"We were exploring whether or not fish have the equivalent of mammalian menopause," Reznick said. "We found that 60 percent of the fish had a significant post-reproductive lifespan, indicating that, yes, fish do have menopause. Indeed, their patterns of growing old are similar to those of mammals."
The researchers' statistical analysis also showed that regardless of which environments the guppies lived in, there were no differences among their populations in the probability of having a post-reproductive lifespan or in its duration.
"This is just what one might predict because these fish provide no care for their young," explained Reznick. "The older fish, after they stop reproducing, do not contribute to the fitness of young fish. As a result, the post-reproductive period is not influenced by natural selection. This result could be of interest to those who study menopause in humans and who have argued that post-reproductive humans can increase their own fitness by contributing to the fitness of their grandchildren and that the prolonged post-reproductive lifespan of humans is, therefore, the product of natural selection.
"But such arguments are difficult to prove by working on a single population or species. Nevertheless, our results show how it would be possible to evaluate whether or not menopause in humans has been shaped by natural selection. Appropriate comparisons, such as those between humans and apes, would help."