Step 1: Researchers isolate DNA from a sample of animal dung. Step 1 shows an illustration of four large, round, brownish dung samples with small inclusions in them. A right arrow points to a partially filled test tube with an illustration of a DNA helix emerging from it.

Step 2: They then use a technique called polymerase chain reaction or PCR to make millions of copies of a region of chloroplast DNA that is variable among plant species. It is called a barcode. Step 2 shows a portion of the DNA helix illustration with a right arrow pointing to an illustration of a PCR machine and another right arrow pointing to a number of DNA snippets called barcodes.

Step 3: Because each dung sample contains DNA from several plant species, PCR will generate DNA fragments with slightly different sequences. Step 3 shows an illustration of a PCR machine with a right arrow pointing to an animation showing three snippets of DNA sequence for each of three different plant species in sequence. The barcodes for plant species 1 are green, the barcodes for plant species 2 are yellow, and the barcodes for plant species 3 are blue.

Step 4: All the barcodes amplified from an individual dung sample are from the same animal. Researchers link all the PCR fragments to a short sequence of DNA specific for that sample. This tag sequence serves as an identifier for that sample and, therefore, the animal that left it. Step 4 shows an illustration of a PCR machine with a right arrow pointing to an animation showing three snippets of DNA sequence for each of three different plant species. The barcodes for plant species 1 are green, the barcodes for plant species 2 are yellow, and the barcodes for plant species 3 are blue. The animation adds a pink DNA segment called a tag to the left of each DNA snippet in sequence.

Step 5: The tagged PCR fragments from many dung samples are loaded onto a sequencing machine to determine their tag and barcode sequences. Step 5 shows the same DNA snippets from the previous step, to which a number of additional snippets are added. A right arrow points to a sequencing instrument.

Step 6: A computer then matches the barcode sequences to reference libraries of plant DNA to build a diet profile for each animal. (The sample tag tells you which animal the sample came from.) Step 6 shows the results of matching the animal barcode sequences with reference libraries of plant DNA. Tag sample 1 comes from an impala and has the barcode sequence C C T A T A G C. It is matched with the plant species Vernonia galamensis, which has the barcode DNA sequence A T C A C G T T T T C C G A A A A C A A C A A G G T T C A G A A A G C G A A A T A A A A A A G. It is also matched with the plant species Solanum nigrum, which has the barcode DNA sequence A T C C T G T T T T C T G A A A A C A A A C A A A G G T T C A G A A A A A A A G. Tag sample 2 comes from a buffalo and has the barcode sequence G G T C T A C A. It is matched with the plant species Panicum maximum, which has the barcode DNA sequence A T C C C T C T T T T G A A A A A A C A A G T G G T T C T C A A A C T A G A A C C C A A A G G A A A A G.