Migrations occur in every major branch of the animal kingdom, from birds and crustaceans to insects and whales. But some of these journeys are amazing feats of endurance, spanning thousands of miles and deadly terrain without rest. Here's what science can tell us about the most extreme migrations, and how animals survive them.
Gif by headlikeanorange
An "Undistractable" Movement
The word "migration" typically brings to mind the seasonal en masse movement of animals from colder climates to warmer ones, but there's no real consensus on the exact definition of what types of movements it includes. In a 2007 overview of the phenomenon — aptly titled "What Is Migration?" — experts Hugh Dingle and V. Alistair Drake note that definitions of animal migration in dictionaries and the scientific literature encompass many overlapping concepts.
Some definitions describe migration as the seasonal movement of animals between favorable and unfavorable regions, often for the purpose of breeding. But migration can also more generally refer to the relocation of an animal that involves longer journeys than those of its normal daily activities. Other definitions say migration is when animals move around single-mindedly, ignoring their usual food-gathering and home-building habits. Finally, migration can be movements that lead to "redistribution within a spatially extended population." In other words, it's when animals move around within a large territory occupied by their own species.
A few years ago, Dingle offered his own definition for migration. According to Dingle, migration has five specific characteristics that separate it from other forms of movement: It is prolonged and takes animals outside of their normal habitats; it is typically linear; it is undistractable, meaning, for example, that you can't get a migrating bird to stop migrating if you wave food in front of it; it involves special start and stop behaviors, such as overfeeding before the big trip; and it requires a lot of stored energy.
Gif by headlikeanorange
But there are always exceptions.
Some animals, such as song sparrows (Melospiza melodia), engage in partial migration, where only some of the population migrates. Insects often perform one-way migration, in which they travel from their birthing grounds to another location to breed and produce the next generation before dying. Species living in mountains, including birds and bats, often conduct altitudinal migration — they move from one elevation to another, such as when spotted owls travel to lower elevations when snow covers the upper parts of the mountain.
Food, Weather, and Sex
The reasoning behind animal migration is simple: Survival, both for the individual and the species. Animals will typically migrate to find food, reproduce or escape bad climate.
For example, in the arctic tundra and other high latitude regions, migratory birds such as the bar-tailed godwit (Limosa lapponica) take advantage of the long days and high abundance of food in the early summer; after breeding, they leave the region to avoid low food availability, as well as winter's short days and low temperatures. Research has suggested that many other birds migrate because of the scarcity of food. And wildebeest are also known to be in a constant search of good food in the Serengeti.
Salmon (Oncorhynchus spp.) are a prime example of animals that migrate for reproductive reasons. The fish live out their adult lives in the ocean, but when it's time to spawn the next generation, they return to their natal freshwater streams (for a comprehensive account of the Pacific Salmon's lifecycle, see here).
Image by Jordan Tan, via Shutterstock
The migration of some baleen whales involves both feeding and breeding. Humpback whales, for example, gorge themselves on krill in high latitude waters during the summer. As winter approaches and the waters get ever colder, the cetaceans travel some 5,000 miles (8,000 kilometers) south to give birth in warm tropical waters, which are more suitable for their calves but lack high quantities of food.
North America's pronghorn antelope (Antilocapra Americana) also migrate to avoid the cold. The "Path of the Pronghorn" runs from Jackson Hole in Wyoming to the Upper Green River Valley in the state. In the summer, the ungulates give birth and raise their young in Jackson Hole, but as autumn approaches they gather to return south to the Upper Green River Valley. They know they wouldn't be able to survive the deep snows of the North, and they must get started on their journey before the snow blocks their path through the Gros Ventre Mountain
Last year, researchers discovered another potential reason for migration: To avoid predators. The common roach (Rutilus rutilus) is a freshwater fish that lives in the lakes and streams throughout Europe; during winter, cormorant birds (Phalacrocorax carbo) roost and breed near the roaches' lake homes. When the birds come along, some 80 percent of the fish migrate to the streams — where there is virtually no food — to avoid getting eaten by the birds, which pick off roaches in the lakes.
Preparing for the Trip
Though some animals stop to eat along their migratory paths, others do not. Instead, they prepare for their trip by essentially fattening themselves up, once their internal clocks tell them to do so. During this "fueling" stage, birds ingest and process food at or close to their metabolic capacity. What's more, organs involved with feeding, including the stomach, gut, gizzard, liver and kidneys, enlarge to support fueling. During takeoff and flight, these organs shrink, while organs that support flight, including the heart, flight muscles and skeletal muscles, grow.
Once they're on their way, migratory animals have several different mechanisms they use to find their destination and stay on the right path, most notably celestial cues and the Earth's magnetic field.
Image by BlueOrange Studio, via Shutterstock
Numerous animals, including fish, insects and possibly birds, can use the sun and the polarized light patterns in the sky or reflecting off bodies of water as compass cues during the day. At night, when the sun and its related polarization patterns are not visible, animals can use the stars as their guide — that is, the rotation center of the night sky (the Pole Star in the northern hemisphere) points them to the geographical north direction.
The Earth's magnetic field also plays a role in migratory navigation. Birds, turtles, fish and other animals seem to be able to detect the intensity of Earth's magnetic field, an ability that may have to do with the bodily presence of the magnetic material magnetite.