I strongly suspect that it was revealed by the illustrator or the scientists who published the paper. What’s the point of a great prank if you’re the only one who knows about it.

On 31 January 2018, Kenneth Chiou and Christina Bergey published a paper in the journal Scientific Reports describing a method for sampling the genome of an animal population via feces: We have developed a method that makes noninvasive population genomics economically and practically feasible for the first time, by exploiting natural, evolutionarily ancient differences in CpG-methylation densities between vertebrate and bacterial genomes to enrich the host genome from feces.That paper, titled “Methylation-based enrichment facilitates low-cost, noninvasive genomic scale sequencing of populations from feces,” included a detailed diagram of the methodological process for sampling the feces (indicated with red arrow):

Source: Snopes.com