Among animal societies in which group members cooperate, certain individuals get a better deal, especially when it comes to reproduction. Social insects such as bees, for example, have a single breeding queen. And other cooperative creatures, including numerous birds and carnivore species, exhibit that same pattern of reproductive dominance by one or a few females. New research has revealed a striking exception to this rule, however. According to a report published today in the journal Science, female African lions have an egalitarian breeding systemsetting them apart from not only other social carnivores but even males of their own species.

Previous observations had suggested that lionesses employ a democratic approach to breeding; pridemates hunted and reared their cubs together, and none seemed to be reproducing more than her fair share. But to determine whether subtle reproductive hierarchies might in fact exist, as in the case of female chimpanzees, veteran lion researchers Craig Packer and Anne Pusey of the University of Minnesota decided to take a closer look. They compared lifetime reproductive variation in females from 31 Tanzanian lion prides to the variation in simulated prides, in which reproductive rate and demography were the same as in the real prides but births were randomly allocated. In all of the real prides, some females had more offspring than others. But the same pattern held true for the simulated prides. The team thus failed to find any evidence of true reproductive despotism among female lions.

To explain the unusual system, the authors point out that unlike females of other cooperative species, these cats can't control one another's reproduction. Lionesses go into hiding to give birth, returning to the pride only after the cubs are several weeks old and less vulnerable to attack. This fact, in combination with the mother's formidable teeth and claws, makes it hard for one lioness to kill a pridemate's newborn. Beyond that, females participate in the communal cub-rearing only if they themselves have cubs. Thus, if one lioness were to eliminate another's cub, she would lose that female's contribution to the rearing of her own offspring. "Lion society provides a distinct alternative to the dog/bird model of cooperative breeding," the authors write, "and reveals the female lion to be one of nature's true democrats."