiles (13). Birds also use different wavelengths of light, for example blue to fly (13).
Migration Can Be Fatal
Migration is difficult and not without risks. Though the benefits over-weigh the problems, casualties do occur. It however has been difficult to estimate fatalities during migration so far (1).
- Nebel,S.(2010)Animal Migration.Nature Education Knowledge 3(10):77.( http://www.nature.com/scitable/knowledge/library/animal-migration-13259533)