The eastern North American population of monarch butterflies is renowned for their annual, multi-generational, round-trip migration from the Oyamel pine forests of central Mexico from the United States to Canada and back. Sadly, the monarch butterfly population is declining, and the future of the monarch’s migratory phenomenon is uncertain.
Scientists can study migration by looking at chemicals deposited in the teeth, bones, teeth and feathers of animals. In the case of monarch butterflies, the signature inherent in its wings reveals where it was a caterpillar, allowing researchers to trace its origin or birthplace.
Isotopes are atoms of the same element that have different masses because they have different numbers of neutrons. For some elements, such as hydrogen and strontium, the ratio of heavy versus light isotopes in the atmosphere varies predictably between locations, giving the locations unique isotopic signatures. Maps of these local isotopic signatures are called isotopes.
Read more: Explainer: what is an isotope?
Isotopes have informed conservation efforts for decades because they help identify where an animal came from.
Strontium (Sr) is an alkaline earth metal with four stable isotopes: 84Sr, 86Sr, 87Sr and 88Sr. The strontium isotope ratio (the ratio of 87Sr to 86Sr) is a new addition to the isotopic toolbox of ecologists. Strontium isotopes will help ecologists pinpoint the origins of migratory animals more precisely and resolve long-standing questions related to migratory interactions and migratory patterns of monarch butterflies.
what the body can reveal
As animals (including humans) feed and drink, they incorporate local isotopic signatures into their bodies. The isotopic signature of animal tissue can be compared to an isoscape map to identify where the tissue formed.
For example, the strontium isotope ratio of human teeth can tell you where a person spent their childhood because teeth form early in life. However, human bones will tell you where they spent the last decade of their lives because bone tissue replaces itself every 10 years.
Some tissue grows in layers over time, such as in tree rings or the ear bone of a fish. These layered tissues reveal where an animal was located at different times of its life, as shown with the giant teeth.
Why do we need isotope geolocation?
Ideally, we would study animal migration by placing small radio transmitters on several individuals and tracking them over long periods of time. However, this approach is not practical in many situations.
For example, we cannot use radio transmitters to trace the origins of illegal elephant ivory or the size of the home range of extinct lemur specimens from a museum. But we can use isotopes to learn something about the lives of these dead animals.
Some animals, such as insects, are very small and can be effectively tracked using tagging methods. Although significant progress has been made in recent years, insects are still too small to be tracked with large-scale radio transmitters. Therefore, isotopes are one of our best tools to answer questions about insect migratory patterns and connectivity. Given the current context of global climate change and population decline, we urgently need to learn more about animal migration so that we can preserve migration for generations to come.
Read more: Monarch butterflies raised in captivity may still be involved in migration
case of monarch butterflies
Hydrogen and carbon isotopes have been used for decades to trace the innate origins of monarch butterflies. These studies have helped guide conservation efforts and inform listing decisions.
For example, isotopes have helped researchers identify which regions of the United States contribute the most monarchs to winter populations. Other studies have shown that some monarchs are choosing non-migratory lifestyles and have shown that an extreme northwestern migration to Canada came from the Midwest.
Strontium Isotope Ratios and Monarch Butterflies
My colleagues and I recently demonstrated how strontium isotopes can be used to study animal migration. We found that strontium isotopes, especially when combined with hydrogen isotopes, can predict a more precise geolocation—about four times better—of a monarch butterfly’s natal origin than using hydrogen alone.
In our study, we created a strontium isoscape map for the reproductive range of the monarch butterfly. This means we now have a ready-to-use tool to estimate the origin of monarch butterflies using strontium.
We anticipate that applying strontium isotopes to both new and archived monarch specimens will enhance our understanding of how monarch migration patterns and connectivity have changed over time, and ultimately guide conservation actions to protect this migratory phenomenon. helps to do.