Migration is a subject that we frequently hear mentioned but we rarely explore the subject in much detail. In this article we will investigate the various migratory habits of different species and exactly how and why these migrations take place.
Migratory behaviour is expressed in many species, including mammals, reptiles, insects and fish. We will look into some of the migratory journeys taken by animal species other than birds.
Mammal Migration
The migratory journeys of the land mammals are very often to do with the available food and water supplies and the suitability of the environment in ensuring that the young survive. Examples of land mammal migrations include the Gnu of Africa, the Bison, the Reindeer of Northern Europe, and the Saiga of the Russian Stepelands.
The examples above all have one thing in common; they generally follow the path of a good food supply. The direction of travel goes along with the food source.
Small migratory journeys are typical of the land mammals - for example, the deer of Britain which migrate to lower altitudes for the seasonal rut, or the snow leopard (Uncia uncia) of central Asia which migrates on the mountainsides from 18,000 feet in the summer to 6,000 feet in the winter. These are examples of ‘to and fro’ migrations; these are migrations which involve short journeys and which usually occur annually with the changes in the seasons.
Insect Migration
Dispersal Migration
Dispersal migrations are those which occur without any seasonal influence at all and which are triggered by insufficient resources. The locust is an insect which gives us a good example of this type of migration. Swarms of locusts have caused a great deal of crop damage while on their migratory journeys and huge swarms have been reported in the past. In 1889 a swarm of desert locusts was observed over the Red Sea which measured approximately 2,000 square miles.
About seven tenths of the world’s surface is composed of the oceans and seas, not to mention the large lakes and rivers that have more individual characteristics. The fishes have evolved to cope with the huge variety of conditions that exist in the natural marine habitats of the world and fish migrations are quite complex in nature.
Most migratory fish can be put into one of these three categories:
1- Anandromous Fishes. These fish breed in fresh water and move to a saltwater environment for feeding. An example is the salmon.
2- Catadromous Fishes. These fish breed in a saltwater environment (i.e. the sea) and migrate to freshwater areas for feeding. An example is the eel.
3- Fish which migrate within the same environment. Fish which live permanently in the same environment and migrate within that environment whether saltwater or freshwater.
Amphibians and Reptiles
One of the most obvious migratory reptiles is perhaps the marine turtle. A good example is the green turtle (Chelonia mydas) which undergoes a migratory journey of around 3,200km; setting off from the Brazilian coast, the green turtle migrates to Ascension Island in the mid-Atlantic and, following nesting, returns to Brazil.
The interesting thing about marine turtles is that they tend to return to the beach where they were hatched. They must therefore have some method of navigation (which is complicated in a marine environment). There are some theories about the navigation of marine animals and these include the detection of salinity (in eels and salmon, for example) where the recognition of freshwater and saltwater environments would be advantageous. The force of current and water temperature are two other cues that are used in navigation.
This lesson has been an exercise in exploring the diversity of animal migratory behaviour. Although migration is highly inspiring in its scope and variety, it must be remembered that it exists solely for a very functional purpose - to increase the survival chances of those individuals who engage in it.
This article has been put together by the distance learning organisation Start Learning who are experts in home study.
If you want to find out more about Animal Behaviour or many other distance learning courses please browse their website: http://www.start-learning.co.uk
Kerrana McAvoy
Academic Director – Start Learning
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