Stable isotopes have helped uncover migratory routes, trophic levels, and the geographic origin of migratory animals. They can be used on land as well as in the ocean and have revolutionized how researchers study animal movement.
What are Stable Isotopes?
Most elements exist in two or more forms, known as isotopes. Isotopes have the same number of protons but differ in their number of neutrons, resulting in different masses. This variation in the relative abundance of stable isotopes results from tiny mass differences that cause the isotopes to act differently in chemical reactions and physical processes. The lighter isotope generally forms weaker bonds than the heavier one and tends to react faster.
Stable isotopes are measured as isotopic deviations from international standards and are expressed as delta (δ) values as parts per thousand. These values are calculated as follows:
δ X = [(Rsample/Rstandard) – 1] x 1000
where X is the element (such as 13C or 15N), and R is the corresponding isotope ratio (13C/12C or 15N/14N). The quotient of the ratios in the sample relative to the standard is the δ value. The numerical values associated with the isotope ratio (such as δ12C) are the atomic masses of the isotopes and are accounted for by differences in the number of neutrons contained in the atom's nuclei. For example, δ12C contains 6 protons, 6 electrons, and 6 neutrons, while its heavier isotope δ13C contains 6 protons, 6 electrons, and 7 neutrons. Therefore, an increase in the δ values denotes an increase in the amount of the heavier isotope component; while a decrease in the values denotes a decrease in the heavy isotope content
'You Are What You Eat'
Stable isotope analysis is based on the principle ‘you are what you eat.' Stable isotope ratios vary among food webs and are incorporated into an animal's tissue via its diet. It is thus sometimes possible to infer the whereabouts of an animal moving between food webs. It is important, however, to choose the appropriate tissue for isotopic analysis, as tissues differ in how metabolically active they are. Keratin-based tissues, such as hair, feather, nail, claw, or bill, are metabolically inert after synthesis. They are usually used to study seasonal movements because an isotopic record reflecting the location of tissue synthesis remains unchanged. Conversely, metabolically active tissues provide dietary and source information for a relatively shorter period. For example, blood plasma and liver turnover elements in a matter of days or even hours, but this process takes up to several weeks in muscle and whole blood, and up to several months or even years in bone collagen. Studies that examine long-term movements, therefore, use metabolically inert tissues, while studies on recent movements (e.g., distinguishing between newly arrived and resident individuals) use metabolically active tissues with rapid turnover rates.
Stable Isotopes Used in Terrestrial Systems
Carbon isotopes can thus be used to reconstruct migratory routes if the geographical distribution of C3, C4, and CAM plants is known and the diet preferences of the study species. Carbon isotopic changes corresponding to dietary changes in a migratory species were first demonstrated in bats. It has since been used in other migratory species, including lesser snow geese, in which it was used to infer the wintering origin. More recently, other stable isotopes, such as nitrogen and strontium, have been added to increase the spatial resolution. Carbon isotopes have also been used to infer the individual fitness of migratory birds arriving at the breeding ground. An intriguing avenue for future research is the isotopic discrimination of exhaled breath (CO2), which does not require sacrificing the study animal to obtain a tissue sample.
One of the most important applications of nitrogen stable isotopes is their ability to determine the trophic level of a species. Nitrogen undergoes an increase (2-4%) in heavy isotope enrichment with each trophic level and can therefore serve as a tool in determining dietary shifts.
Deuterium (the heavy hydrogen isotope) has revolutionized stable isotope analysis in the study of animal migration. Unlike other stable isotopes, which require analysis on a species-by-species basis, deuterium signatures can be used to create a continental isotopic map, which is applicable to all migratory species. Deuterium ratios vary strongly with weather conditions, resulting in highly predictable spatial variation across continents. Constructing continental maps that predict deuterium levels is therefore relatively straightforward, given a large amount of existing data on continental weather patterns. These maps detailing variation in deuterium were quickly utilized by biologists to determine migratory origins of species. Deuterium ratios in feathers and claws were shown to be effective indicators of breeding latitude in birds.