How do scientists track any animal migration?
Local and long distance movements of animals have been tracked by biologists with a wide variety of marking systems. All of these methods, including the Monarch Watch tagging project, require 1) an initial capture of the animals and then 2) some form of recapture farther along the migratory route.
Are there any limitations to these classic mark-recapture techniques?
Since we recapture such a small percentage of the initially marked animals, the amount of data recovered with these techniques is often very limited even with lots of effort. Ideally, we would be able to skip the initial capture and marking step and still be able to obtain information on the origin and movement of any individuals captured along the migration route.
Are any new techniques being developed to track migration?
Recently, researchers at Environment Canada have developed a new and promising means of studying migratory species using a naturally occurring chemical signature, stable hydrogen isotopes.
What are stable isotopes and how can they help us?
Stable isotopes are atoms of slightly different versions of the same element. Atoms are classified as part of the same or different elements based on the number of protons they contain, right? So, by definition, all the atoms of one element will have the same number of protons. But, atoms of the same element can have different numbers of neutrons. This, then, is what isotopes are: atoms of the same element that contain different numbers of neutrons. Both of the isotopes we'll be talking about are of the element hydrogen, and both of them are stable. In other words, unlike radioactive isotopes that decay quickly (that is, lose their extra neutrons quickly), hydrogen's stable isotopes keep the same number of neutrons for a long, long time.
All atoms of the element hydrogen (H) contain one proton but how many neutrons do they contain?
Most naturally occurring atoms of hydrogen contain only one neutron, but there is a tiny but measurable percentage that contain two neutrons. So, there are two naturally occurring isotopes of the element hydrogen that interest us. The rarer of these isotopes of hydrogen (the one with two neutrons) is called deuterium (D) and occurs naturally in rainwater across North America. A strong deuterium concentration gradient in rainwater occurs across North America (Canada, United States and Mexico). That means that the ratio of the two isotopes in rain water (HDO to regular H2O) is actually different in different places. Some of the factors that affect what the value of the ratio is exactly include things like
1) the local temperature where the rain is falling (most important) - hot or cold
2) the relative humidity of the area - dry or humid
3) where the water came from in the first place - Pacific Ocean, Atlantic Ocean, or Gulf of Mexico.
Thus, the isotope ratios of rainfall in desert areas that are hot and dry (SW) are different from those found in regions that are hot and humid (SE) or areas that are cold (Canada and NE). Of course, you never find a lot of deuterium anywhere, but, as a general rule, the hotter it is, the more deuterium you find in rain and the colder it is, the less deuterium you find. Scientists are still studying why this difference occurs exactly, but it is probably because a water molecule that contains deuterium (HDO) is slightly heavier than a water molecule that contains only regular hydrogen (H2O). The heavier molecule will need extra energy in the form of heat (in other words, warm local temperature) to go from liquid to vapor phase - that is to go from water to water vapor. This step is necessary before the water vapor can recondense and come down as rainwater again.
How can the deuterium content of rain tell us anything about migrating animals?
Previous studies have shown that the deuterium content (isotope ratios) of local rain is reflected in tissues of shallow rooted plants. In turn, insects that feed on these plants permanently incorporate that isotope ratio in their body parts. Also, birds that feed on those insects, in turn, incorporate the deuterium signal into their own tissues. This is pretty nifty, and means that plants and insects and other animals raised in Minnesota will have different isotope signatures in their body tissues from those raised in Kansas, Georgia, or Quebec. These differences in the isotope ratios should act as a kind of location marker and should be able to tell us the geographic origins of animals captured anywhere along their migratory route. For example, if we found a butterfly in Mexico and ran tests that showed that it had a relatively large amount of deuterium in its wings, we would know that it came from somewhere pretty warm. We wouldn't know exactly where the butterfly had come from, but we'd be able to at least rule out really cold places like Canada, northern United States and anywhere else that tends to stay cold, like high on a mountain. The researchers at Environment Canada have already used this technique to study migration in songbirds.
What do scientists need before they can use this technique to trace Monarch origins?
To use hydrogen isotope signatures to trace the origins of Monarchs we need data from two sources, controlled experiments and field studies. We conducted the controlled studies last year at the University of Kansas. Milkweed plants, Asclepias curassavica, were started from seed. Flats containing the seeds were divided into three groups and each group was given "deuterium labeled" water of a different concentration. Once the plants were large enough, we reared Monarch larvae on each group of plants. Samples of the plants and the adult Monarchs from each group were dried and then sent to Canada for analysis of their hydrogen isotope ratios. These specimens have been analyzed and our colleagues in Canada, Len Wassenaar and Keith Hobson have told us that the isotope ratios in the adult Monarchs were nearly identical to those of the water and the plants! The project is clearly off to a good start.
In the summer of 1996 we solicited help from volunteers with the field portion of the study. We asked for volunteers from each state and province east of the Rockies to rear Monarchs from eggs on naturally occurring milkweeds whose only source of moisture was rainwater. There were over 160 applications to participate in this project, and, from these applicants, we selected 99, based mostly on location and experience. Each successful applicant was sent a Monarch rearing kit containing eggs, instructions and data sheets. The applicants were asked to return a dried sample of the milkweed used to rear the Monarchs as well as three adult male and female Monarchs. Most of the volunteers successfully reared the Monarchs, and 86 samples of milkweeds and Monarchs from throughout the eastern Monarch breeding area are now being analyzed for their hydrogen isotope ratios or "home signals". Because it's still early in the data analysis, we are not yet certain whether we will ask for additional volunteers in '97. There were some regions where there were no volunteers this past season so we may ask for volunteers from a few selected areas next summer. We will keep you posted.
Once our colleagues in Canada finish analyzing the specimens and establishing the "home signals" for Monarchs throughout eastern North America, it will be possible to use the deuterium content of Monarchs to answer questions about the geographic origins of the butterflies that reach the discrete roosting sites in Mexico. We may also be able to determine whether Monarchs attempt to return to their region of origin. Results of the project will be added to this web site and presented in the Monarch Watch Season Summary.
This field project integrates information from a number of subject areas such as meteorology, atmospheric physics, geography, physiology and ecology and has proven to be very interesting to students. It was also one in which students, teachers and parents could work together to contribute to a unique scientific study. In the coming months we will develop a curriculum on this subject.