Researchers say naked rodents break rules on mammary glands, pups

A breeding female from a Cornell laboratory colony of naked mole-rats offers proof that biologists' "one-half rule" for mammary glands does not apply to all mammals. Unlike humans and most other mammals, nursing naked mole-rats often have many more pups than mammary glands. Nicola Kountoupes/University Photography

By Roger Segelken

Only hungry babies and grown-up biologists worry whether there are enough mammary glands to go around.

Naked mole-rat mothers don't worry. Even when a female produces more than two dozen pups and has "only" a dozen mammary glands, or mammae, to feed them, naked mole-rat society has a way of keeping peace in the underground nests, according to the cover-story article in the August 1999 Journal of Mammalogy by Paul W. Sherman, Stanton Braude and Jennifer U.M. Jarvis.

"These animals break all our rules," said Sherman, a professor of animal behavior at Cornell. "Mammals are just not supposed to have so many more babies than mammae," he said. "Most mammals follow the one-half rule. That is, they produce about one-half as many young in each litter as they have mammae. In general, females have enough mammae for each young in the largest litters to have his or her own. It even works for humans, where our average litter size is one, but twins sometimes occur."

Nobody told naked mole-rats about the biologists' long-standing rule, however. Breeding female mole-rats have an average of 12 mammae and as many as 28 pups at a time.

Yet, there are no tantrums in the African burrows where naked mole-rats come from or in the popular zoo exhibits where most in the United States now live. (The newest opened this month in Syracuse's Burnet Park Zoo.)

"This disparity had us puzzled," said Sherman, who maintains six laboratory colonies of the highly social and cooperative rodents at Cornell. "We watched their nursing behavior and realized what's going on. They share. The young take turns nursing from the same mammary. There are no fights, and there's plenty of milk to go around."

No food fights among baby mole-rats, that is.

But vicious fights-to-the-death periodically occur among mole-rat females that are vying to become the colony's only breeding female, or "queen." The largest combatant usually wins the honor of producing all the babies for the colony and being attended to, in insectlike cooperative fashion, by all the other colony members. That quirk in naked mole-rat society also helps explain the equanimity at feeding time, Sherman said.

"The queen is larger to begin with, and she is kept very safe and healthy by her colony mates," Sherman said. "She is able to concentrate all her physiological resources on those huge litters, on gestation and lactation. And she is able to spend plenty of time with her pups.

"This is one more reason why Heterocephalus glaber (naked mole-rats) are so interesting to biologists," Sherman said. "They live like social insects, the adults share food and defense tasks, they have the largest litters of any known mammal, they are closely related, and now, it appears, the young are willing to share mother's milk. These animals have evolved to break the rules because of their extreme sociality."

But why not have 28 mammae, one for each pup? The biologists think that would cause more harm than good. "We hypothesize that larger numbers of mammary glands would incur increased costs, due to infections (such as mastitis) and mammary cancer," Sherman said. "Since the pups will share, and since their mother can spend enough time with them to feed every one, there is no necessity to have a mammary for every pup."

The study was supported, in part, by the National Science Foundation of the United States, the South African Research Council and the Alfred Sloan Foundation. Braude is an assistant professor of biology at Washington University in St. Louis and the International Center for Tropical Ecology. Jarvis is a professor of biology at the University of Cape Town, South Africa. Sherman is the author of more than a dozen papers and several books about H. glaber, including Biology of the Naked Mole-rat. The Journal of Mammalogy article is titled "Litter Sizes and Mammary Numbers of Naked Mole-rats: Breaking the One-half Rule."