Editor’s note: This report expands on reporting we started when we first visited this question in 2012. The audio story includes interview excerpts from the Curious City Fecal Matters! live event of March 2015.

There’s a natural cycle to urban life that can’t be ignored; as the snow melts away and the citizenry emerges from winter burrows, residents spend more time outdoors, and with that, there’s more opportunity to ponder the animals’ rhythms and cycles, including the less seamly ones.

Chicagoan Kelley Clink reflected on life’s natural processes, particularly as she potty-trained her pup two springtimes ago. She wondered how poop management worked on a larger (Ok, institutional) scale, and she then sent us this question:

What happens to all the, um, ‘animal waste’ from the Lincoln Park Zoo?

“My dog at the time was pooping in various places,” Clink said. “Sometimes I’d pick it up and throw it in a dumpster, and sometimes if he pooped on my rug I’d take some toilet paper and flush it inside. So it made me think, ‘Gosh, with all these animals are they flushing it? Putting it in the dumpster? Where is it going?’”

Well, the answer can be summed up like this: Lincoln Park Zoo tosses the poo, it studies the poo, and it stores the poo (in the hopes of studying the poo even more someday). That may not be returning poo to “the great cycle of life,” but it’s how the stuff is dealt with, regardless. If you can hold your nose for a short bit, here are the details.

Toss it

The first thing to note is that zoo poop is not so easy for journalists to access, so you’re spared first-hand accounts of the nitty-gritty, behind-the-scenes scraping, shoveling and the like. The Lincoln Park Zoo tells us raw animal waste is considered biohazardous, so we could not actually go anywhere near it to follow its journey.

But, the Lincoln Park Zoo confirms that the bulk of the animal waste is pretty much handled like garbage; it’s hand-removed by staff, thrown into dumpsters or bags, and compacted along with all the other garbage, according to General Curator Dave Bernier. He says the zoo uses a waste management company to cart everything away.

Some zoos have opted to use the feces for composting, even selling the material as fertilizer in their gift shops for use in home gardening. Bernier said he’s heard talk of doing similar things at Lincoln Park, but he says there are some considerable barriers to doing so. He said it would require hiring staff and the park currently doesn’t have space or a back-lot for such an operation. Besides, Bernier said, “We have a hard time getting people to like the smell of our aardvark, I can’t imagine they’d like this feces brewing somewhere.”

What can you do with zoo poo? Study it!

But the old heave-ho doesn’t apply to the zoo’s entire supply of animal feces. Dave Bernier says a portion of the poo is studied for insights into the animals’ physical and emotional well-being. In some respects, Bernier says, the zoo treats feces as a “management tool” to monitor animal health. For example, zoo keepers look for obvious changes in the consistency, color or amount of feces animals produce.

Bernier says that a few years ago the zoo “had a camel that had loose stool. Normally they’re well-formed pellets of stool — just think of chocolate-glazed donut holes.” He explains that staff looked into that camel’s diet and realized it was eating too much of the free-growing plant material in its space.

“So we ended up cutting back some of the plants they could reach in their exhibit and then their stool normalized again,” he said.

But the zoo keepers take an even closer look at feces, too, performing diagnostic tests in an on-site laboratory.

Rachel Santymire, director of the zoo’s Davee Center for Epidemiology and Endocrinology, oversees and studies about 10,000 poop samples a year from about 50 animal species at the zoo. Santymire says each sample is a clue into an animal’s emotional health.

“Animals can hide certain behaviors,” Santymire said. “I can look inside the animal — they can’t lie to me! — and I know exactly how they’re reacting to whatever they’re encountering … all from poop.”

For example, Santymire can tell whether an animal is pregnant by detecting changes in its hormonal levels. She can also get a sense of whether an animal feels stressed out — all by looking for the hormone cortisol.

Bernier says those fecal tests can be used to make important decisions, such as changing an animal’s living situation.

“I had a singly-housed female antelope which normally lives in groups and she seemed perfectly fine. But she was alone because her cage-mate had recently passed away,” Bernier said.

He wondered if introducing another antelope to her cage would ultimately be a positive change, or if it would stress her out. They tested the theory by slowly introducing a new antelope friend. All the while, staff collected and tested samples of both animals’ feces — before the introduction and after it. Bernier said the cortisol levels spiked and then dipped after the introduction.

“But ultimately both of their stress hormone levels went down below their baselines when they were together,” Bernier said. He adds that, without this kind of testing, staff could not have known whether it was a positive or negative change because the animals showed no outward signs of stress.

“Animals are meant to mask any kinds of injuries illnesses or deficiencies because a lot of them are prey animals or have to survive in a social setting,” Bernier said.

But, as zoo staff often say: Hormones don’t lie.

Institutional poo hoarders

With about 10,000 poop samples a year making their way through Santymire’s lab, you’d suspect she has a complex storage system for all that waste; however, Santymire says the setup’s quite simple. It involves refrigeration. And lots of it.

First, the animal care staff collects samples from the animals like you might pick up after a dog, using sealed, plastic bags. The staff puts those samples in refrigerators all around the zoo, and Santymire collects the new material every month. She then weighs out portions of the poop, shoves them into test tubes, and then places the tubes into carefully labeled boxes, according to species. Santymire says each box holds 100 poop samples, and she’s got 10 standard, 21-cubic-foot freezers full of poo boxes.

Why keep all that poop at the ready? Well, Santymire says, it stays fresh for a long time, making the samples good material for follow-up questions she comes up with.

“Instead of throwing away samples when we’ve published our results, I look at the tubes and say, ‘Wow, I can ask and answer another question with these poop samples. I cannot throw them away. I admit it,” she said.

The kicker: What are the grossest offenders?

Questioner Kelley Clink wasn’t just interested in the Lincoln Park Zoo poo’s ultimate destination. She tossed us a quick follow-up that we couldn’t resist: Which animal is the worst to clean up after?

We put the question to both Santymire and Bernier.

Santymire’s nominee: The Fishing Cat

“Imagine a cat that eats mostly fish. If you boil the feces you can clear out the fecal lab,” she said. “No one wants to be around when you’re working on fishing cat poop.”

Bernier’s pick: The Pygmy Hippo

“Special note on the hippos … They’re the messiest of all animals,” he said. “Because our hippos here are a river species — they’re pygmy hippos. So they advertise their territory with feces. But instead of just dropping the feces, they use their tails like a propeller and they spray it all over the place.”

(Shawn O’Dell/YouTube)

Logan Jaffe is Curious City’s multimedia producer. Follow her @loganjaffe. Jennifer Brandel founded Curious City, and is now expanding the project as Curious Nation. Follow her @jnnbrndl.