Sci. Aging Knowl. Environ., 9 January 2002
Growing Old in Style
For turtles and trees, getting old doesn't necessarily mean relentless deterioration
Abstract: Wrinkled and sluggish, turtles rarely inspire envy. Yet turtles and some kinds of trees, fishes, whales, and hydras might age at a glacial pace--or perhaps not at all. Scientists are just beginning to explore the latter possibility, and so far they can't say whether aging stops or just slows drastically. In any case, discovering how these Methuselah organisms resist the ravages of time might help us curtail our own aging.
Old But Unbowed
Even with help from a miracle-working plastic surgeon, no Hollywood star grows old as gracefully as the turtles in the backwoods of eastern Michigan (Fig. 1). Still vigorous in their 60s and 70s, the "elderly" reptiles outreproduce youngsters one-third their age. The bristlecone pines of the western United States' Great Basin manage even more impressive age-defying feats. Although they look a little worn, trees that have survived for nearly 5000 years are as spry as saplings and show no decline in fertility.
As any pet owner can attest, creatures age at different speeds, and maybe bristlecone pines and turtles simply fall to pieces much slower than, say, goldfish and hamsters. However, studies of organisms as diverse as hydras and whales raise the possibility that aging might not simply slow down: In some creatures it might stop altogether.
Gerontologists, ecologists, and other researchers are awakening to the possibility of slow or nonexistent aging, says molecular biologist Caleb Finch of the University of Southern California in Los Angeles. "It's a completely ignored part of the biology of aging," he says. But it's starting to rouse interest. In September 2000, Finch and ecologist Robert Ricklefs of the University of Missouri, St. Louis, convened the first symposium on slow aging (in Los Angeles--not far from Hollywood, appropriately enough); the National Institute on Aging will host a similar gathering in February 2002.
This newborn field of study is too young to supply many answers. No one can say whether the creatures of interest fail to age or crumble at a barely perceptible pace. As a result, scientists aren't sure whether to call the phenomenon "negligible senescence" or "slow aging." Nor can they explain how these organisms defy a process that is generally deemed inescapable. Proving that some organisms are indeed impervious to aging would upset both popular and scientific assumptions about growing old. Research just under way might answer some of the questions of apparent agelessness, but many other mysteries await scrutiny.
The best evidence for arrested aging comes from a long-term survey of the Blanding's turtles that plod around a reserve near Ann Arbor, Michigan. Ecologists began studying populations there in 1953, capturing and marking the saucer-sized turtles and measuring their rates of reproduction and survival. Some of the turtles alive today are at least 60 years old. That's a feat by itself, but what's more surprising is that these codgers show no signs of deterioration. In fact, the oldest females deserve the description "chronologically gifted." They lay bigger clutches of eggs and breed more often than do younger females. Because older females are no larger, the disparity can't be due to differences in body size, says population ecologist and current head of the project Justin Congdon of the Savannah River Ecology Laboratory in Aiken, South Carolina. What's more, the oldsters are less likely to die in any particular year than are younger turtles. "When they die, they get killed," he says. "They don't get old and die."
On only one measure do the older turtles score lower: egg survival. To Steven Austad of the University of Idaho in Moscow, that difference is telling. He argues that the decline in the number of eggs that hatch supports evolutionary biologist W. D. Hamilton's aphorism that "senescence will always creep in." But Congdon disagrees, saying that the disparity in hatching success might result from random differences in nest location and weather. He plans to test for differences in viability by raising the eggs of old and young animals under identical conditions.
Besides Congdon's turtle project, a few other studies also suggest that aging stops altogether. According to research by Ronald Lanner of the U.S. Forest Service in Placerville, California, and Kristina Conner of the U.S. Forest Service in Starkville, Mississippi, bristlecone pines up to 4713 years old show no decline in pollen viability, seed weight, shoot length, or diameter of the water-conducting tracheids--all measures of tree youthfulness (Fig. 2).
Additional circumstantial evidence from extremely old plants and animals supports the contention that some species are impervious to age. Trees dominate the longevity records, but some deep-sea fishes, whales, birds, and turtles might outlive us. The next time you order orange roughy swimming in butter, for example, remember that the fish might have been swimming in the ocean since just after the Civil War. Bowhead whales might ply the seas even longer, possibly more than 200 years.
Freedom from senescence wouldn't necessarily translate into immortality, Finch is quick to point out. "The reality is that life in the natural world is extremely risky," he says, and nonaging organisms would eventually die from predators, disease, or accidents. Even a 5000-year-old tree can go up in flames. The collection of plants and animals, fish and fowl with record-breaking life-spans looks haphazard, but Austad sees a common factor. Most species live in protected habitats or carry around their own formidable protection: Turtles sport tanklike shells, birds can fly, and whales are too bulky for most predators to tackle. Impervious defenses might be necessary to evolve extended life-spans.
A handful of reports doesn't clinch the case for negligible senescence, however. "There is good evidence that some [organisms] are extremely long-lived," Austad sums up, "but there's poor evidence that they aren't aging." Finch agrees that available data don't establish whether we are seeing slow aging or no aging. To resolve the issue, Finch and Austad say, researchers need to show that age brings no increase in mortality and no decline in fertility and physical performance. Congdon's findings come closest to this standard, suggesting that the turtles improve with age, but he hasn't proved that older animals remain hale and fit. Are their immune systems still potent? Are the animals troubled by human scourges of old age such as cataracts, clogged arteries, cancer, or arthritis? How do they match up against young turtles in stamina and brain function? Nobody knows.
Searching for the Fountainhead
Field studies to gather these basic data are expensive and time-consuming, and they sometimes run into an unusual problem: The subjects outlive the scientists spying on them.
Whether aging idles or merely creeps, these Methuselahs clearly resist the deterioration that sends most creatures to the grave. How do they tap into the fountain of youth?
Here, scientists know next to nothing--for several reasons. Most slow-aging creatures are unfamiliar and rarely studied. The common lab denizens--rats, mice, Arabidopsis, nematodes, and Drosophila--were chosen for their ability to "grow up and die in a hurry," as Austad puts it. To identify slow-aging candidates, scientists need information on longevity, mortality, and reproduction. Field studies to gather these basic data are expensive and time-consuming, and they sometimes run into an unusual problem: The subjects outlive the scientists spying on them--and for such studies, continuity in data collection is crucial. To take one example, Congdon belongs to the third generation of researchers to survey turtles at the Michigan site, and he would retire if he could locate a successor. It's hard to find people who are willing to commit to such grueling labor--especially because the publication schedule for such work doesn't match tenure timetables.
However, much of our ignorance stems from the newness of the field. Because scientists are just starting to investigate the possibility of slow or negligible senescence, they are only beginning to collect demographic, biochemical, and genetic data on long-lived organisms. Nobody has yet tried to pinpoint the causes of longevity. Expect the mechanisms to be subtle, Finch says. The cells of a weed that sprouts and dies within a few days are very similar--genetically, physiologically, and structurally--to the cells of a bristlecone pine that endures for millennia. He suspects that disparities in aging rates might arise from differences in gene expression rather than the presence of unique youthfulness genes.
Birds might prove particularly useful for identifying the mechanisms of slow aging, according to Austad. They can live two or three times as long as similarly sized mammals, despite a feverish body temperature, a roaring metabolism, and the blood sugar levels of a diabetic (Fig. 3). Austad thinks their longevity stems from their high resistance to the free radicals and browning reactions thought to cause aging (see "Aging Research Grows Up"), and he suspects that other long-lived creatures might battle senescence in the same way. Directly testing resistance to oxidation and browning damage wouldn't be difficult and would require only a small tissue sample, Austad says. From there, scientists can start tracking down the genes involved.
Finding the mechanism behind slow aging is more than an intriguing puzzle. It might bring valuable medical payoffs, providing a way to curtail our own deterioration. Plastic surgeons needn't lie awake fearing for their jobs: Such advances aren't imminent, and there will be plenty of wrinkled faces to refurbish in the meantime. But someday we might all grow old as gracefully as a turtle.
January 9, 2002
Mitch Leslie writes about science in Albuquerque, New Mexico, where he has stumbled upon the secret to remaining young: denial.
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