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Around a third of mole-rats may make up a fatter, lazier class within the rodents' society.

T. Jackson

Mole-Rat "Caste" Lives the Good Life

Couch potatoes who snack while others do the chores aren't the only freeloaders in the animal world. Many animal groups have to deal with individuals that live the high life while others do all the work. A new study suggests that lazy and fat mole-rats comprise a physiologically distinct "caste" within their colonies. But the loafers aren't completely useless: Though a burden to their colony, they may play an important role in helping the group branch out into new territories.

Two species of African mole-rats are the only mammal species thought to be eusocial, meaning that some individuals do not reproduce and instead perform a specific function in the colony. Often scientists think of these social groups as having physiologically distinct castes, such as the sterile worker bees. Behavioral physiologist Michael Scantlebury of the University of Pretoria in South Africa and colleagues wanted to confirm anecdotal reports that some Damaraland mole-rats indeed differed physically and behaviorally from their peers.

Trapping mole-rats in their tunnels, Scantlebury's team injected each animal with water containing stable isotopes and released them. Two to five days later, the researchers recaptured the mole-rats and determined how dilute their isotopes had become--a measure of their energy expenditure. They found that some mole-rats were heavier: About a third of the population weighed nearly 60% more, on average, than did the rest. And during the long, dry season, the fatter mole-rats expended about 30% less energy (corrected for their extra mass) than their counterparts did.

Things changed in the rainy season. The formerly lazy mole-rats ratcheted up their digging to the point where they were expending about the same amount of energy as the other members of the colony. Scantlebury says that rain--a rare treat in this arid region--seems to trigger once lazy mole-rats to dig while the soil remains soft. These former freeloaders may then be able to contribute to the colony by expanding tunnels to find food and other mole-rat colonies, the team reports 6 April in Nature.

Not everyone buys the caste argument. "'Castes' implies that it's determined for the rest of your life," says evolutionary biologist Stan Braude of Washington University in St. Louis, who has done work on both naked and Damaraland mole-rats. It's more likely, he says, that many mole-rats go through a phase of life where they become lazier and then later slim down and put their noses to the grindstone.

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