National Geographic : 2010 Nov
• and different biologists define it differently, depending in part on what sorts of animals they study. Joel Berger, of the Wildlife Conservation Society and the University of Montana, who works on the American pronghorn and other large terrestrial mammals, prefers what he calls a simple, practical de nition suited to his beasts: "Movements from a seasonal home area away to another home area and back again." Gen- erally the reason for such seasonal back-and- forth movement is to seek resources that aren't available within a single area year-round. But daily vertical movements by zooplankton in the ocean---upward by night to seek food, downward by day to escape predators---can also be consid- ered migration. So can the movement of aphids when, having depleted the young leaves on one food plant, their o spring then y onward to a di erent host plant, with no one aphid ever returning to where it started. Dingle, an evolutionary biologist who studies insects, o ers a more intricate de nition than Berger's, citing those ve features (persistence, linearity, undistractibility, special start-and- stop behaviors, stored energy) that distinguish migration from other forms of movement. For example, aphids will become sensitive to blue light (from the sky) when it's time for takeo on their big journey and sensitive to yellow light (re ected from tender young leaves) when it's appropriate to land. Birds will fatten themselves with heavy feeding in advance of a long migra- tional ight. e value of his de nition, Dingle argues, is that it focuses attention on what the phenomena of the wildebeests and the sandhill cranes share with the phenomenon of the aphids and therefore helps guide researchers toward understanding how evolution by natural selec- tion has produced them all. on the Great Plains of western Canada is a peculiar but illuminating case. A young Canadian biologist named Dennis Jørgensen, now employed by the World Wildlife Fund, studied movements of the prairie rattle- snake (Crotalus viridis viridis) on the outskirts of Medicine Hat, Alberta, near the northern limit of its range, and found the snakes migrating ambitiously each spring and fall. e average round-trip by his animals was about 5 miles, though an earlier study detected Canadian rattle- snakes migrating as far as 33 miles. In Arizona, by contrast, rattlers don't travel nearly so far, because they don't have to. e driving logic of the Canadian migrations is related to cold winter temperatures (always di cult for reptiles) and the scarcity of really good den sites in which to survive hibernation. " ere aren't many dens that can support sur- vival over winter on this landscape," Jørgensen told me. An ideal den must be deep under- ground, where the earth is warm, but accessible from the surface via burrows or natural ssures. Such refuges are few and far between. "Because of that, what you get is very large aggregations of snakes at these communal dens." Picture a serpentine tangle of a thousand snakes, piled together cozily, calm and sleek in their subterra- nean nook, jointly awaiting the signals of spring. When surface temperatures rise to a comfortable threshold, they emerge. For a while they bask in the sunlight, crowded like bronzed tourists on the Costa del Sol. But the rattlesnakes are hungry. What's their next imperative? To get away from one another, nd food, and mate. So they migrate radially---in all possible directions away from the den---like starburst embers from a Fourth of July rocket. Jørgensen used small radio transmitters, sur- gically implanted, to chart this pattern, tracking the individual routes of 28 di erent rattlesnakes during 2004 and 2005. More recently, on a blaz- ing summer day, he took me back to one of the den sites, in a slumping bank above the South Saskatchewan River. e slumping had opened deep underground cracks in which roughly 60 prairie rattlers had wintered. From the riverbank we turned toward the uplands and began retrac- ing the migration route of one of his animals, an ambitious female he had labeled E. Not far upslope were three rounded boulders, lichen-covered, with a hole beneath. Snake E had arrived here on May 8, Jørgensen said; she rested, basked, and took o again on May 27. She ascended this steep bench (we started climb- ing) amid the sage and crumbling gray mud, then slithered back down the slope (we plunged a er her), crossed this dirt road, crossed this moist gulch full of goldenrod and skunkbrush Contributing Writer David Quammen is the author of 11 books, including e Reluctant Mr. Darwin. Nebraska-based Joel Sartore specializes in covering biodiversity. is is his 30th story for the magazine.