magine swimming 7,500 miles to give birth; walking 70 miles in sub-zero temperatures to get food; or packing up the family to fly from the North Pole to the South Pole, just for a few extra hours of daylight. All without a single road sign to get you there.
Such scenarios are common for the gray whale, the emperor penguin, and the Arctic tern, respectively—along with others in the animal kingdom, where migration has evolved over hundreds of thousands of years to enhance a species’ chance of survival. Why do they migrate? And could it be that the evolution behind these mass movements is still at work, happening in our national parks, right before our very eyes?
In the Everglades and along the Carolina coast, Florida manatees are in the northernmost part of their range. When water temperatures drop below 68 degrees Fahrenheit, the manatee’s thin layer of fat makes it hard to regulate body temperature, and a sluggish metabolism makes it even harder to forage. So the gentle sea cow begins its leisurely migration in search of warmer waters.
That’s when Everglades National Park becomes a fascinating place to observe migrating manatees. Following in the “footsteps” of their ancestors, some head south to the park’s White-water Bay. These migrants and others that summered on nearby seagrass beds move inland during cold weather where they “bottom rest” in warm water trapped in the depths of basins, canals, and rivers. After the cold fronts pass, they also hunker down in shallow bays that heat up rapidly when warm temperatures return.
Knowing precisely where manatees migrate is important to preserving their habitat, so the National Park Service recently partnered with the U.S. Geolo-gical Survey to track manatees with satellite transmitters and document their movements. This information should bring conservationists closer to protecting an endangered species that wanders outside protective park boundaries.
Like the manatees, pronghorn know no borders. The antelope-like ungulates in Grand Teton make up the second largest migrating herd in the Western hemisphere, traveling about 150 miles twice a year. The park’s high mountain meadows make for perfect summering grounds, but heavy winter snows force the group into lower elevations, where they can munch happily on sagebrush until spring arrives again.
Perhaps the greatest modern-day mystery about pronghorn migration is how to preserve it. Historically, at least eight migration routes existed—today, only two corridors remain, and they are being squeezed tighter as housing developments spread across what used to be wide-open ungulate territory.
Out at sea, the Pacific gray whale has less concern with cramped corridors as it does with hunger and the challenge of giving birth to healthy calves. The toothless giants swim nearly 7,500 miles between feeding grounds in the Bering Sea, where they rummage along a sandy ocean floor for amphipods, tubeworms, and other bottom-dwellers, and the warm lagoons of Baja California, Mexico, where they breed and females return more than a year later to give birth.
Some biologists believe gray whales calve in Baja because killer whale populations are lower and the threat to newborns minimal—a strategic, intelligent example of “outdistancing predators,” says Joel Berger, a biologist with the Wildlife Conservation Society. Professor Dawn Goley of Humboldt State Uni-versity in Northern California offers another view. Because females give birth after swimming south, fasting all the way, calves are actually born without fat and would have trouble thermo-regulating anywhere colder than the balmy waters of Baja. So why make the return trip? In the spring, 24-hour Arctic sunlight leads to an intense amount of food production. Evolution has provided the whales with the instinct to return to the Arctic for this not-to-be-missed occasion.
On a slightly smaller scale, consider the green darner. This dragonfly is common throughout the United States and known to migrate along Lake Su-perior, laying eggs in parks like Pictured Rocks National Lakeshore. Oddly enough, not all green darner populations migrate—and entomologists aren’t sure why. Amy Maskey, a biological technician and acting entomologist at the park, has a hunch that migrating increases reproductive potential. Individuals traveling north extend the species’ breeding season, and their larvae develop faster than the resident larvae. Could this be evolution at work?
“The evolution of a migrant population would help ensure the survival of the species as a whole by allowing some of them to retreat to a more favorable climate,” Maskey says. “What I can’t say is which came first…the resident or the migrant. I don’t know that the question has ever been addressed before.”
Some migration rituals have existed in this country for more than 6,000 years, making these ancient movements part of our country’s natural heritage. As humans affect the land between parks, protected areas within the parks provide refuge for weary travelers, whether they are resting, just passing through, or settling into their final destination.
