GUESSING someone's age is never easy, what with all the latest advances in dermatology and surgery. But at least most of us just have to deal with our own species. Spare a thought for Craig George who studies the bowhead whales of the Arctic Ocean.

Eight years ago, he got an inkling that the animals might live to be truly ancient. But how to prove his suspicion? George could find no reliable way to estimate their age. Finally, he tried looking deep into the whales' eyes. What he found surprised even him.

It all started back in 1992, when George, who is lead researcher with Alaska's North Slope Borough Department of Wildlife Management, heard that an old stone harpoon head had been found by Inupiat whalers inside a bowhead they were butchering. It looked like the sort of tool that was used before the native Alaskans came into contact with Europeans in the mid-1800s and switched to metal harpoons. "People recoiled a bit," says George. Even he was sceptical. "Then we started seeing more. That's what really got me excited."

To date, Inupiat whalers have found at least six more traditional whaling harpoon heads, made of stone and ivory, in the blubber of their prey. Comparisons with harpoons at the Smithsonian Institution in Washington, DC, date the points back to the 18th century, and George notes that the whales that survived those attacks were probably already mature when first struck. That could make bowheads among the most long-lived animals on Earth. What was needed was proof.

Unfortunately, gauging a bowhead's age proved unusually difficult. None of the techniques used successfully with other whales would work. Killer whales, for example, have been followed so closely over the past 30 years that researchers know individuals by sight and have built up an accurate picture about how they grow and mature. The age of blue whales can be estimated by counting growth layers of bony plugs on their eardrums. Other whales give up their secrets through the carbon isotopes in their baleen, which oscillate annually as they migrate from one region of the ocean to another feeding on prey with specific isotopic constituents. But bowheads are have not been well studied, their inner ears show no signs of layering, and the isotope oscillations in their baleen peter out after a dozen years or so.

George seemed to have reached an impasse. Then, a few years ago, he heard about the work of Jeff Bada, a biochemist at the Scripps Institute of Oceanography in San Diego. Bada had developed a new technique to estimate the age of marine mammals based on amino acids in lenses of their eyes. There seemed to be no reason why it wouldn't work with bowheads.

Many marine mammals, including whales, have spherical lenses-an adaptation that lets them focus well under water. Each is an onion-like structure, consisting of dozens of protein layers, the first of which is laid down while the animal is still in the womb. Once formed, it begins a type of decay that can be used to estimate the time since it was deposited.

The key is aspartic acid, one of the amino acids in lens protein. Like almost all amino acids, it can occur as one of two stereoscopic isomers, three-dimensional mirror images of each other, known as L and D forms. Only L acids are laid down, but these spontaneously switch back and forth between the D and L conformations at a known rate--a process called racemization. The longer this goes on, the closer the ratio between the D and L forms approaches one-to-one. Using liquid chromatography, Bada is able to measure the ratio of a sample taken from the centre of the eye. This produces an estimate of how long it's been since the first layer in the lens nucleus was laid down.

George had access to the frozen remains of 48 bowheads harvested between 1978 and 1996. He thawed out the eyes, removed the lenses and sent them to Bada. Working with Judy Zeh from the University of Washington, Bada came up with age estimates that left even veteran bowhead biologists stunned. Four of the whales were more than 100 years old, and one topped 211. That rivals the 200-year-old giant tortoises and giant clams that hold the record for the longest lived animals. Even with a margin of error of 25 years, that would make bowheads the oldest mammals around, period. And the whales George's team were measuring were only dead because hunters had killed them.

Many questions remain. Are bowheads still reproductively active into their second century? If not, what other evolutionary pressures could have caused such a long lifespan? To biologists, creatures that produced few offspring over a long period of time are known as "K-strategists," as opposed to the "r-strategists" that live short, fast lives and produce large numbers of offspring. It is possible, says Kerry Finley, an independent biologist who studied bowhead behaviour for 14 summers in the Canadian Arctic, that a relentlessly unforgiving environment has led to a species that invests an unusual amount of effort in a slow growth pattern, rather than rapid reproduction. George, too, suspects that bowheads may simply be the ultimate K-strategists.

But there may be other forces at work. Studies of species as diverse as yeast, humans and insects suggest that death rates often fall at the extreme end of regular lifespans. James Vaupel of the Max Planck Institute for Demographic Research in Rostock, Germany, and a team of demographers have analysed a wide variety of longevity studies. Their findings challenge the conventional wisdom that post-reproductive periods should be brief because there is no evolutionary advantage to living any longer.

According to Charles Krebs, a population ecologist at the University of British Columbia in Vancouver, a long life makes sense for a species like the bowhead. A socially sophisticated and relatively intelligent animal, bowheads tend to travel and hunt together, and recordings of bowhead "songs" suggest they exchange information to help each other avoid pack ice. It could be that the experience that accumulates with old age proves beneficial to the individual, and perhaps to other whales. Basically, says Krebs, "the older guys are not only bigger, but wiser. They know where to look for food and how to avoid predators."

Of course, even the old ones weren't able to escape the whalers' harpoons. Although the 8,000-strong bowhead population of the western arctic is growing at about three per cent a year, recovery elsewhere is proving elusive. In Canada's Eastern Arctic, for example, there are probably no more than 1,000, and possible a few as 500 left.

Despite the low numbers, Inuit whalers of the region were recently granted permission to harvest one or two bowheads every year or two. Finley says the surprising longevity figures suggest that decision was misguided. "There will never be sufficient funds to quantify the life parameters necessary to manage the species so finely," warns Finley. "To play statistical brinkmanship with such odds is irresponsible. The consequences won't be known for a long time."

For further reading:

The Bowhead Whale, Editors John J Burns, J. Jerome Montague and Cleveland J. Cowles. The Society for Marine Mammalogy, Lawrence KS, 1993

Age and growth estimates of bowhead whales (Balaena mysticetus) via aspartic acid racemization. John C. George, Jeffrey Bada, Judith Zeh, Laura Scott, Stephen E. Brown, Todd O'Hara and Robert Suydam. Canadian Journal of Zoology. Vol. 77; 571-580.

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