The lazy ones get to travel and mate

Lounge around. Gorge yourself on munchies. Road-trip occasionally, looking for mates.

That description might fit your average male college student. But this life of Reilly is being lived by some other mammals, Damaraland mole rats, furry burrowing rodents that live in colonies in southern Africa.

Along with their cousins, the naked mole rats, Damaraland mole rats are the only mammals considered to be eusocial, meaning that as with ants and some other insects mole rat reproduction is a cooperative affair, with a division of labor. Not everyone gets to do it.

Eusocial organisms often divide up other kinds of labor as well, with castes of workers. But not all of the mole rats are pulling their weight. South African researchers say there is a caste of lazybones, referred to, charitably, as "infrequent workers." These slothful mole rats can make up as much as 40 percent of a colony yet do only about 5 percent of the work.

Michael Scantlebury and Nigel C. Bennett of the University of Pretoria analyzed the energy expended by the lazy mole rats and their more industrious counterparts, using biological tracers for the animals' metabolism. Their study, published in the journal Nature, found that the two groups had very different levels of energy consumption. Most of the time the lazy animals did little besides eat.

In eusocial insects, castes are recognized by anatomical differences. "This is the first time it's been shown that things are happening physiologically as well as morphologically," he said.

The researchers also found that there was a method to the mole rats' laziness. Anecdotal evidence had suggested that when the soil was moistened by rain, the fat animals would dig new tunnels, looking for love by connecting with another colony. The energy studies provided further evidence that this occurs: the lazy rats' energy expenditure increased markedly after rainfall.

So although the mole rats sit around most of the time, draining the colony of resources, they are actually building up their reserves for those brief periods when they act as dispersers. "The colony puts up with them," Dr. Scantlebury said, "because they offer the chance of spreading the genes and creating future colonies."