Every year, thousands of animal species migrate southwards to warmer climates, and they remain there during the winter. As spring approaches, they begin their journey back home. If their migratory home is a better environment, why don’t animals stay here year-round? What is the advantage of returning to a region with a cooler climate?
Animals head back from warmer climates for the vast amounts of food. Whales return to their feeding grounds for food and because the cooler temperatures allow better skin cell regeneration. Many birds return so that they do not have to compete with food sources against native birds.
Migration expends a great deal of energy, and some species are known to travel thousands of miles along their migratory routes. The benefits of returning home must outweigh the cost of such a mammoth journey.
The definition of migration is:
The seasonal movement of animals from one region to another.
Not all species migrate, but those that do, do so for specific reasons. This could be better food sources, nesting grounds or requirements of offspring that differ from the adults. Some animals only migrate a short distance, perhaps only one or two states. Many species will migrate hundreds or even thousands of miles from Canada and North America to the Southern states of California and Florida or even further south into Mexico, Central or South America.
The summer regions are dependant on the requirements of each animal species. Some species need to go a short way to find better feeding ranges, whereas other animals must travel further to particular areas for breeding or nesting.
Migration for a lot of animals is a huge endeavor, especially for small animals such as hummingbirds. The reward for migrating must outweigh the physical cost of the journey. There are many risks associated with migration, including predation, exhaustion, and death due to disease or parasites prevalent in the tropical regions.
Species that do not migrate for winter tend to employ other adaptive behaviors such as hibernation. This process whereby the animal’s metabolic and heart rates will slow, decreasing their core body temperature. This means they use less energy and can survive for long periods without food. North America’s bear species hibernate during winter. They spend the last weeks of summer digging out a shallow den to spend the winter months in a deep sleep. Females will even give birth during these months.
There is a theory among scientists and archaeologists that early humans would have hibernated during harsh winters, based on fossil evidence and interruptions in bone growth. It was discovered that bone growth was interrupted for months at a time, suggesting that early humans were hibernating as recently as 400,000 years ago. In modern times, we do not need to hibernate since we can keep our homes warm, and we have year-round access to food to sustain us.
Some animals, such as squirrels and bats, will use a mild form of hibernation called torpor, which allows them to sleep deeply for several days or weeks then wake for a day to relieve themselves and feed. They will then go back into torpor for another period. This continues throughout winter until the temperatures warm and there are better food sources to sustain normal waking and sleeping patterns.
Benefits of Returning
Most animals migrate for breeding or feeding purposes, but many whales living in colder waters will migrate to the tropics each year for calving and another reason – skin care. During the springtime, the waters of the coast of California and Florida welcome several species of whale migrating from Arctic waters. It was long believed that the reason for this migration was that the warmer climate was better suited to calving, however, recent studies have found that whale anatomy is best acclimated with cold regions.
Additionally, there is far less food available to whales in warmer water, so this would not be beneficial to rearing young. In fact, some whale species will fast during their migration due to reduced feeding opportunities.
In 2011, scientists in the research journal Marine Mammal Science proposed that whales migrate to revive their skin metabolism. In the Arctic cold waters, whales will divert blood flow away from the skin to conserve body heat, but this reduces the molting rate of dead skin. Returning to warmer waters allows for better blood flow to the skin, increasing the rate of molting and allowing better skin cell regeneration.
Since they are not well adapted to live in warm waters for long periods, the whales will return north again. Here, although the temperatures are far cooler, there is a greater abundance of food, and their bodies are adapted to the frigid conditions thanks to a thick layer of blubber.
For other animals, breeding would not be successful in their home region. Atlantic salmon are anadromous, meaning they live in both freshwater and saltwater. An adult female will journey from waters around Greenland, down the East coast of Canada to the Labrador Sea for their first summer. Older adults can be found further south in the rivers of Maine.
Atlantic salmon will begin their migration in spring back to the same river they hatched in. Some will even return to the same stretch of river. Scientists believe salmon use olfactory navigation (smell) to find their way. Salmon eggs must be hatched in rivers as the fry and smolt (juveniles) are adapted to freshwater. Once they reach one year old, their bodies go through an internal change, allowing them to survive in saltwater.
At this point, Atlantic salmon will leave their home river for the first time and begin the long journey north. Some adults do this each year, but several adults will remain at sea for several years. Salmon travel to the ocean to feed and grow. There is much more food here so that they can grow quickly. Larger females produce more eggs, so this is beneficial to ensuring more eggs hatch successfully.
One of the biggest benefits of migrating back north is food. As winter approaches, many species, mostly birds, will migrate from the cold North American states down towards the tropics of Central and South America. This is because, during the winter, food abundance falls to levels that would not sustain the population.
Hummingbirds, Canada geese, and orioles all migrate to southern US states or into Mexico and Central America. Here there is better food availability, so they spend their summer months here, but it is not beneficial to stay year-round. Although there are better food sources here than in their home regions, these species have to compete with native species living in the tropics year-round.
Migratory birds, therefore, return to their home regions at the beginning of spring. This timing is important as this is when there is a sudden increase in the abundance of food as the weather warms and the days grow longer. This migratory pattern ensures that these animal species have plenty of food sources available all year round, so the energy expended in migrating long distances is worth it in the long term.
Monarch butterflies are perhaps one of the most well known North American migrators. Each year they travel thousands of miles from Canada and Northern US states down to Mexico. In springtime they fly back to North America. The reason for this is a very specific requirement of Monarch caterpillars.
When Monarch butterflies arrive back in the US, they begin laying eggs, but they only lay eggs on the leaves of milkweed plants. The leaves of milkweed are the only leaves monarch caterpillars can eat when they hatch. If the eggs are laid on another plant, the caterpillars will starve.
Milkweed grows in large numbers in the US and Canada in early summer, just the right time for the Monarchs to lay their eggs.
Bowler, J. (2020, December 21). Early Humans May Have Hibernated Through Long Winters. Retrieved from Science Alert: https://www.sciencealert.com/early-humans-may-have-hibernated-through-long-winters
Lothian, A. J., Newton, M., Barry, J., Walters, M., Miller, R. C., & Adams, C. E. (2017). Migration pathways, speed and mortality of Atlantic salmon (Salmo salar) smolts in a Scottish river and the near-shore coastal marine environment. Ecology of Freshwater Fish.
New Hampshire PBS. (2021). How and Why Animals Migrate. Retrieved from NatureWorks: http://www.nhptv.org/natureworks/nwep4c.htm
NOAA Fisheries West Coast Region. (2020, February 21). Why do whales migrate? They return to the tropics to shed their skin: First suggested for killer whales, skin molt may drive long-distance migration for all whales that forage in cold waters. Retrieved from Science Daily: https://www.sciencedaily.com/releases/2020/02/200221125111.htm
Pokras, M. (2010). Why do birds return north after winter—why don’t they just stay where it’s warm? Tufts Journal.
U.S. Fish & Wildlife Service. (n.d.). Fish Migration: Atlantic Salmon. Retrieved from U.S. Fish & Wildlife Service: https://www.fws.gov/fisheries/fishmigration/atlantic_salmon.html#:~:text=Unlike%20their%20Pacific%20cousins%2C%20Atlantic%20salmon%20do%20not,Atlantic%20feeding%20grounds%20%28arrows%29%2C%20usually%20near%20western%20Greenland.