Scientists have developed a method to determine where animals obtain essential amino acids. They discovered that all life forms leave traces or ‘fingerprints’ in amino acids during biosynthesis. With these fingerprints, which are based on naturally occurring isotope variations, it is possible for the first time to distinguish between algal, bacterial, fungal and plant origins of amino acids through tissue samples. This discovery makes it possible to find out what animals have been feeding on without observing them directly or examining their stomach content....