X-ray imaging as a time-saving, non-invasive technique for diet analysis

Dietary patterns of animals have a long-recognized importance in ecology and evolution, with numerous and diverse applications. While many methods of diet assessment exist, the most common method of direct diet examination for most small vertebrates is stomach-content analysis, using labor-intensive surgical removal of the gut following death. Methods that can reduce the time required to collect diet information without necessarily sacrificing specimens could prove invaluable for a range of applications. We evaluated digital X-ray imaging as a non-invasive method for examination of stomach contents of small fishes. Based on both a feeding experiment and examination of field-collected preserved specimens, we found that digital radiography consistently revealed the presence of moderate- to high-density prey items in the stomach, such as small arthropods. Moreover, X-ray imaging allowed for rapid identification of some particular prey items such as detritus, dipteran larvae, ostracods, hard-shelled molluscs, and small fish. However, this method failed to detect some low-density prey items present in some stomachs, and could not be used for precise taxonomic identifications in most cases. Overall, we found that digital X-ray images can be quickly acquired from anesthetized or preserved animals, permit rapid identification of certain prey items, and facilitate digital data archives. Future studies that employ this method should first “ground-truth” the radiological signatures of prey items observed within a given study using stomach-content analysis, which then permits rapid data collection strictly using X-ray images. This method can provide information useful for determining the inclusion of certain prey items in diets, even quantifying particular taxonomic groups of prey (% occurrence, % by number). Thus our results indicate that for certain study goals, X-ray radiography may provide a time reducing, non-invasive technique for diet analysis of small vertebrates.