According to classical evolutionary theory, animals faced with low chances of survival should do best by following the "live fast and die young" strategy. But new studies of guppies suggest that we need more sophisticated theories to describe how evolution shapes life span.
When predators abound, it doesn't make much sense to invest energy in long-term survival. Instead, animals should mature quickly and reproduce prolifically. Studies in the wild and the lab have mostly supported this idea, but guppies living in waterfall-strewn streams in Trinidad seem to buck the theory. Evolutionary biologist David Reznick of the University of California, Riverside, and his team gauged the internal aging clock of lab-reared offspring of wild-caught guppies. Animals from waters with guppy-hungry predators aged more slowly than did fish from pools cut off from predators by waterfalls--an apparent contradiction of the classical theory. Fish from high-predation environments also reproduced for longer periods than did guppies from low-predation regions, again contradicting the conventional view, the researchers report in the 28 October issue of Nature.
Other aspects of guppy ecology were consistent with classical theories. The swimming ability of predator-prone guppies declined more rapidly with age--an example of neuromuscular aging--than in their predator-free cousins. And, as expected, guppies from predation-prone waters matured more quickly and had more offspring than did those from safer environments.
The unexpected outcome reflects the fact that predation does more than just increase mortality, says Reznick. It lowers population density, leaving more resources for the fish that survive, for instance, which might facilitate a longer life. "The ecological background is important," he says.
"The original theory is really simplified," says comparative gerontologist Steven Austad of the University of Texas Health Sciences Center in San Antonio. "It makes tons of assumptions, and in previous studies those assumptions have been met." The new work shows that "the universe of outcomes is really quite complicated," he says. Guppies might have some more wisdom to share about how lurking danger--and other forces--shape aging.