Most animals reproduce until death, maximizing their number of offspring. Which begs the question: If animals are just vehicles designed so genes can reproduce themselves, as Richard Dawkins' selfish gene theory suggests, what is the evolutionary advantage in an animal living beyond its fertility window?
Go ask grandma.
The grandmother hypothesis posits that it is evolutionarily worthwhile for a female to live beyond the birth of her children and grandchildren so she can help raise her descendants. The theory has mostly been used to explain human menopause, but it's not well supported in the rest of the animal kingdom.
Support for the grandmother hypothesis is lacking, in part, because we have observed menopause in so few animals: lab rats and mice, opossums, pilot whales and some primates, such as gorillas.
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Now, we can add guppies to the list. Guppies that die of old age—rather than predation—experience menopause. A study published in the January issue of PLoS Biology, shows that female guppies live up to 15% of their life after they are no longer able to reproduce.
Lead author David Reznick, a biologist at the University of California Riverside, captured wild guppies from environments in which there are no predators—such as pools above natural barriers like waterfalls—and from habitats with strong predation pressure. He then raised two generations of offspring in identical lab conditions Eventually, Reznick compared the grandchildren of fish from the two environments, examining genetic differences between the populations.
In earlier work, Reznick found that, contrary to what evolutionary theory would predict, guppies whose grandmothers hailed from high predation environments lived longer than those whose grandmothers were from predator-free environments. In this study, Reznick found that females in both populations have identical post-reproductive lifespans, indicating that the longer total lifespan of female guppies in high-predation habitats is the result of a longer reproductive period, not a longer post-reproductive period.
Reznick's work neither supports or negates the grandmother hypothesis. One wouldn't expect that guppies make good grandmothers—the species doesn't raise its young. In fact, a female will eat her offspring if given the opportunity. This makes it even more strange that guppies have a post-reproductive lifespan at all.
"Proponents of the grandmother effect certainly love their hypothesis, but there are no compelling data to support the notion," said Craig Packer, associate professor of biology at the University of Minnesota, via e-mail. "My research on this topic in Savanna baboons and African lions showed no evidence whatsoever for any adaptive significance to post-reproductive lifespan."
Reznick concludes that post-reproductive life is not the result of evolutionary selection pressures but is, rather, an artifact of the varying rates of decline for different body parts.
"There's an old song we used to sing when we were kids, called 'The Grandfather Clock,' about this clock that is so well designed that every piece of it wore down at the same rate," Reznick said. "One day every part of the clock failed and the clock stopped."
"Apparently, our bodies aren't built that way," he continued. "The different components of our bodies wear out at different rates."
"The reproductive tissues in these animals clearly wore down and stopped working before the somatic components—muscles and organs—stopped working."