Last March Scholastic Press released the picture book Owen & Mzee: The True Story of a Remarkable Friendship, the story of an extremely unlikely pairing. Owen, a young hippo who became separated from his mother when the devastating 2004 tsunami struck the coast of Kenya, was rescued and brought to a wildlife sanctuary, also home to a 130-year-old giant tortoise named Mzee.

Just under a year later, there are 400,000 copies in print. Last month Scholastic followed up with a sequel, Owen & Mzee: The Language of Friendship, and Owen & Mzee: Best Friends, a board book adaptation. Both titles have a 100,000-copy first printing.

There seems to be no limit to the appeal of this story. Harcourt's Mama, a nearly wordless book depicting the heartwarming tale, was published last spring and won the 2006 Society of Illustrators Silver Medal and was named a 2006 Boston Globe-Horn Book Award Honor Book. And next month, Simon & Schuster gets into the act with A Mama for Owen by Marion Dane Bauer, illustrated by John Butler.

The start of this craze is attributable to Craig Hatkoff's discovery, along with his then six-year-old daughter Isabella, of a newspaper photo of the hippo and tortoise, snapped by photojournalist Peter Greste. They paired up with Greste and Dr. Paula Kahumbu, general manager of the Kenyan sanctuary, to tell their story. The same team worked on the Scholastic sequel as well.

And Owen and Mzee are stars in their homeland as well. The Postal Corporation in Kenya recently printed one million stamps bearing their image to commemorate the second anniversary of their friendship.

Asked what it is about this friendship that has captured the attention of so many, Ellie Berger, Scholastic's senior v-p and publisher, said, "I think it is both the story of the incredible pulling together of a community to rescue Owen from the tsunami, and the unique and enduring bond that formed between a baby hippo and a wise old tortoise. It is a relationship that continues to baffle scientists." And, obviously, to sell books.