"Natural-Born Mothers" by Sarah Blaffer Hrdy, in Natural History (Dec. 1995), American Museum of Natural History, Central Park West at 79th St., New York, N.Y. 10024.

Despite Medea and other, more recent murderous moms, nothing is more synonymous with nurturing than motherhood. But researchers who study mammal mothers of various species now take a much more expansive view, reports Hrdy, an anthropologist at the University of California at Davis. Motherly behavior that just a few decades ago would have been looked upon as deviant is now thought to be as "natural" as tender loving care.

Motherhood, Hrdy writes, "is not as straightforward a matter as just turning on the milk. Mothers have to factor in recurring food shortages, predators, and social exploitation by members of their own species. Faced with poor conditions, a mother must weigh babies in hand against her own well-being, long-term survival, and— most important—the possibility of breeding again under better circumstances."

Take the cotton-top tamarins of South America, for example. These pint-sized monkeys can give birth as often as twice a year to twins whose combined weight adds up to one-fifth of the mother’s. Only with the help of fathers, older offspring, or transient adults, who carry the babies when the mother is not suckling them, can the mothers cope. A researcher at the New England Primate Center found that 57 percent of cotton-top mothers without such help abandoned their young.

Abandonment is but one strategy. A pregnant house mouse that encounters a strange male likely to pose a threat to her offspring "may reabsorb her budding embryos," Hrdy says. Among the langur monkeys of India, a young mother with many fertile years ahead of her may, under persistent assault from strange males, "simply stop defending her infant, leaving more intrepid kin—usually old females that have not reproduced for years—to intervene."

Other mammals stretch the meaning of motherhood even further. A biologist who monitored a population of black-tailed prairie dogs in South Dakota found that lowweight mothers sometimes abandoned their litters, letting other prairie dogs eat the pups and occasionally even joining in the feast themselves.