Science: Penelope's Secret

For six years the earnest curators of New York's Bronx Zoo have busied themselves with the delicate problem of platypus family life. Platypus reproduction is a baffling business, for platypuses are not quite mammals. Their blood is warm and they have mammal-like fur, but they lay soft, reptile-type eggs about ¾ in. long. From the eggs hatch blind, hairless little "larvae" that nurse by licking milk from their mother's mammary pores. Only after several months do they frisk out of their burrow as furry platykittens.

Even in their native Australia, only one platypus couple (Jack and Jill) have bred in captivity, and...