The Deuterium Example

Linking Shorebird Habitats

The different isotopes of an element have different numbers of neutrons and therefore a different atomic mass. Many of the isotopic forms do not decay with time; hence the term, “stable isotope.” Biologists are able to capitalize on two important traits of stable isotopes. First, the isotopic ratios of a subject’s tissues reflects isotopic ratios from items it ingests. Second, the local isotopic ratios vary spatially across the face of the earth. Combining knowledge of these two traits, biologists are able to make inferences about nutritional relationships and the migration of birds, mammals, and fish.

To understand the approach, let’s look at hydrogen. Hydrogen occurs in two stable isotopes: hydrogen-1 and hydrogen-2 (deuterium, or D). Both forms, when combined with oxygen, create water. But water created from deuterium is heavier and condenses to form rain more readily than water created from hydrogen-1. Because there is more precipitation near the equator, and less near the poles, ratios of the hydrogen isotopes vary by latitude (see maps below). In general, the further away from the equator, the less deuterium is found in the precipitation. When rain falls in an area, the plants absorb the rainfall, and the plants, as well as organisms in higher levels of the food chain, reflect the isotopic ratios of the precipitation. These ratios then can be measured in plant and animal tissues. Thus, scientists can predict the geographic origin of a plant, or of an insect or bird that may have eaten the plant, by relating the ratio of deuterium in the organism to the mapped ratios of deuterium in the environment.

Patterns of delta D measurements in growing-season average rainfall in North America. (Courtesy Len Wassenaar. Updated to 2001 by L. Wassenaar, pers.comm. See earlier version in Hobson, K.A., and L.I. Wassenaar. 1997. Linking breeding and wintering grounds of neotropical migrant songbirds using stable hydrogen isotopic analysis of feathers. Oecologia 109:142-148.)

Approximate patterns of delta D measurements in growing-season rainfall in Argentina. (Approximated using linear regression.) (A. Farmer)