monarch decline
Isotopes to the rescue
glider theory

Isolating isotopes
You can't see isotopes. You can't feel them. But they're here. Think of them as mutant but harmless atoms of a single element that have different masses. The common isotope of hydrogen has one proton and one electron. But the hydrogen isotope deuterium also has a neutron, and is almost twice as heavy as the common hydrogen isotope.

Chemically, these isotopes of hydrogen cannot be distinguished -- when combined with oxygen, they both make water. But because deuterium is more massive, water containing deuterium (called "heavy water") evaporates more slowly and rains out more quickly.

. howzit work?
How do you weigh atoms? With a mass spectrometer, of course. © 1999, Fred Senese.

That difference in behavior gives scientists an opening big enough to drive a mass spectrometer through. Isotope-ratio mass spectrometers measure the ratio of stable isotopes of an element in a sample, and show what relative percentage of each mass is present. Isotope-ratio mass spectrometers are particularly valuable for analyzing light elements like hydrogen, oxygen, and carbon, which just so happen to be the most common elements in living things.

(Stable isotopes are different from the radioactive isotopes formed during nuclear decay. Radioactive isotopes change over time into other isotopes. Eventually they finally become stable isotopes, which cannot decay further.)

Geologists have been analyzing stable isotopes for decades. Lately, climatologists have joined in the fun. Now it's the biologists' turn.


Monarchs dining on milkweed nectar.
Photo by Karen Oberhauser. © 1987, Monarchs in the Classroom, Monarch Butterfly Sanctuary Foundation.

Mind your monarchs
dining out The first study of insect migration using isotopes was performed by two Canadian researchers, who reported their results in December, 1998 (see "Natal Origins of Migratory Monarch... " in the bibliography). The goal was to compare isotopes in monarchs found in Mexico to the isotopic "signature" of various possible breeding grounds in Canada and the United States.

The field rearing was done by volunteers and students, under the coordination of Monarch Watch, at the University of Kansas.

Milkweed -- the host of the monarch caterpillar -- and the monarch adults raised at each field site were gathered and analyzed for hydrogen isotopes in a mass spectrometer by Leonard Wassenaar and Keith Hobson of Environment Canada, Saskatoon, Canada.

Later in the same migratory cycle, 597 butterflies that died naturally were collected at the 13 known wintering sites in Mexico, and put through a similar analysis. The researchers analyzed wing membranes because their isotopic composition does not change after they are formed at the breeding site. Thus the isotopic content of wintering individuals is a good marker of the geographic origin of the monarchs.

The isotope results showed that about half of all monarchs originated in the Midwestern corn and soybean belt.

Reading rainfall
Isotopic analysts "read" clues in the form of different isotope ratios. But why are different ratios present in the first place? The monarch researchers pin the difference on rainfall.

Recall that the isotopes of an element are chemically identical, but physically different. When water evaporates from the ocean, it carries both hydrogen and the heavier isotope, deuterium. The heavier water, containing deuterium, condenses and forms rain sooner than the light (regular) water. Since most water vapor in the atmosphere comes from oceans near the equator, rain contains more deuterium near the equator than near the poles. Although mountains and wind patterns cause local variations, in general, the further north, the less deuterium.

When plants take up water, they show a similar isotope pattern. Similarly, the butterflies that eat those plants "inherit" this pattern of isotopes, making it relatively straightforward to compare plants and butterflies to determine where the butterflies hatched.

The Canadian research project was the first attempt to probe the migratory patterns of an insect with isotopes, says Wassenaar, an isotope specialist. He and Hobson have also used the technique to track the breeding grounds of migratory birds.

A current project looks at scaup, a waterfowl in central North America. Scaup numbers are declining in certain regions, but since they breed over a large, remote part of northern Canada, they are difficult to tag. "It would be nice to find a way to relate individuals that are shot to where they are coming from without requiring physical tagging," Wassenaar says. By identifying the isotopic signatures of different breeding locations, scientists will be better positioned to understand and perhaps correct the decline.

The explosion in use of stable isotopes reflects advances in technique -- and machines that don't "take a Ph.D. in physics to operate," Wassenaar says.

Tracking Teeth! Desktop-sized stable isotope mass spectrometers are handy in anthropology, too.


back more
The Why Files
There are 1 2 3 4 5 pages in this feature.
Bibliography | Credits | Feedback | Search

©1999, University of Wisconsin, Board of Regents.