High-ranking baboons' fertility woes.

Being on top doesn't guarantee reproductive success, according to a long-term study of 138 wild baboons.

In the world of monkeys, females of a high social rank get first dibs on food and other scarce resources, and subordinates cower in their presence. Field studies have produced conflicting accounts of whether these dominant dames have more success bearing children than their subordinates.

Researchers, including chimpanzee expert Jane Goodall, have followed five baboon troops living in the Gombe National Park in Tanzania for 25 years -- the average length of a baboon's life span and a much longer period than any previous studies have covered. The researchers find no relationship between lifetime reproductive success and rank, they report in the Jan. 5 Nature. Similar numbers of offspring from subordinate and dominant baboons survived.

Compared to low-ranking baboons, the powerful females had shorter periods between births. Also, their infants survived longer, and their daughters matured more quickly. However, their higher rate of miscarriages and fertility problems offset these advantages, says coauthor Craig Packer of the University of Minnesota in St. Paul.

The scientists collected data on the baboons' menstrual cycles, births, and deaths. They determined the animals' ranks each year by observing aggressive interactions and behaviors while eating. A baboon gets its first rank when a juvenile, but rank changes over a lifetime.

The reproductive difficulties of the dominant females ensure that baboons don't become a hyperaggressive species, the researchers note. The top-ranking females probably have high amounts of androgen, the male sex hormone, which causes their miscarriages and aggressive behavior, Packer suspects.

Stress probably does not contribute to the miscarriages, as subordinate animals experience the most stress, he adds.