On International Sloth Day the Cincinnati Zoo recounts the confusing tale of Moe

CINCINNATI -- The Cincinnati Zoo and Botanical Garden is celebrating International Sloth Day on Thursday with a webcast featuring its beloved two-toed sloth, Moe -- if Moe is awake for the party.

On this day of celebration, it should be noted that this notoriously slow-moving mammal threw caretakers a curveball earlier this year.

This spring, the Cincinnati Zoo received permission from the Association of Zoos and Aquariums to breed Moe as part of the Species Survival Plan. But after multiple attempts with the ideal male suitor, Moe showed no visible signs of pregnancy. Upon closer inspection, Moe revealed why she will never have babies: ‘She’ is actually a 'he.'

"The organization that gave up Moe in 2006 rescued him as an abandoned orphan, labeled him as a female and said, 'Here’s your female sloth," said Sarah Dapper, head keeper of the zoo’s interpretive collection. "We thought, 'Great!' because Moe was our only sloth at the time, and no one thought to question this organization.”

Moe looks down on (From L to R) Sarah Dapper, Erin Curry and Jessye Wojtusik. (Photo by Christine Charlson)

The revelation began after they brought in Moe’s male suitor and set up video cameras to monitor night time breeding activity for the nocturnal couple. Surprisingly, Dapper said Moe always seemed to be the aggressor initiating contact, a behavior absolutely unheard of among females of the species.

"In the wild, usually male sloths are the ones that pursue the females," she said. "And we thought, wow, look at Moe! So everybody got a little chuckle out of it."

While Moe clearly recognized his own role in breeding, his sex may have remained a mystery had it not been for the zoo's Family Center for Conservation and Research of Endangered Wildlife's -- also known as CREW -- intervention with ultrasound equipment to check for possible pregnancy. Dapper said the test revealed no fetus, but instead an unusual double structure. As little information exists about the anatomy of sloths, Dapper said CREW noted its findings then repeated ultrasounds over the course of several weeks.

"They finally brought Dr. Terri Roth in, really to give me the bad news I think, and she said, 'I’m sorry, Sarah, I think Moe has testes.' I said, 'What? Moe’s a girl.' And she said, "No, Moe’s a boy."

The mix-up isn’t as uncommon as it may seem, explained CREW reproductive physiologist Erin Curry. She said unlike many other mammals, sloths tend to be androgynous from the outside, making it almost impossible to determine sex by a glance.

"When we’re told an animal is a male or a female, we normally don’t question it. We assume the information is correct," Curry said. "Plus, we have no reason to believe otherwise. So when we see these two structures that shouldn’t be there, you raise your eyebrows and Google it to see, so I think it took a while for it to sink in."

The plot thickened when Dapper said it dawned on her the pair remained relatively hospitable. Aside from a few spats, she said the two seemed to be getting along fairly well, another behavior unheard of for two male sloths in such close proximity.

"So I went out and examined the other sloth very closely and compared his genitalia to Moe’s, and sure enough, they were very different externally," she said. “And we realized not only was Moe not a female and turned out to be a male, that the other male sloth was actually a female."

Perhaps nature would have revealed the mix up, Dapper said, leading to even more confusion when they discovered the supposed male caring for a newborn baby sloth. She said even with the role reversal, the pair failed to get pregnant. But Moe will have other opportunities, Dapper said: He’s genetically valuable because he came from the wild. She said she hopes to get a new female in next year for breeding.

"We’re all hoping for a baby sloth," Curry said.

The next potential suitor will go through a bit closer inspection, said CREW post-doctoral scientist Jessye Wojtusik. After testing Moe week after week, she said she’s discovered what is and is not an anomaly in a male sloth, and she now looks at the mix-up as a learning curve.

"So I was still seeing these two structures, still seeing it the next week," Wojtusik said. “I still didn’t know what it was. Certainly there is a good lesson learned. We will always check for genitalia. We won’t take anyone’s word for it again. We will always have to check on our own."

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