Among meerkats--mongooses that inhabit dry regions of southern Africa--raising the kids is a group effort. The funny thing is, though, most of the offspring belong to a single mom. This dominant female gives birth to around 80 percent of the litters born in her group. In certain cooperative breeding animals, such a dominant female is the only one capable of breeding; the subordinates don't have the proper hormone levels to be fertile. Yet in meerkat society, all of the females are physiologically equipped for the task.

So why do the subordinates stick around to care for someone else's kids, when they could be passing along their own genes? Researchers have come up with two theories. One, known as the "limited control" model, holds that power struggles between the dominant female and the subordinates determine the frequency with which the subordinates breed. In contrast, the "optimal skew" model posits that the dominant female fully controls subordinate breeding, and allows them to breed only insofar as it entices them to stay with the group and assist in rearing her own litters. Previous studies of similarly organized societies had had supported the optimal skew model, but a study published today in the journal Nature suggests that meerkat behavior instead supports the limited control model.

Tim Clutton-Brock, a behavioral ecologist at the University of Cambridge, and his colleagues tracked reproductive cooperation and conflict among more than 400 meerkats belonging to some 20 groups over a seven year period. Their observations did not fit with the predictions of the optimal skew model. For example, subordinates that did manage to breed did not necessarily help raise the dominant female's litters, nor did they always stick around, won over by the breeding "bribe." Moreover, whereas the optimal skew model predicts that the dominant should sometimes kick subordinates out of the group--when they are breeding too much, for instance--Clutton-Brock and his colleagues found that booted individuals were often non-breeders.

Because various versions of these models exist, the Clutton-Brock results do not yet close the case on the mystery of cooperative reproduction. But in a commentary accompanying the Nature report, Walter D. Koenig of the University of California at Berkeley and Joseph Haydock of Gonzaga University note that the meerkat research casts doubt on whether optimal skew, which successfully explains reproduction in certain insect societies, can also explain the patterns seen in vertebrate reproduction.