For female guppies, there's more to life than making babies.
A new study finds that guppies experience menopause just like humans and other animals. The study is the first demonstration of menopause in fish and raises the question of why some female animals live beyond their fertile years at all.
It was previously thought that fish don't experience menopause because they produce eggs throughout their entire lives. Birds and mammals, in contrast, have a finite number of eggs that they are born with.
Guppies typically reproduce about every 30 days and give birth to litters approximately 20 times throughout their lives.
The researchers found that as female guppies aged, they began to skip litters or even stop reproducing for extended periods of time, effectively ceasing to reproduce after a certain age.
In other words, the guppies were going through a fish version of menopause.
Living longer to have more babies
David Reznick, a biologist at the University of California, Riverside, and colleagues compared the life history of 240 guppies taken from mountain streams in Trinidad. Some of the guppies were from high-predation environments, while others were not.
The researchers divided the life history into three segments: birth to first reproduction, first to last reproduction and last reproduction to death.
In a previous study, Reznick found that guppies from high-predation environments live longer and start reproducing at a younger age than those that were at less risk. The current study was meant to explore how and why this happens.
Using evolutionary theory, the researchers were able to make specific predictions based on what they knew about the reproductive life history of guppies.
After giving birth to their litters, female guppies don't stick around to provide maternal care.
The researchers therefore predicted that if evolution by natural selection was responsible for the longer lifespan of the high-predation guppies, then it should only affect their reproductive years. This is because, from an evolutionary point of view, those are the years that are most crucial for the success of the species.
Scientists call an animal's ability to produce offspring "fitness."
"Guppies have no postnatal care for their young, so from a fitness perspective, when they last give birth, they might as well be dead," Reznick told LiveScience. "The only reason for them to live longer would be that different parts of their bodies break down at different rates."
The researchers also predicted that the post-reproductive lifespan — the period after menopause — of both the high- and low-predation guppies would be the same.
The study found both predictions to be true.
"The older fish, after they stop reproducing, do not contribute to the fitness of young fish," Reznick said. "As a result, the post-reproductive period is not influenced by natural selection."
Reznick wonders if the same might be true for other animals, including humans.
Why have menopause at all?
Scientists have long wondered why some female animals live beyond their reproductive years at all.
According to one hypothesis, sometimes called the "grandmother effect," it's so that females past their reproductive prime can help care for their offspring or relatives. So far, however, humans are the only species in which this effect has been observed.
Menopause has been observed in other animals such as Japanese quail, laboratory rats and mice, opossums and other primates such as gorillas, but most of these animals lack well-developed family networks and engage in very limited, if any, maternal care.
On the other hand, female lions and baboons — animals which rear dependent young and live in complex social groups like humans — don't experience menopause at all and die soon after giving birth to their last young.
Reznick's study suggests that like guppies, some of the animals going through menopause and experiencing significant life spans beyond their reproductive years may be doing so because of factors other than natural selection, like good nutrition and health.
The study was detailed in the Dec. 27 online edition of the Public Library of Science (PLOS), Biology.
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