Why Do Animals Migrate?
Animal migration refers to the movement of animals over a long distance, usually in line with changes in the seasons. This movement exists in all the main animal groups, which includes birds, fish, insects, amphibians, crustaceans, mammals, and reptiles. A simple movement of animals over a substantial distance cannot be considered a migration. A migration is animal movement due to reasons like changes in the season such as when birds in the Northern Hemisphere escape to the south during winter. A migration can also occur if there is a major change in the habitat of an animal such as when a young one leaves the habitat of birth and moves to adult habitats.
The definition of the phrase “animal migration” is more of a guideline than a definitive statement as migration can occur in a number of ways depending on the species. Four proposed concepts are usually used as the general guidelines for looking for signs of migration. These signs are: movement in a relatively straight line, relocation of a species on a massive scale, and a movement that redistributes members of a species in a population. The fourth sign is the aforementioned seasonal movement.
Animals that Migrate
Birds
About 18% of the 10,000 species of birds in the world migrate due to changes in the weather seasons. Most of these birds make a north to south journey. The summer in the north is usually a season for the birds to feed and breed while the winter sees them move south to warmer places. Other species make an annual migration from the north and south hemispheres. For example, the Arctic tern makes the migration from the north to the south every year, which I a distance of about 12,000 miles.
Fish
Unlike birds, fish do not always migrate over long distances since they may end up in the same location. For example, if fish inhabit a huge lake and end up switching habitats, then the migration is rather short because they are still in the same water body. However, there are fish species that go for longer distances of up to hundreds of miles. In total, at least 120 fish species such as salmons move between freshwater and saltwater habitats. Other fish species include forage species such as capelin and herring that migrate within the North Atlantic Ocean, sardines in South Africa, and many more.
Insects
Migration of insects usually happens among winged insects such as dragonflies, butterflies, and locusts. A species of the dragonfly known as the wandering glider or the globe skimmer (Pantala flavescens) makes the trip from Africa to India across the ocean. The glider’s migration is the longest crossing of its kind among insects. Other insects that migrate include the painted lady and the monarch butterflies. However, for these two butterflies, the group that begins the migration is not the same one that completes the journey. The reason for this is that the butterflies mate and reproduce along the way so the newer generations are the ones that complete the migration.
Mammals
This group exhibits the largest terrestrial migration of mammals. A good example of this migration is the famous wildebeest migration in Africa’s Serengeti National park. Aside from the wildebeest, other species that migrate include zebras and gazelles. Interestingly, these groups can alter their direction depending on the environmental conditions such that they move towards the rain.
Other Groups
Other animals such as cetaceans, which includes dolphins, whales, and porpoises also migrate. Others include some species of bats (such as the Mexican free-tailed bat) and some reptile and amphibian species. Crustaceans that migrate include the stunning Christmas Island red crab, which migrates in the millions every year.
Reasons for Animal Migration
Reproduction
One of the most common reasons for migration is for animals to find suitable breeding grounds for reproduction. An example of such an animal is the Atlantic salmon, which begins life in a river and then moves to the ocean upon reaching maturity. However, it still heads back to the river when ready to reproduce and the cycle is repeated. Crustaceans such as many species of crabs live in the deep seas but come to shallow waters for breeding before going back to deeper waters. Amphibians such as frogs and toads alternate between ponds for breeding and larger lakes for living.
Hibernation and Escaping Harsh Weather
Hibernation is crucial to the survival of some animals. A good example of such an animal is the little brown bat. During the summer, these creatures live in trees while they migrate to caves for hibernation in the winter.
Most of the bird species that migrate do so because of changes in the seasons. The aforementioned Arctic tern is a perfect example of such a bird. Due to its migration, the bird gets to experience two summers in a year instead of one.
Look for Food
Another common reason is a decrease in food levels. A perfect example is the wildebeest migration in the Serengeti. During seasons when food is scarce on one side, the animals begin moving to greener pastures in other places. Along the way, the direction may alter depending on where it is raining, which is where food will be in abundance. By doing this, they make sure that they give time for the land they left behind to recover so that it will be able to provide food when the herds return. Food is linked to reproduction since most animal species will migrate to breed in places where there is enough food for the survival of their offspring.
How Do Animals Know Which Direction to Migrate In?
Experts in the field are still not sure how animals know the right directions although a few theories have been suggested. One such theory states that animals use landmarks to tell the direction. Examples of such landmarks include rivers and lakes. Other scientists have suggested that the sun and the stars are used in determining the direction. The acute sense of smell of some animals may also be how the animals determine the proper course while others use the magnetic field of the earth.
