Sylvia Atsalis, a co-author of the study and a primatologist at the Brookfield Zoo in Chicago, said, ''Like humans, gorillas seem to go not only through menopause, but also through perimenopause, during which we have documented some hormonal changes and during which there is reduced likelihood of conception.''
Some scientists theorize that menopause is evolutionarily adaptive, giving grandmothers an opportunity to help with child care or leave mothers more time to care for existing offspring. Others see it as merely an artifact of the increased life span of animals in captivity, having no survival value for a species.
In the study, researchers monitored the menstrual patterns of 30 gorillas in 11 zoos by measuring daily fecal samples for progesterone, the hormone whose levels increase sharply just after ovulation. Of the animals, 22 were older than 30, the age at which gorillas begin to have reduced pregnancy rates and poor survival of offspring. Of the 22, five had clearly passed through menopause and seven were in perimenopause. The oldest, a menopausal 51-year-old, last gave birth at age 11, but mated with her partner until she was 49.
The scientists also observed sexual behavior -- touches, stares, sexual displays for the male, copulation and masturbation. Although they plan to do so, the researchers did not measure hot flashes.
''In humans, hot flashes and so on vary considerably in their manifestation among populations worldwide,'' said the other co-author, Susan W. Margulis, curator of primates at the Lincoln Park Zoo in Chicago. ''They are not universal. Therefore, we would not necessarily expect to find the same symptoms universally in gorillas either.''
The study, to be published in The International Journal of Primatology, began with a 41-year-old gorilla, Alpha at Brookfield. A mother of seven, she had not given birth in 13 years. Yet once a month she showed considerable sexual interest in her partner, staring at him intently, tossing hay and sometimes sitting in his lap.
Fecal analysis showed that she was still cycling, but at hormone levels that would have made pregnancy unlikely no matter how successful her flirtations. As in women approaching menopause, the aging gorillas had cycles that varied in length, and hormone production fell off compared with younger animals.
''This is not a phenomenon of captivity,'' Dr. Atsalis said. ''It happens because under optimal conditions gorillas are reaching their maximum life span, just as it happens in humans when health and nutrition improve.''