In August this year a team of marine biologists came up with an ingenious method of radiocarbon dating sharks in Greenland. Doing so, they found one female specimen was 400 years old. She was a five-metre shark, probably born sometime around the death of Queen Elizabeth 1.
While the discovery made for some heady news releases, and excited anyone in the field of radiocarbon dating, it also raised a pretty serious question. That is, how do some animals live for so long, while others—like humans—only get a few decades?
To understand, we'll need to get a little technical. The concept of senescence refers to biological ageing. The precise mechanism behind human senescence is very complicated; however, it's fair to say we experience the progression of ageing differently to many other species. Theoretically, some animals will never die of natural causes.
Negligible senescence is basically defined by a lack of ageing, the absence of functional decline. Such a phenomenon explains some seriously long lifespans, such as the alleged 255-year-old Aldabra giant tortoise, which died in 2006 at India's Kolkata Zoo. Or the 507-year-old ocean quahog clam that was caught off the coast of Iceland in the same year.
So how are they living so long?
Dr Sarah Milton of Florida Atlantic University attributes negligible senescence to "adaptations that permit both extended anoxic survival and recovery." In other words, the ability to survive long periods without oxygen, as well as being able to recover rapidly from those periods. Both set organisms up for surprisingly long lives.
This definition doesn't include the extra biological time amassed by cases of suspended animation—such as the Tardigrade, otherwise known as That Water Bear everybody loses their shit over. Tardigrade have relatively short lifespans but can theoretically exist forever based on their ability to take biological timeouts, which is cheating.
If you're looking for the poster child for negligible senescence you need to think bigger, specifically, the saltwater crocodile.
According to American physicist and popular science pundit, Dr. Michio Kaku, crocodiles have no recognised finite lifespan. Instead, they just get bigger and bigger until they're inevitably killed out by "starvation, accidents, or disease." This is the reason we don't happen to see crocodiles the size of Boeing 747s in the wild.
Dr Kaku also claims the standard 70-year crocodile lifespan defined by textbooks is basically because "zookeepers die at 70." While this may sound like some sensationalist science reporting, there are undeniable facts we have to take into account.
For a start, saltwater crocodiles have the well-documented ability to survive without oxygen for over two hours. They also do not require lengthy recovery times following these anoxic periods. Animals such as whales can survive three hours at the bottom of the ocean hunting for squid, but require approximately double that time to recover. Crocodiles simply come up to breathe and can immediately submerge themselves again.
All of the mechanisms supporting anoxia, such as hypometabolism and the induction of various protective proteins, are associated with some serious longevity in animals.
It should also be noted that crocodiles don't appear to deteriorate with old age. They are as vigorous at 70 years old as they are at five, suggesting that they don't experience a decline after reaching sexual maturity.
Some unusually enormous crocodiles have been found in the wild, indicating that they grow for the entirety of their lives. As Billy Collett of the Australian Reptile Park explains, "[once they] hit three and a half or four metres, their growth rate slows right down to about an inch or two a year. Obviously to grow another metre takes a very long time." This rule of thumb makes it easier to speculate on the age of crocodiles that are longer than six metres.
Queensland is actually home to the world's largest captive crocodile, Cassius. Measuring in at around five and a half metres long, he's estimated to be more than 110 years old.
When I asked Billy what he thought of the proposition that saltwater crocodiles could not biologically die, he was sceptical. He did concede though that there's not a lot of research around the ageing process for crocodiles.
In captivity, stress appears to stem their lifespan. In the wild, most crocodiles are relatively young because the market for handbags in the early 20th century wiped out anything older than 100 years.
As for humans, the scramble to illuminate ageing has focused research on organisms that display negligible senescence, or no senescence at all. Organisations such as Strategies for Engineered Negligible Senescence Foundation and the Methuselah Foundation are pouring millions of dollars into regenerative research.
And though you can't live forever, just yet, science is already offering a flicker of hope for those feeling the weight of time. Efforts to reprogram our chromosomes with proteins from the biologically immortal planarian flatworm have actually produced results.
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