First mammal joins the eusocial club

For years, scientists have observed that just one female naked mole rat in a colony of about 80 animals does all of the breeding. Several generations of sterile offspring take care of the queen and her newest pups. This sort of community is typical of insects like bees, termites, ants, and wasps—not mammals—but some scientists have resisted grouping the naked mole rat with the so-called eusocial insects.

Although naked mole rats appeared to live eusocially, they hadn’t met a crucial criterion: permanent, physical traits that distinguish certain castes of a colony. For instance, queen ants have wings, but worker ants don’t.

After 10 years of data gathering and analysis, an international team of scientists reports that specialized vertebrae make the breeding female of a naked mole rat colony longer than her cohorts. This makes the rodent the first undeniably eusocial mammal known, the researchers conclude in the Nov. 21 Proceedings of the National Academy of Sciences.

“If there is a difference which is permanent, like in bone structure, I’d say, ‘Yes,’ that really does look like something eusocial,” says evolutionary biologist Bernard J. Crespi of Simon Fraser University in Burnaby, Canada. He had previously argued that evidence was lacking for labeling naked mole rats as eusocial. Now he says the newly published data have convinced him otherwise.

The lower spine of a queen naked mole rat lengthens after her first or second pregnancy, becoming at least a third longer than those of her colony mates, the researchers found. The longer abdominal area enables a pregnant queen to carry up to 27 fetuses and still fit through the narrow tunnels of her home in sub-Saharan Africa.

“I was always under the impression that when naked mole rats reach a certain maturity, they stop growing and that’s it,” says coauthor Rochelle Buffenstein, an ecophysiologist at the City University of New York. But now, she and her colleagues in Paris and Cape Town, South Africa, hypothesize that hormonal levels during nursing or the late stages of pregnancy trigger a growth spurt.

Why animals as different as naked mole rats and termites developed a similar style of community life remains a puzzle. According to evolutionary biologist Stan Braude of Washington University in St. Louis, scientists now must zero in on environmental factors required for eusocial structures to emerge.