The Homing Instinct: Meaning and Mystery in Animal Migration. Bernd Heinrich

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      Dispersing to “anywhere but here” generally applies to nonmigratory species that have no encoded or learned directions to go to but may have innate instructions to move in more-or-less straight lines rather than potentially going in circles in order to achieve distance. In Africa, dung-ball-rolling scarab beetles race away from their often thousands of competitors at a dung pile at night by using the swath of stars of the Milky Way galaxy as a reference. Swarms of insects feeding at dung and carcasses also attract predators, and as soon as they finish feeding, many distance themselves from those predators. I’ve observed blowfly larvae at animal carcasses keeping to almost perfectly straight lines in their getaway at dawn, by steering directly toward the direction of the rising sun. Mass movements sometimes observed in some rodents, such as lemmings and gray squirrels (as in 1935 in New England) following a population explosion after a superabundance of food, may be another example of dispersal to get to a better place, though not necessarily a predetermined one.

      On the other hand, “true” migrants are able to utilize ideal conditions in two places, provided they vary predictably. Arctic terns, Sterna paradisaea, breed throughout the Arctic, then fly to Antarctica to escape winter when food availability declines and to arrive in spring and food again, a round-trip distance of nearly seventy-one thousand kilometers. Gray whales, Eschrichtius robustus, also feed in the Arctic in the summer but then travel eight thousand kilometers along the coastline to Mexico to bear their calves in warm waters.

      Dispersers are not neatly differentiated from migrants, although the first commonly rely on passive mechanisms as opposed to the migrants, which move to specific goals by their own powers of locomotion. There are all gradations in between. Each case is unique, and there are thousands. Let’s look at more ways of getting to a good place, as represented by eels, a grasshopper, aphids, and ladybird beetles.

      Eels. There are many species of eels, but the American, Anguilla rostrata, and the European, A. anguilla, the species with which we are most familiar, live most of their lives in freshwater ponds and lakes. For reasons that are not clear, though, they do not reproduce in their home areas. To the contrary, both disperse (or “migrate”) thousands of kilometers on a one-way trip to spawn and then die in the mid-Atlantic.

      Just as birds and some insects use air currents, eels use water currents to help them leave their lifetime homes. After eels leave their freshwater homes and head for the ocean to spawn and die, their larvae then drift in the ocean currents for years. But eels’ dispersal behavior is anything but passive. Eels fatten up to prepare to leave their home haunts in the bottoms of freshwater lakes and streams. Before they head for the sea, they absorb their digestive tracts and transform themselves by greatly enlarging their eyes and turning silvery on their ventral side. The latter transformation produces counter-shading that reduces their visibility to predators below them in the open ocean waters.

      The eels’ dramatic changes in behavior, morphology, and physiology that enable them to switch from living in freshwater habitat to open ocean highlight the operation of strong selective pressures. But why do they leave their homes in freshwater ponds where they grew up and lived most of their lives? Their one-way, once-in-a-lifetime migrations to their Sargasso Sea spawning grounds can’t be to find a better feeding ground, or to escape competition. However, I suspect what the behavior accomplishes superbly is that the adults, which are predators, do not come in contact with and feed on their own young. Is it a mechanism that has evolved because it reduced predation on themselves?

      There are over six thousand publications on eels, but the life cycle of these economically important food fish is still murky and has a long history of speculation. For centuries, nobody ever saw a baby eel, and even now, their spawning has not been witnessed. Aristotle presumed that eels grew from earthworms. The first young of eels, transparent leaflike forms, were found in the open Atlantic Ocean. Gradually, as more of these leaflike creatures were collected, it was noted that they varied in size, and that the smallest ones were found south of Bermuda in the Sargasso Sea, which was therefore presumed to be the site of their origin, that is, the eel spawning area.

      The eel larvae, after hatching in the waters of the Gulf Stream, drift north. Like plankton, they move at the whim of the prevailing oceanic current. As they grow from about five to six centimeters in a year, they take on a more eel-like form but remain transparent. They are by then able to swim and, presumably by scent, find and swim up a river. Unlike salmon, however, these transparent “glass eels,” as they are known at this larval stage, can have no specific home stream scent to follow, because they have experienced only the scents of the ocean.

      The female glass eels at this stage, in early spring, migrate up rivers and streams along the East Coast of America. After two months in a river they grow to about ten centimeters. Now known as “elvers,” they are no longer transparent, and they enter lakes and become eels. These female eels live in lakes for eight years or more, fattening up (the males stay in the saline estuaries). When they achieve the right amount of fat, these females become sexually mature. Each develops a clutch of three to six million eggs, and then one fall the gravid females start their journey downstream back to the ocean, to the Sargasso Sea to spawn. Since the males don’t live in fresh water, somewhere in the ocean the females then apparently meet the males for fertilization.

      As the Gulf Stream continues north beyond North America, the larvae of the European eel, which originate from the same apparent spawning area, the Sargasso Sea, as the American eels, continue their journey. Finally, in two or three years, they reach the coasts of Europe, where they then also seek rivers and streams. Migrating upstream, they become pigmented, and after growing to adulthood, they migrate back into the open Atlantic and make their sixty-five-hundred-kilometer return to the Sargasso Sea, where they spawn on average several million eggs, and then die. Only one of several million of them will make the return journey to grow to a reproducing adult in fresh water.

      Grasshoppers. One of the best-known insect dispersers, the “migratory locust” (the grasshopper, Schistocerca gregaria), engages in some of the most spectacular mass movements in the animal kingdom. On the African continent, this species has been famous since biblical times. Swarms of the “locust” have blackened the skies, and as those in the vanguard settle onto the earth and consume every green thing where they land, the rest fly over them until they reach more green, while those behind then take flight and do the same, and so a horde of hundreds of millions moves along, stripping all vegetation in its path. Predators cannot put a dent in those hordes. Additionally, the migratory locust is distasteful to potential predators because when it migrates, it is not choosy about what it ingests and takes up toxins from poisonous plants, which it incorporates into its tissues. The grasshoppers’ bright red-orange and yellow coloration, like that of many insects including the monarch butterfly, reminds potential predators of its distastefulness.

       Nymphs and adults of the two phases of the migratory grasshopper

      Although this distinctively colored grasshopper appears to arrive suddenly, it is often there all along, but in a different guise. It has a green solitary grass-fed form that blends in with its food and that is palatable to predators. For a long time scientists thought that the grasshopper “migrants” that appeared so suddenly were a unique species, one arriving from an unknown origin and heading for an unknown destination. Now we know that the migrants are a “phase” of a common species that changes its color, form, and behavior in response to crowding. Proof comes from experiments: to create these “migrants” from isolated individuals one takes a nymph (immature stage), puts it in a jar, and has a motor-driven brush tickle it continuously. The constant tickling mimics the crowding, which in the case of S. gregaria is the signal evolution has “chosen” to trigger the nervous system to alter the hormones that result in development into the restless migratory phase of a different color, wing length, and behavior. It is a good example СКАЧАТЬ