Why not stay put? In evolutionary terms, that seems an easier option for migrating animals. The evolutionist might argue that animals need to follow the food supply, but since natural selection is supposed to be so clever, why not evolve hibernation or a smaller stomach? Sure, animals need to follow the temperature, but why not just evolve thicker fur or feathers like snowy owls?
For every spectacular case of animal migration, biologists can find other animals that stay put in the cold, like the snowshoe hare, or go into hibernation, like bears or insect pupae, surviving the cold months of winter. Plants stay put whether in the Arctic or the desert. Why should animals travel thousands of miles through precarious weather or trackless seas? It doesn’t make evolutionary sense. Let’s look at some recent findings about animal migration.
Fish: European Eels
In Evolution: Still a Theory in Crisis, Michael Denton describes the amazing life cycle of European eels. It’s an example of “baroque” design in the animal world, he argues, totally inexplicable by any kind of “selective pressure” that might be alleged to explain it. Since we last wrote about eels in October, more has been learned about the spectacular migration of these freshwater fish out to the salty ocean. We had asked, “Are eels equipped with magnetosensing, like salmon, sea turtles, and Monarch butterflies?”
The answer is, “yes” — researchers at the University of Miami have found.
Scientists are closer to unraveling the long-standing mystery of how tiny glass eel larvae, which begin their lives as hatchlings in the Sargasso Sea, know when and where to “hop off” the Gulf Stream toward European coastlines to live out their adult lives in coastal estuaries.
In a new study by the University of Miami (UM)’s Rosenstiel School of Marine and Atmospheric Science in collaboration with the Norwegian Institute of Marine Research’s Austevoll Research Station found that these glass eels (Anguilla anguilla) can sense Earth’s magnetic field and use it like a compass controlled by an internal “biological” clock to orient themselves towards the coast. [Emphasis added.]
Notice that two senses cooperate in this skill: the ability to sense Earth’s magnetic field, and a biological clock to know when to change direction in relation to that field. The discoveries were made by monitoring eel orientations in a test facility where the ambient magnetism could be controlled.
“It is incredible that these small transparent glass eels can detect the earth’s magnetic field. The use of a magnetic compass could be a key component underlying the amazing migration of these animals,” said Cresci, the study’s lead author. “It is also the first observation of glass eels keeping a compass as they swim in shelf waters, and that alone is an exciting discovery.”
Not surprisingly, the paper in Science Advances doesn’t speculate about how this ability might have evolved. They simply state the design-friendly facts:
Glass eels have a magnetic compass, and their orientation abilities appear to be linked to the tidal phase. This is preliminary evidence that magnetic compass–guided movement behavior could be tuned by an endogenous rhythm in the early life stages of a fish. This compass-guided movement, regulated by an endogenous rhythm, may be present in many migratory species.
Speaking of timing, it appears that salmon like to migrate in groups. Phys.org says that even when conditions are stable, observers don’t see sockeye salmon cueing off environmental conditions individually. Instead, pulses of fish are seen migrating together, perhaps for the added protection of a group.
Birds: Golden Eagles and Shearwaters
Humans aren’t the only beings familiar with a generation gap. Golden eagles have it, too. Bird watchers with the American Ornithological Society found that young and old eagles have “counterintuitive” migration habits. Science Daily explains the findings:
Migration is tough, and birds do everything they can to optimize it. How do factors like weather and experience affect the strategies they choose? A new study from The Auk: Ornithological Advances shows that older, more experienced Golden Eagles actually migrate in poorer weather conditions and cover less ground than their younger counterparts, but for a good reason — they’re timing their efforts around raising the next generation of eagles.
The authors give a tip of the hat to evolution, saying, “Because of the costs of migration, there is selective pressure to capitalize on variation in weather to optimize migratory performance.” But as we said earlier, selection pressure should work to make the eagles stay put, not make them go through such hardships to reproduce.
The shearwater is a migratory bird that carries with it a travel journal of sorts. Richard Banati, a nuclear physicist in Australia, decided to take a look at the feathers with X-ray fluorescence microscopy, and found something unexpected: bands of zinc, calcium, bromine, copper and iron. He believes these provide clues to this species’ migratory habits as they fly a figure-8 path between the coasts of Siberia, Japan, and Tasmania on a 60,000-km route over open ocean. Writing in The Conversation, he says:
Like the annual growth rings of trees, birds’ feathers lay down growth bars during their moult. (Moulting is the process of shedding old feathers, making way for new ones to grow.)
While bars simply show growth, the patterns of chemical elements tell us about the bird’s life during the growth period of the feather. They can indicate environmental exposures in a bird population, perhaps before impacts such as illness and death are clear….
The chemistry of feathers might become a tool for watching our environment.
Imagine the surprise of boaters when a blue whale overturned their sightseeing boat off the coast of San Diego (see the video at BBC News). Is this sport to the giant beasts, the largest animals that have ever lived, something like tipping cows to farm boys? Nobody knows what was going through this whale’s mind, but we do know that whales are also master migrators, covering thousands of miles through clear and murky ocean routes. Remember Isabela, the blue whale that clocked a record 5,200 kilometers?
The World Wildlife Fund has been having fun with “whale cams” attached to humpback whales and minke whales, Live Science reports. This is giving scientists unprecedented views of the social lives and feeding habits of these animals. The whale cams show that migration can be vertical as well as horizontal: “whales will range from rolling lunges near the surface to dives up to 1,148 feet (250 meters) deep to eat krill (small crustaceans), their main food source.”
Another team publishing in Science Advances found a novel technique to monitor humpback feeding habits: radiocarbon. “While the whales mostly relied on Antarctic-derived energy stores during their annual migration, there was some evidence of feeding within temperate zone waters in some individuals.” Differences in radiocarbon, measured in the whales’ baleen plates and skin, apparently come from different abundances of radiocarbon between polar and temperate waters. The study provided the first evidence that some individuals were supplementing their diet with trips into geographically distant food webs.
Amazing Daily Commuters
Daily migrations can be as interesting as annual migrations. The average human commuter drives about 5 to 13 miles per day, according to National Geographic. Compare that with blue herons, which fly up to 20 miles per day in search of food. A tropical Atlantic fish called the French grunt swims about a kilometer per day. Then there are the golden jellyfish of Palau, which follow the sun each day across a lake to support their photosynthetic algae partners.
One of the most amazing daily commuters, though, is also the smallest: plankton. Reporter Liz Langley calls it the largest vertical migration of its kind, in terms of biomass. An embedded video clip teases, “The world’s largest migration isn’t what you think.” In the video, Dr. Erika Montague says that all the plankton in the world outweigh all other sea animals combined. Tiny jellyfish, shrimp, comb jellies, and other organisms lumped into the collective we call plankton are not just passive drifters; they have the ability to move vertically through the water column. You can see them flapping their fins or pumping their water bells like hard-working swimmers.
Oceanographers estimate that these tiny sea creatures might move as much water as the wind and the tides. “This happens all around the planet, in every ocean,” Montague says. “It’s amazing.” It is.
Photo: Golden eagle, by Tony Hisgett from Birmingham, UK, via Wikicommons.