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Marvelous Migrations at All Scales


Think of the simple reasons for animals to migrate back and forth from one area to another. They might need to look for food. They might need to get away from cold or heat as the seasons change. They might want to get away from predators or parasites. If any of these were universal natural laws, though, we wouldn’t find so many exceptions.

Picture the bison in Yellowstone suffering through some of the harshest winters in America, yet enduring hot August days in their shaggy coats as tourists snap pictures. The swans stay, but the sandhill cranes migrate. The coyotes and rabbits stay, while their fellow mammals, the elk and deer, migrate off the plateau in winter. Some crows migrate, but others do not. Is there a design story in this mixture of phenomena?

As we recently shared in a post about a European bird, technology is opening new windows on animal migratory habits that were never before possible. Let’s begin by looking at a few of the more spectacular examples of animal Olympians:

  • Homing pigeons can find home when released from an unfamiliar location.
  • The bar-tailed godwit, a shore bird, can fly 11,000 km, the longest known example of non-stop flapping flight.
  • The globe skimmer dragonfly completes a 15,000 km circuit in four generations.
  • Monarch butterflies cover 9,000 km in four generations; painted lady butterflies fly almost 10,000 km from England to Africa and back.
  • 15 million free-tailed bats migrate up to 1,500 km each year between Mexico and the US.
  • Zebras make the longest walking migration in Africa: 500 km (longer than wildebeest circuits).
  • The arctic tern flies from pole to pole each year, a circuit of 50,000 km.
  • Sea turtle hatchlings can find their nesting beach after 30 years away. Loggerhead turtles cross the entire Pacific from Australia to South America.
  • Whales, tunas, eels and turtles cross ocean basins during their life cycles.
  • The greatest biomass movement on earth is the daily swimming up and down in the water column of billions of planktonic animals. (Cyrus Martin, Current Biology)

That’s a lot of targeted motion! Migrating animals come in all sizes, from the mightiest whales to the smallest copepods in ocean plankton. Even bacteria can cover relatively large distances with their flagella.

In September, Current Biology published a special issue on migration. Migrating animals appear in a wide variety of animal groups:

  • Vertebrates: salmon, eels, tunas, sea turtles, numerous bird species, wildebeest, zebra, deer, bats, whales
  • Arthropods: butterflies, moths, dragonflies
  • Crustaceans: copepods and other zooplankton
  • Plants: the seeds of plants can “migrate” through air and water, sometimes crossing oceans. It’s a fascinating topic, but this article will focus mainly on animal migration

Mechanisms for navigation are also very diverse. Bees and flies use a sun compass. Salmon and sea turtles guide by the earth’s magnetic field and also use olfaction. Dung beetles navigate at night by the Milky Way (Current Biology). Birds use landmarks, sun angles and stars, and olfactory cues. Mammals and birds can “follow the leader,” staying with an individual that has made the trip before. Whales may use infrasound. Animals coordinate the external cues with their internal circadian clock; for instance, monarch butterflies time their migrations to the angle of the sun. Some animals, like birds and eels, migrate in massive flocks; others, like sea turtles, go it alone but end up together at the same places.

Involves or Evolves

Kenneth Lohmann, an expert on sea turtles, notes that long-distance migration “involves considerable costs in terms of energy and risk. Thus, for such behavior to evolve, the benefits must be high.” Even if a large benefit were easily discernible, though, Darwinians face high hurdles explaining particular cases:

Different species that are closely related evolutionarily, and sometimes even different populations of the same species, exhibit very different scales and patterns of migration. Much of this variation is under genetic control, but the genes involved are largely unknown. A new and burgeoning field of migration research seeks to unravel the genetic architecture of migration. This is challenging because migration involves a complex, interrelated suite of physiological, morphological and behavioral traits that are mediated by an inherited ‘migratory gene package’, which in turn can be viewed as part of a ‘migratory syndrome’. [Emphasis added.]

Some aspects of migration appear to be genetically inherited, because Pacific golden plover chicks can find the Hawaiian islands from Alaska over the trackless ocean, and butterflies can find the same trees their great-grandparents did, never having flown there before. The fact that sea turtles can find their feeding grounds as hatchlings implies they are born with an inherited genetic map. But routes can also adapt quickly to changes in the environment. Lohmann points out two cases of rapid alteration of inherited maps:

Migratory adaptations arise and vanish rapidly. For example, monarch butterflies were introduced into Australia only about a century ago, but now have a migration in which the timing and direction are altered by 6 months and 180° relative to the North American population from which they presumably descended Another interesting example involves the central European population of a migratory bird, the blackcap (Sylvia atricapilla), which evolved a new migration route to the British Isles in only three decades. The direction of first migration appears to be encoded by at most a few genes and the new route appears to be based on a novel, genetically programmed orientation preference that is spreading rapidly through the population.

Evolution: Explanation or Assumption?

Most of the articles in this special edition of Current Biology invoke the phrase “has evolved” as a magic wand, never explaining how things evolved. Other than asserting that migration “has evolved repeatedly in numerous species occupying diverse ecological niches,” Lohmann has little to say about evolution. Migration might keep parasites from evolving more virulent forms, he says, and might increase the health of the migrating population; but other than those suggestions, he offers no explanation for the origin of precision mechanisms that make migration possible: celestial navigation hardware and software, olfactory organs with accuracies of parts per trillion, and sensors that can detect the intensity and angle of the earth’s magnetic field lines. Merely possessing the equipment, though, would never provide an advantage for animals physically incapable of long distance travel. Evolution needs to explain the whole animal as a migration-capable machine.

In their article on “The Ecology of Migration,” Alerstam and Bäckman agree that migration offers various “selective advantages” to animals, “like exploitation of seasonal and ephemeral resources, and avoidance of predators, diseases and competitors.” But a potential selective advantage does not create the tools to exploit it. Instant long-distance communication across the globe could be called an advantage to humans, but would that account for the emergence of the iPhone? The authors say that populations that migrate reproduce better than those that don’t, but they fail to explain how magnetic maps and olfactory systems as sensitive as those in Pacific salmon came to be.

The Closest Attempt

The closest attempt to invoke a neo-Darwinian explanation for any aspect of animal migration can be seen in Reppert and de Roode’s entry, “Demystifying Monarch Butterfly Migration.” Those who remember Illustra’s film Metamorphosis, with its multiple challenges to Darwinism, will want to watch this show! Reppert and de Roode link specific mutations with selective advantages:

Monarchs have become a textbook example of warning coloration and plant-derived toxicity to predators. Monarch larvae sequester cardenolides from their milkweed diet, and broadcast their toxicity through the black, white and yellow stripes in larvae, and the bright orange marked with black and white accents in adults. Classical studies have demonstrated that avian predators quickly learn to associate the bright colors with bitter taste and emesis, leading to prey avoidance.

Cardenolides exert their toxic effects in most animals by interfering with the Na+/K+-ATPase sodium pumps, but monarchs and other milkweed specialists have mutations that reduce cardenolide binding. Evolution of cardenolide insensitivity evolved in a step-wise manner during the macroevolution of milkweed butterflies, with monarchs having at least two non-conservative mutations that strongly reduce their sensitivity to cardenolides

The high level of resistance to cardenolides allows monarchs not only to feed on milkweeds, but also to sequester cardenolides for their own defenses. Although the sequestration of cardenolides by monarchs has been mostly studied in the context of predation, recent studies suggest that these toxins also provide protection against infection with a virulent protozoan parasite…. Monarchs reared on milkweed species with higher concentrations of heart-arresting cardenolides experience lower infection rate, less parasite growth and fewer disease symptoms than those reared on milkweeds with lower concentrations. Furthermore, when given a dual choice between milkweeds that vary in their anti-parasitic effects, infected female monarchs prefer to lay their eggs on the anti-parasitic species…, thereby reducing infection and disease symptoms in their offspring. These studies have also shown that despite their high level of insensitivity, monarchs are not fully resistant to cardenolides, with highly toxic cardenolides reducing adult monarch lifespan.

That’s it. In the whole series of articles, that is the closest that any author comes to an actual neo-Darwinian explanation for any trait in any species, other than to tell us, over and over, that everything “has evolved.” But notice: this example says nothing about navigation or migration itself. It only talks about how mutations in the butterfly allow it to ingest cardenolides without dying of heart attacks. And they got this ability by breaking things: breaking the binding link that makes the toxin interfere with the sodium pump. That may protect them from birds, but doesn’t account for the origin of the time-compensated sun compass, the inherited map, and the ability to fly unerringly 3,000 miles to the exact trees where their ancestors roosted three generations back.

Design advocates agree that the innate mechanisms for migration can adapt to changing environments, as shown in the Australian monarch butterflies. You can call that “evolution” in a limited microevolutionary sense. But in the only cases we know of where navigation equipment and the ability to use it came into being, it was designed by intelligent minds. Try telling our ace fighter pilots or ship captains that the radar instruments that allow them to go far afield and back again “have evolved” simply because they possess a “selective advantage.” If their routes change, they will most likely tell you that the capability to modify routes is built into the system.

Photo: Monarch butterfly, by liz west from Boxborough, MA [CC BY 2.0 ], via Wikimedia Commons.