Why put up with a lazy rat?

Lounge around. Gorge on munchies. Occasional road trips. Yes, it sounds just like some people you know. But this way of life is also common among other mammals, like the Damerland mote rats that live in southern Africa. Lazy rats can make up as much as 40 percent of a mole-rat colony, but do only about 5 percent of the work. Now, researchers at the University of Pretoria have found out why hardworking rats let their lazy neighbors get away with it. Damarland mole rats are the only mammals that are eusocial, which means their reproduction is a cooperative affair with a caste-like division of labor. Most of the time, the lazy rats do nothing but eat. But when it rains and the soil gets moist, the fat rats dig new tunnels and took for mates by connecting with another colony. So when the rats are just sitting around eating, they are actually building up energy reserves for those brief trips. "The colony puts up with them," says researcher Michael Scantlebury, "because they offer the chance of spreading the genes and creating future colonies."