Tracking the movements of animals that migrate thousands of miles has never been easy. Over the years, biologists have doggedly tried to trace the paths of everything from birds to butterflies to fish to seals to elk to sea turtles. They've banded and tagged these animals, and have hoped that one in a thousand might be recaptured somewhere at the other end of its journey. They've attached radio transmitters to them and followed them with satellites. They've teased out patterns of variation in their DNA to try to match wintering and summering populations. Still, we know distressingly little.

But now a promising new method has been added to the mix. A recent stampede of researchers is seeking to use stable isotope analysis to answer questions that have long remained intractable with other methods. In fields such as geology and geochemistry, archaeology, and paleontology, such analysis is old hat. It's enabled practitioners to reconstruct paleoclimate, determine diets of fossil humans, date ancient materials, and much more. In ecology and physiology, plant researchers picked up isotopes and ran with them 20 years ago, using them to work out photosynthetic pathways, study nutritional physiology, and trace chemical fluxes through ecosystems.

But now ecologists are using isotopes to reconstruct diets of animals, trace nutrient flow through their tissues, judge their physiological condition, and track long-distance movements. In 1997, two teams independently published papers tracing intercontinental flights of neotropical migrant birds isotopically. One was a group led by C. Page Chamberlain of Dartmouth College. The other was Keith Hobson, a research scientist with the Canadian Wildlife Service and adjunct professor at the University of Saskatchewan in Saskatoon, and his colleague Leonard Wassenaar of the National Water Research Institute of Environment Canada. With birds, fish, sea lions, and polar bears, Hobson has used isotopes to infer diet composition and nutritional condition, determine trophic relationships in food webs, map population subdivision, and trace artificial contaminants in food chains.

In 1998, two high-profile papers coauthored by Hobson finally put isotope use in tracking animal movement squarely on the scientific map. He and Wassenaar published a continent-wide study of monarch butterfly migration, "Natal Origins of Migratory Monarch Butterflies at Wintering Colonies in Mexico: New Isotopic Evidence," in Proceedings of the National Academy of Sciences. With Peter Marra and Richard Holmes in Science's "Linking Winter and Summer Events in a Migratory Bird by Using Stable-Carbon Isotopes," he tracked migrant birds from the Caribbean to New England. In each case, the isotopes provided information important to conservation efforts.

But let's step back a bit. Recall from Chemistry 101 that isotopes are alternate versions of chemical elements, created when varying numbers of neutrons produce different molecular weights. For instance, the isotope carbon-13 has one extra neutron beyond carbon's normal complement of 12. Some isotopes are radioactive and decay, but stable isotopes may exist in nature in constant ratios. If you know the natural abundance ratio of an element's isotopes (called an "isotopic signature"), you can use that information in various ways.

For instance, the isotopic signature of carbon in plants varies predictably with the plant's type of photosynthetic machinery. Plants with a C4 pathway (such as grasses) have higher proportions of carbon-13 than C3 plants like oak trees, while CAM (Crassulacean acid metabolism) plants like cacti have variable but intermediate values. When animals eat plants, isotopic signatures are incorporated into their tissues - for instance, from milkweeds to the monarch larvae eating them. Signatures may continue up the food chain from herbivores to carnivores. Thus Marra, Hobson, and Holmes found that warblers consuming herbivorous insects in Jamaica were gaining isotopic signatures and taking them with them when they migrated to New Hampshire to breed. Warblers wintering in C3 mangrove habitats migrated earlier than those in C4 scrub habitats, and arrived first at the breeding grounds - which spells a big reproductive advantage for a territorial songbird.

Carbon isotopes tell us directly about an animal's diet, but because plant communities are often dominated by species with one type of photosynthetic pathway, and are often geographically structured (e.g., deserts have many water-efficient CAM plants), carbon signatures often can indicate geographic location. In Mexico, Theodore Fleming of the University of Miami used carbon isotopes to trace the diet of nectar-feeding bats as they shift seasonally and geographically between C3 plants, CAM agaves, and cacti during their long-distance migrations. In Africa, Paul Koch of the University of California at Santa Cruz measured signatures deposited in elephant tusks to infer movements between habitats throughout their lives. The within-population variation in his data cast doubt on the feasibility of earlier hopes to use isotopes to determine origins of poached ivory. In the ocean, carbon signatures vary with distance from coastlines, and Koch, Hobson, and others have used these patterns to determine movements of animals like salmon and seals.

Nitrogen isotopes can quantify trophic structure and nutritional condition. Animals higher in the food chain, and animals that are starving, show heavier nitrogen ratios because nitrogen-15 is preferentially held in tissues and nitrogen-14 is preferentially excreted. Hobson has worked out trophic relationships in seabirds, marine mammals, and Arctic ecosystems using nitrogen, and has demonstrated nitrogen-15 enrichment in geese that fast while nesting.

Hydrogen isotopes (deuterium), like carbon, can help trace long-distance movements, but for a different reason. Hydrogen signatures in rainfall vary systematically across North America, becoming lighter as you move northwest. Because the rainfall signature gets passed up from plants to animals, Wassenaar and Hobson could use hydrogen in combination with carbon to pinpoint geographic origins of monarchs collected at Mexican winter roosts. "Ironically," Hobson says, meteorologists "had known about this pattern for years. But people hadn't clued in to the fact that the patterns are transferred up the food web."

Researchers say there are many more elements whose standing patterns of variation in nature remain underutilized or even unknown. "We've only just scratched the surface of learning about [large-scale] natural isotope patterns that exist in nature," Hobson says.

Most people currently using stable isotopes in animal ecology began their careers in fields where the technique was already established, and Hobson is no different. A physics major in college, he helped build a radiocarbon dating lab at Simon Fraser University, then did stable isotope research in human archaeology. For his Ph.D., he applied his isotope knowledge to his passion for birds, working out trophic relationships among Arctic seabirds. "And I was very surprised," he says, "to learn that very few other people were doing the same thing."

Wassenaar, who has a geochemistry background, echoes Hobson's surprise, in recalling the origin of their collaboration on the monarch project: "It was in one of those coffee-time discussions, and I said to Keith, has anyone ever made use of those [hydrogen rainfall] patterns in biology? We thought, 'probably.' But we looked, and we found there was nothing!"

"It was remarkable to me that there were these patterns sitting here waiting to be used," Hobson adds, "and here we were running around trying to track animal movements with transmitters and things."

Exploration of isotopic opportunities in ecology has been driven by recent dramatic improvements in technology. New isotope ratio mass spectrometers make it possible to process huge numbers of samples with an ease unheard of just several years ago. "Now it's almost: load a sample, push a button, get a number," Wassenaar says.

Hobson and Wassenaar foresee an imminent boom in use of hydrogen - perhaps the most useful isotope for tracing movements, but inherently difficult to work with. Machines coming on the market this year should greatly speed and simplify hydrogen analysis, they say. "It's a truism to say that in the next year to two years to three years we'll see more measurements . . . than have been made in the entire period leading up till now," Hobson says. "It's a mind-boggling, astounding thing."

But rapidly growing scientific fields attract bandwagon-jumpers, and many isotope veterans voice concern that newcomers may see the technology as a silver bullet and overreach in making unjustified inferences. "The tendency has been to come to expect a lot out of the field," Hobson explains, "and to be so enthused about potential applications that [some] people are pushing the limitations of interpretation."

In particular, some warn against interpreting field data without doing lab experiments to test assumptions. Assuming that all constituents of an animal's diet are equally incorporated into tissue, or that constituents are routed equally to different body tissues, for example, may not be justified. It's also necessary to measure when and how quickly isotopic signatures change within tissues.

And for movement-tracking studies, one needs a good map of standing variation from the field. In Wassenaar and Hobson's work, monarchs were grown in the lab with deuterium-spiked water to verify faithful transmission of isotopic signals from water to plant to insect, and an unusually accurate field map was created thanks to the cooperation of 100 volunteers from the Monarch Watch program, who raised caterpillars on milkweeds throughout eastern North America.

Assuming that precautions are heeded, isotopic analysis has several advantages over other methods of tracking animal movements. Unlike genetic data, it can provide habitat information, and can be used with populations that aren't genetically subdivided. Unlike mark-recapture techniques, you don't have to wait years to recover needle-in-haystack individuals. Unlike radio tracking, you can work with tiny animals like insects, and can build up larger sample sizes. Koch, whose isotopic work with seals ended up supporting data that colleagues were gathering with satellite radiotelemetry, points out that isotopic data can both corroborate results from other methods and point out inconsistencies and areas for further research.

Current work in Hobson's lab includes studying birds' eggs to determine how much nutrition comes from body stores versus directly from the diet, and mapping migratory pathways of yellow warblers using isotopes to complement DNA analysis. In addition, he and Wassenaar will map breeding origins of lesser scaup shot by hunters, to help determine whether hunting pressures are contributing to this duck's population decline.

Last year, Hobson and Wassenaar put on the first scientific meeting aimed specifically at the use of stable isotopes in animal ecology. Its success in addressing a "pent-up demand," they say, has spurred a second meeting next May in Germany. With such momentum, the outlook appears promising for isotopes to teach us much more about the movement and ecology of animals.

Jay Withgott recently left the academic track to pursue a career in science and environmental writing. His interests range widely from evolution to ecology and behavior to natural history of birds, reptiles and amphibians, plants, and arthropods.

Caleb Brown is an illustrator and biologist living in Montana. By day he drives a delivery van, and by night he draws pictures with his computer.

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Endlinks

NWRI Stable Isotope Laboratory - the Web site for Wassenaar, Hobson, and Geoff Koehler includes information on the lab's activities, staff publications, and the 2000 Stable Isotope Ecology Conference.

ISOGEOCHEM - an online resource for isotope researchers, which includes a thorough catalog of stable isotope labs throughout the world and a discussion group.

Monarch Watch - the volunteer organization that assisted Wassenaar and Hobson with their butterfly study.

Journey North - a highly informative site detailing volunteer efforts, especially those of schoolchildren, to help track migrating wildlife.

Birding with a Purpose - HMS Beagle review of the Web site of the Cornell Laboratory of Ornithology, an organization focused on the study of bird migration.