ST. LOUIS (AP) – The St. Louis Zoo has tested 42,000 stool samples in 20 years of operation from more than 150 zoos and other organizations around the U.S. to study the hormonal health of animals.

The St. Louis Post-Dispatch reports the zoo’s endocrinology lab has taken samples from more than 60 animal species. The National Zoo in Washington, D.C., and the Brookfield Zoo near Chicago are the only other zoos that accept outside fecal samples.

Feces contain hormones that are excreted through the body’s filtering process. The hormones pass through blood, saliva, urine and hair, but Corinne Kozlowski, lead endocrinologist at the St. Louis Zoo, says fecal matter is the easiest to collect.

Gideon Erkenswick, a doctoral student who conducts research in the lab, says fecal samples provide a phenomenal amount of information.