Noah obviously didn't think of it, but Ogden Nash might have: A 22-year-old female African elephant named Gretchen and a 2-year-old female Nile hippopotamus called Cleopatra are living amicably together at the Cincinnati Zoo, which says it is the only U.S. zoo to house those two species together.

"The two play like children," says the head keeper of the elephant house, Herb Kingsbury. "When Gretchen is lying down, Cleo will lick her around the ears and bite her ears playfully. They go in the pool together. They live in their own little world."

It's a world that Cleo, an export from the Toronto Zoo, entered with trepidation last year. One look at Gretchen's imposing trunk and Cleo was all set to pack hers. Instead, she dove to the bottom of the enclosure's pool. Now the two get along swimmingly even though Gretchen outweighs Cleo by 6,300 pounds, give or take a snack.

Cincinnati's experiment began nearly 10 years ago when Gretchen's keepers saw her reaching over a large iron barrier and playing with a hippo neighbor named Maudie. Eventually Gretchen and Maudie became roomies. Their friendship ended with Maudie's death in 1985, but the bereft Gretchen has found solace with Cleo. Says Kingsbury, who knows a peculiar twain when he sees one, "It's like they grew up together."