快猫短视频

GUESSING someone鈥檚 age is never easy, what with all the latest advances in
dermatology and surgery. But at least most of us only have to deal with our own
species. So 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
for more than two centuries鈥攂ut how could he prove his suspicion? He knew
of no reliable way to estimate their age. Finally, he tried looking deep into
the whales鈥� eyes, and what he saw surprised even him.

It all started when George, who is lead researcher with Alaska鈥檚 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. But
George was sceptical; perhaps the story had been made up. 鈥淭hen we started
seeing more. That鈥檚 what really got me excited.鈥�

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

Unfortunately, gauging a bowhead鈥檚 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 of
how they grow and mature. The age of blue whales can be estimated by counting
growth layers in the bony plugs on their eardrums. Other whales betray their age
through the carbon isotopes in layers of their baleen, or bone. Ratios of these
oscillate annually as the whales migrate from one region to another feeding on
prey with distinctive isotope profiles, and this is reflected in the bone they
lay down. But bowheads are little 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 had reached an impasse. Then, a few years ago, he heard about the work
of Jeff Bada, a biochemist at the Scripps Institution of Oceanography in San
Diego. Bada had developed a new technique for estimating the age of marine
mammals from the amino acids in the lenses of their eyes. There seemed to be no
reason why it wouldn鈥檛 work with bowheads.

Many marine mammals, including whales, have spherical lenses, an adaptation
that lets them focus well underwater. 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, chemical changes begin in each layer 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, in a
process called racemisation.

The longer this goes on, the closer the ratio between the d and l forms
approaches 1:1. Using liquid chromatography, Bada is able to measure the ratio
in a sample taken from the centre of the eye. This gives an estimate of how long
it鈥檚 been since the first layer in the lens was laid down.

George had access to the frozen remains of 48 bowheads killed 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 was 211. That rivals the 200-year-old
giant tortoises and giant clams that hold the record for the longest-lived
animals. And even with a margin of error of 25 years, this would make bowheads
the oldest mammals on Earth. And the whales George鈥檚 team measured were only
dead because hunters had killed them.

Life in the slow lane

Many questions remain. Are bowheads reproductively active into their second
century? If not, what other evolutionary pressures could have caused such a long
lifespan? Biologists call creatures that produce few offspring over a long
period of time 鈥淜-strategists鈥�, as opposed to the 鈥渞-strategists鈥� 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
created a species that maximises its chances of surviving and reproducing by
growing slowly. George, too, suspects that bowheads may 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. So the risk of an individual whale dying in a given year
might actually decrease after a certain age. James Vaupel of the Max Planck
Institute for Demographic Research in Rostock, Germany, and his colleagues have
analysed a wide variety of longevity studies. Their findings challenge the
conventional wisdom that animals should die soon after reaching the end of their
reproductive lifespan, 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. Socially sophisticated and relatively intelligent animals, bowheads
tend to travel and hunt together, and recordings of their 鈥渟ongs鈥� suggest they
exchange information to help each other avoid pack ice. It could be that the
experience that accumulates with age is beneficial to individual whales, and
perhaps to others. 鈥淭he older guys are not only bigger, but wiser,鈥� says Krebs.
鈥淭hey know where to look for food and how to avoid predators.鈥�

Of course, even the oldest animals weren鈥檛 able to escape the whalers鈥�
harpoons. And although the 8000-strong bowhead population of the western Arctic
is growing at about 3 per cent a year, elsewhere recovery is proving elusive. In
Canada鈥檚 eastern Arctic, for example, there may be as few as 500, and probably
no more than 1000 individuals. Despite the low numbers, Inuit whalers in the
region were recently granted permission to kill up to two bowheads a year. It is
impossible to say whether this is sustainable: the data is simply not available.
But, says Finley, the surprising longevity of bowheads suggests that even
without interference the population will grow extremely slowly, so the decision
looks misguided. 鈥淭o play statistical brinkmanship with such odds is
irresponsible. The consequences won鈥檛 be known for a long time.鈥�