On a steep slope, hip deep in bamboo grass, in the heart of the Madagascar rain forest she saved, Patricia Wright is telling a story. “Mother Blue is probably the oldest animal in this forest,” she begins. “She was the queen of group one, and she shared her queendom with what I think was her mother.”

The animals she is describing are lemurs, primates like us. They are the unlikely product of one of nature’s reckless little experiments: all of them—more than 50 living lemur species—derive from a few individuals washed from the African mainland into the Indian Ocean more than 60 million years ago. The castaways had the good luck to land on Madagascar, an island the size of Texas 250 miles off the southeast coast of Africa. And there they have evolved in wild profusion.

Wright, a late-blooming primatologist from the State University of New York at Stony Brook, has made lemurs her life, tracking bamboo lemurs and sifaka lemurs that live in a handful of social groups in Ranomafana National Park. The story she is telling, to a work party from the volunteer group Earthwatch, is one episode in a running saga from 20 years of field research in Madagascar. If her tone evokes a children’s story, that may be apt. Wright is a matriarchal figure, with straight auburn hair framing a round face, slightly protuberant eyes under padded eyelids, and a quick, ragged grin. The business of conservation has made her adept at popularizing her lemurs, using all the familiar plotlines of wicked stepmothers, families broken up and reunited, love, sex and murder.

A female sifaka lemur perches on a branch over Wright’s head. The graceful creature, a little bigger than a house cat, has a delicate, foxlike snout and plush black fur with a white patch on her back. Her long limbs end in skeletal fingers, curved for gripping branches, with soft, leathery pads at the tips. She turns her head, her stark, staring, reddish orange eyes glowing like hot coals. Then she bounds aw...