For example, while queen bees attempt to control reproduction within the hive, female worker bees sometimes kill the eggs and larvae of the queen.
Cambridge's Young is part of a research team that has been conducting field studies of meerkat societies in the Kalahari Desert of southern Africa for over a decade.
The researchers hope to gain a more complete understanding of a sometimes puzzling phenomenon: the evolution of cooperative societies.
Meerkats have a cooperative breeding system, meaning the adults help care for and feed other individuals' young as well as their own. Such systems have been studied in a variety of mammals and birds.
In the meerkat version, this helpful behavior is set in the context of a strong social hierarchy. In each group of up to 30 adults, around 80 percent of the litters are produced by a single, older female.
For the dominant female, the evolutionary advantage of this arrangement is clear. Not only does the top meerkat mom produce the most pups, she also has an entire group of "baby-sitters" to help raise them.
Feeding the young is particularly important during the first three to ten weeks of life, when pups' chances of survival depend largely on their food intake.
Killing her helpers' litters is therefore a way for the dominant mom to maximize the care given to her own offspring.
Such top-down control over reproduction is a common feature of cooperative animal societies. But low-status female meerkats are far from passive, Young says.
Although dominant females seek to monopolize reproduction, their efforts sometimes fail and subordinates may become pregnant.
When this happens, the subordinates may turn the tables and kill the pups of both the power-holder and others of their own status. Their young then receive the larger share of group care giving.
"Reproductive sharing arises not simply from dominant control over their subordinates but from a tactical power struggle among multiple group members, each with a degree of control," Young said.
Dark Side of Cooperation
Cooperative social systems can be at least partially explained by the theory of kin selection.
This theory argues that if an individual's reproductive opportunities are limitedas is the case for all adult female meerkats other than the dominant animalplaying a baby-sitting role may be a sensible strategy.
An animal can help pass along its own genes by ensuring the survival of its closest relatives.
Most of the time that's exactly what subordinate animals are doing, says biologist John Hoogland of the University of Maryland in College Park. But low-rank animals can try to work the system to their own advantage in other ways.
"There are always going to be subordinate individuals looking out for themselves," Hoogland said. "We might call it the darker side of cooperative breeding systems."
University of California, Berkeley, biologist Walter Koenig says the Cambridge meerkat study goes beyond previous reports by demonstrating how reproductive control may extend both up and down a hierarchy.
"Such findings certainly raise interesting questions about what it means to be a dominant versus a subordinate and the extent to which such labels are meaningful," he said.
The biologists agree that infanticide by subordinate females may occur in a larger number of species than has been reported. Newborns are often concealed, making observation of the behavior difficult.
Koenig said this and similar findings in both mammal and insect societies may "help pave the way for a unified theory of social evolution "
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