AT ROUGHLY 5 A.M., José is at the opposite bank. After a few minutes burrowing into the dark mud, he beelines it for the lodge, followed by the muskrat, or Sebastien, as I've come to call him, whose head is covered in leafy greens. Sebastien and José are not seen all morning.

Scandalous!

There can be no doubt now that Sebastien is shacking up with José.

Back at my apartment, I make a few calls. It turns out I am not the only one to notice the partners. Julie Larsen Maher, staff photographer with the Wildlife Conservation Society, has observed José and what looked like a muskrat. She even has a digital pic, slightly blurry and taken from far away, which she e-mails me. Like flood victims, they are perched at the tippy top of the original lodge as the rising brown waters threaten to wash away their home. It's not exactly the photo I imagined—a close–up of José and Sebastien climbing out of a limo on the Côte d'Azur after–hours—but it does suggest a beaverly imbroglio.

Darrin Lunde also knows about José. He's the collections manager for the department of mammalogy at the American Museum of Natural History, and he's done extensive research for a book about beavers.

"Could a beaver and a muskrat live together?" I ask.

"Sure, that's not uncommon," he says. "They're well known to share lodges."

"And could they …" I stutter, struggling to find the appropriate scientific term.

"God, no!" he replies. "They're completely different genera."

That's cool, I guess. New York City has plenty of space for all kinds of breeds.