Live Slow, Die Old
Since then, evidence of negative senescence has been stacking up.
In the case of moss campion, the plant has evolved a strategy of slow, deliberate growth. Doak believes it spends much of its early energy building an extremely long tap root that helps ensure water and nutrients later on, but slows the plant’s above-ground growth in the meantime. In the moss campion’s tundra home, “it’s very hard to get established,” says Doak. But once it is, its chances of surviving and eventually reproducing are high. There’s not much that will kill moss campion. The plant is so flat and low to the ground, and its leaves so tiny (less than half an inch long), that caribou and Dall sheep have a hard time eating it.
To Doak, it makes sense that natural selection would, in this case, act against aging. “Random catastrophes aren’t going to kill you, and it’s worth your while to put your investment in yourself rather than just in putting out offspring,” he says. Rather than “live fast, die young,” the campion strategy is more “live slow, die old.” Really, really old.
With some organisms, really old can mean millennia. High in the White Mountains near the California-Nevada border live some of the oldest trees in the world. Their trunks thick and gnarled, their oldest needles, born when JFK was president, still hanging on, these bristlecone pines are nearly 5,000 years old. Living five millennia is quite a feat, but what’s even more surprising is that these trees show no sign of decline. They are more likely to survive environmental stress than their younger cohorts, and they continue to reproduce at a steady rate. Their measured growth allows them to build extra-durable wood that resists rot, drought and lightning. In other words, in this case, natural selection appears to favor avoiding senescence entirely.
But plants are hardly the only organisms defying the aging process. Studies of turtles and lizards have also turned up negative senescence. One long-term study of three-toed box turtles in Missouri found that the animals were still reproducing well into their 70s.
In the mammal world, naked mole rats are the longest-living rodents. They can reach nearly 30 years of age in captivity. Scientists have found that breeding females “show no decline in fertility even well into their third decade of life,” according to a 2008 study published in the Journal of Comparative Physiology B. That makes sense, says Doak: “They live underground, in a resource-poor environment. They live cooperatively, meaning that your only chance to reproduce is after you’ve lived for a while and moved up the social strata.” Natural selection in this scenario favors individuals that live longer.
A New Threat
Doak’s moss campion research has lately turned up more than just evidence for negative senescence. He’s also found signs that global warming may be exerting a tangible influence on death’s odds. Close monitoring of the Alaskan moss campion plants over the years reveals that what’s most likely to kill the plants today is climate. “In winters when it’s quite cold but there are warm periods, the plants lose the blanket of snow that covers them,” Doak explains. They come down with the equivalent of freezer burn; ultimately, they die from being freeze-dried. “We’ve been seeing more and more of that over the course of our study,” he says.
While global warming represents a hurdle for the plants, Doak himself faces a more existential challenge. “It’s very difficult,” he admits, “to show that senescence doesn’t ever occur.” To prove conclusively that something doesn’t age would itself require human immortality. And, unfortunately, negative senescence in humans remains elusive.
[This article originally appeared in print as "Forever Young."]