When scientists write papers for publication, they often forget to reference information. In the following passage, indicate where a citation should be made.
The carbon (δ13C), nitrogen (δ15N), and sulfur (δ34S) isotope ratios of humans, other animals, and microbes are strongly correlated with the isotope ratios of their dietary inputs (1–5). There are limited differences (≤10/00) between heterotrophic organisms and their diet in either the δ13C or δ34S values. Hydrogen (δ2H) and oxygen (δ18O) isotope ratios of organic matter, however, are more useful, because δ2H and δ18O values of precipitation and tap waters vary along geographic gradients. Although differences in the δ2H and δ18O values of scalp hair have been noted in hu-mans, less is known about diet–organism patterns of δ2H and δ18O values. Four potential sources can be important: dietary organic molecules, dietary waters, drinking waters, and atmospheric diatomic oxygen. Hobson et al. provided evidence that δ2H values of drinking water were incor-porated into different proteinaceous tissues of quail. Other research showed that the δ2H values of bird feathers and butterfly wings (both are largely keratin) and water in the region in which the tissue was produced are highly correlated (14, 15). Kreuzer-Martin et al. showed that ~70% of the oxygen and ~30% of the hydrogen atoms in microbial spores (~50% proteinaceous) were derived from the water in the growth medium, whereas the remainder was derived from the or-ganic compounds supplied as substrate. (With permission from National Academy of Sciences, USA)