"Lazy Slob" Mole Rats Are Key to Colony Growth, Study Says

Nicholas Bakalar
for National Geographic News

April 5, 2006

A few lazy rats are getting away with doing practically nothing while
others do all the work. And no, this has nothing to do with what goes on
in your office.

Between 60 and 75 percent of a population of Damaraland mole rats (Cryptomys damarensis)—rodents native to southern Africa (see map)—are "frequent workers."

These industrious rats perform 95 percent of the work, while the remaining "infrequent workers" enjoy the results, new research shows.

Damaraland mole rats, along with the closely related naked mole rats, are the only known mammals that are eusocial—that is, they live in social colonies with a division of labor among members.

A queen rat rules, and worker animals tend to her needs, including one or two males that mate with her.

But some of the workers carry more of a load than others, according to findings published in tomorrow's issue of the journal Nature.

Infrequent workers get busy only during wet seasons when the ground is soft enough to extend the burrows the animals live in. At these times the infrequent workers actually do more than the frequent workers.

"The real question is why the frequent workers put up with and feed these lazy slobs," said John R. Speakman, a professor at the University of Aberdeen in Scotland and co-author of the study.

"The answer is that some of their genetic material is locked up in the slobs as well, because all the members of a colony are related."

Rats in Overdrive

Speakman, tongue planted not too firmly in cheek, draws an analogy to explain.

"One way to think about this is to imagine the infrequent workers are like older teenagers. They do nothing around the house, and they eat all your food. Yet you tolerate them, because they are your only way to spread your genes into the wider world.

"This is not an exact analogy, because the degrees of relatedness are different, but you get the general picture."

The researchers trapped and measured some of the animals near the town of Hotazel, South Africa (see map).

They found that infrequent workers are larger than frequent workers and have a higher percentage of body fat.

One way for a rat to ensure that his or her genes will continue into the next generation, Speakman explained, is to "work like fury supporting the queen."

But that requires lots of energy, and there's none left over to dig more burrows and extend the colony's productive range.

Another technique for ensuring gene survival, therefore, is to sit around getting fat until the wet season, when it's time to dig. And that's exactly what a small minority of mole rats do.

"To be a good disperser," Speakman continued, "you need lots of muscles to dig new tunnels without guaranteed energy supplies and lots of fat to support your digging activity.

"You can only get these if you laze around all day doing not very much work. When it rains and the soil becomes moist, however, the infrequent workers go into overdrive, and their work rate exceeds that of the frequent workers."

Above or Below?

But some experts see a few unanswered questions in the team's findings.

"Is all [mole rat] dispersal below ground?" asked Rodney L. Honeycutt, professor of wildlife and fisheries biology at Texas A & M University.

There is, he said, evidence that the closely related naked mole rats (Heterocephalus glaber) extend the colony's range above ground.

"It's unclear whether or not this dispersal increases during the rainy season," he said.

Honeycutt also wonders whether the infrequent workers are older than the frequent workers and whether they might be the progeny of a different queen.

"If they are," he said, "any gains in terms of inclusive fitness would be considerably less than for the frequent-worker group."

Stanton Braude is an instructor in the behavioral ecology department of Washington University in St. Louis.

"This is a very interesting finding," he said. "It makes sense that the dispersers will save energy rather than contribute to the colony."

But like Honeycutt, Braude questions the team's assumptions about how the rats extend their territory.

"The only glaring error I see in the paper," he said, "is the assertion that in both species the animals can only disperse by extending the burrow.

"The authors … ignore work by me and [colleague] Justin O'Riain documenting that naked mole rat dispersers move above ground, sometimes for an amazing two-plus kilometers [about a mile]."

Michael Scantlebury, a researcher at the University of Pretoria in South Africa and the lead author of the study, disagrees with the reasons infrequent workers venture above ground.

He asserts that there is good evidence "that immediately after a period of heavy rainfall, infrequent workers make prospecting forays away from the colony in an attempt to mate with members of other colonies or found new colonies."

And the rest of the time, apparently, they kick back. At least for Damaraland mole rats, laziness might have its rewards.

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