POST GAZETTE (Pittsburgh, Pennsylvania) 07 May 08 Secrets of the very old - Looking for the whys of long lives (Mark Roth)
When Big Mo died in March at the Pittsburgh Zoo & PPG Aquarium, the yellow-footed tortoise was reputed to be more than 70 years old.
What a piker.
In the world of tortoises, Big Mo was venerable, but hardly a record-holder. The largest of the tortoises, the Galapagos and Aldabra varieties, have been known to live to 150 and possibly longer.
When the Galapagos tortoise Harriet died two years ago in the Australia Zoo, she was not only rumored to be 176, but to be one of the tortoises that were picked up during Charles Darwin's original expedition to the Galapagos Islands.
But even if the oldest tortoises only go back as far as the Spanish-American War rather than the Civil War, their life spans are still amazing, and they raise an intriguing question: What is it about tortoise biology that makes them so long-lived?
The same thing can be asked of some even more intriguing creatures in the Methuselah Club, including the rough-eye rockfish (up to 205 years), the bowhead whale (211 years) and the ocean quahog clam (225 years).
While the age of tortoises is often obscured by missing or incomplete records, scientists have been able to measure the age of these ocean creatures precisely, said Steven Austad, an animal longevity expert at the Barshop Institute of Longevity and Aging Studies at the University of Texas Health Science Center at San Antonio.
In the bowhead whales, researchers have been able to chart the slow change in the orientation of amino acids in their eyelids, he said, while the rockfish and quahog lay down age-related rings, the rockfish in an ear bone and the quahog on its shell.
The tortoises, bowhead whales, rockfish and quahogs all share one trait common to long-lived animals, Dr. Austad said -- they can stay out of the way of predators.
The whale is too big to have many enemies, the tortoise and clam have hard shells and the rockfish lives deep in the ocean where there is less competition.
Some experts also believe that the laid-back lifestyle of the tortoises and their smaller relatives, the turtles, contribute to their longevity.
Turtles and tortoises spend a lot of their lives hibernating, sleeping and estivating (a sort of summertime hibernation), says John Iverson, a biology professor and turtle expert at Earlham College in Indiana. "The idea is that turtles live life slower and therefore live it longer," he said.
But others say it is not clear how big a role slowed-down metabolism plays in longevity.
Several decades ago, the prevailing theory was that every creature on the planet was granted about the same number of lifetime heartbeats, so that those with high metabolisms, like insects and mice, would die much sooner than those with slower biological activity, like elephants and whales.
But that theory has now been largely discredited, says Jaoa Pedro de Magalhaes, a geneticist and postdoctoral fellow at Harvard University, because there are too many exceptions to the rule.
For instance, he and Dr. Austad said, birds generally have much higher metabolisms than mammals, yet have an average life span that is three times longer than that of mammals.
Parrots can live longer than 50 years, Spanish canaries can live more than 20 years and even the frenetic hummingbird can live up to 14 years, they said.
Dr. de Magalhaes said he did a survey two years ago of hundreds of species of mammals, and "what we showed is there is really no correlation between metabolic rate and life span in mammals."
If metabolism is no longer the key, size may still make a big longevity difference, though, he said. "If you're an elephant, you're not going to be eaten by a fox."
Elephants live up to 80 years, Dr. Austad added, and would probably live longer, but their unusual dental inheritance does them in. Most elephants get only 24 molars, which move from the back of their mouths to the front as they age. After the last molars wear out, most elephants only have a limited time before they starve to death.
Tortoises also get a size benefit. Once they pass a certain growth threshold, their size, shell and lack of predators give them a fairly secure future.
At the time of his death, Big Mo weighed 86 pounds. One of the best known of the current Galapagos tortoises, dubbed Lonesome George for his lack of a mate, has now reached five feet in length and weighs 200 pounds.
Once an animal achieves a longer life span, evolution grants it another gift, the scientists said -- the ability to reproduce much later in life.
Human females go through menopause, but still can reproduce much later in their lives than other primates. And tortoises, including Big Mo, remain sexually active for years.
Even though he was never able to produce offspring, when Big Mo was paired with a mate, "he'd put everything he had into breeding with this female" late into his life, said Henry Kacprzyk, reptile curator at the Pittsburgh Zoo.
Another biological feature that some feel contributes to tortoise longevity is that they have very long telomeres.
Telomeres are the protein caps at the ends of chromosomes. When cells are grown in the lab, the telomeres shorten each time the cells replicate, eventually becoming so stubby that the cell culture dies off.
Researchers are divided over whether this same process affects life span in living organisms outside the lab.
"I'm a little bit skeptical about the idea that telomeres contribute that much to aging," said Dr. de Magalhaes, given the fact that mice, which live about four years, have longer telomeres than humans.
In the laboratory, he added, scientists have been able to create mice with short telomeres and with long telomeres, and "the mice with long telomeres don't have a significant difference in life span."
Others believe these chromosomal end caps could be important.
Mark Haussmann, a biology professor at Kenyon College, has studied telomeres in a long-lived bird species called the storm petrel, and has found that the birds that survived the longest into adulthood also had the longest telomeres at birth.
A similar study on human subjects by Richard Cawthon at the University of Utah found that those with longer telomeres outlive others in the study group by an average of five years.
Telomeres may not cause life span changes directly, Dr. Haussmann said. Instead, he thinks it is likely that when they shorten, "telomeres are kind of a scoreboard in the cell that is saying this cell is becoming more damaged or less healthy."
The damage they are signaling could be oxidative stress, he said -- specifically, the injury to tissues and blood vessels caused by oxygen molecules known as free radicals.
Creatures that live a long time may be especially good at repairing oxidative damage, he and others said.
In fact, a new study done by Swarthmore College biologist Patrick Baker shows that painted turtles seem to be especially good at healing themselves.
In the wild, painted turtle hatchlings spend the winter in shallow nests. The cold temperatures reduce their metabolism to a crawl, and sometimes freeze their bodies. The adults, meanwhile, winter at the bottom of ponds, where they survive in almost no oxygen.
Dr. Baker duplicated those conditions in the lab by putting hatchlings in subfreezing temperatures or in nitrogen gas for two days. When he restored them to normal conditions, tissue tests showed almost no oxidative stress damage.
This ability to neutralize normal tissue damage protects the turtles during hibernation and may also lengthen their lives.
Dr. Haussmann suspects the same repair mechanism exists in the birds he has studied. "They have to keep things like free radicals under wraps, because if they didn't, with their high level of metabolism from flying, they would succumb [to damage] more quickly."
And someday, the scientists say, these biological repair capabilities may offer some clues for how to expand human life span.
We might even live as long as Lonesome George.
Secrets of the very old