Nature’s superlatives are inherently fascinating. The fastest. The biggest. The smallest. The strongest. The tallest. This post is about another one of those superlatives: the oldest. Specifically, this post is about the oldest vertebrates. Many of the oldest vertebrates are marine creatures, a happy coincidence given my expertise.
For reference, the life expectancy of any given person living in the United States is approximately 79. In the animal kingdom that’s pretty old – only a tiny fraction of non-human animals regularly live that long (think tortoises, whales, elephants). But again, we’re talking about superlatives; so how old can humans get? The oldest person on record was Jeanne Calment, who died at the ripe old age of 122 in 1997. Demographers estimate the maximum human lifespan to be 125, based on mortality data. But if you think that’s old, we’re just getting started. The oldest non-human vertebrates are much older indeed.
Let’s start with birds, some of my favorite organisms. There are records of captive flamingos, condors, parrots, and cranes living into their eighties, but no wild bird is known to live that long. However, this brings up an important issue in gerontology: how do we verify the age of wild animals? The most intuitive way is to identify or label an individual with a unique mark so that scientists can track the individual over time. The most well known way of doing this is bird banding, where a metal band with a unique number is affixed to a bird’s leg, like an ankle bracelet of sorts. But to get age information on a banded bird you need two things: to know roughly how old the bird is when it is banded and to recapture the bird at some later date. These are both difficult endeavors. Birds aren't easy to age: while many birds sport distinct juvenile and adult plumages, once the bird reaches adult plumage there is often no way to tell exactly how old the individual is; birds’ feathers don’t gray like human hair does. Recapturing a banded bird at some later date is even more of a challenge: banded birds are almost never recaptured. For example, the recapture rate of previously banded songbirds is a minuscule 1.3%.
Seabirds have much higher recapture rates, however, due to their breeding site fidelity. If conditions are right, most seabirds return to the same island – and often the exact same location on the island – annually to breed. Seabirds also happen to have long lifespans; some albatross don’t even start breeding until they are nearly 10 years old. The oldest known seabird is also the oldest known wild bird on the planet. She is a Laysan Albatross named Wisdom and she is at least 66 years old. Wisdom was banded as an adult in 1956 on Midway Atoll in the north Pacific by US Fish and Wildlife Service biologist Chan Robbins. Even more remarkable: she’s still reproducing; earlier this year she hatched her 41st chick! By comparison, the oldest human to conceive and give birth naturally was 59. Through her spectacular longevity and long reproductive history, Wisdom has become possibly the most famous wild bird in the world; there have been books written about her and she even has her own Facebook page (as a funny aside, we actually share 11 friends in common)!
As far as albatross go, Laysan Albatross like Wisdom are some of the smallest, and in longevity, size matters. In general, the larger the species, the longer lifespan. It is, therefore, interesting to ponder how old some of the great albatross wandering the Southern Ocean might be. These flying giants are about twice the size of a Laysan Albatross and have been recorded to live longer than 50 years, but we simply don’t know what their maximum lifespan is. My guess is a good deal older than 60, possibly older than 70. Only time and data will tell.
If larger animals tend to have longer lifespans, then baleen whales, the largest animals on earth, should be really old. This proves true for the Bowhead Whale, a large arctic filter feeder. They are enormous, weighing up to 100 tons (200,000 pounds), making them the second heaviest animal on the planet behind the Blue Whale. The first hints at the Bowhead Whale’s extreme longevity came about in an unusual way – modern Inuits discovered harpoons from 19th century hunters in the bodies of whales they captured. Simple arithmetic ages those whales at over 100 years old.
For bowheads that do not have harpoon fragments lodged in their body, scientists have used a technique called aspartic acid racemization to age individuals. The basic premise is this: aspartic acid is an amino acid (a building block of proteins) that exists in two different forms called D and L enantiomers, which are essentially mirror images of each other. Living tissue produces only the L, but not D, enantiomer. In metabolically inactive tissue, such as the lens of the eye, the L enantiomers slowly convert to D enantiomers – a process called racemization. The ratio can be used to age metabolically inactive tissue, and thus age the individual. Of course, analysis of the eye lens tissue requires lethal sampling: the animals must be dead for the sample to be taken. Luckily for scientists, Inuit hunters agreed to donate Bowhead Whale eyes for research. The results of the aspartic acid racemization were stunning: 4 of the 24 males tested (17%) were over 100 years old. One special individual, 95WW5, was estimated to be 211 years old, by far the oldest mammal on record. To put that into perspective, this whale was born around the time The Bill of Rights was ratified.
Recently however, the Bowhead Whale was dethroned as the longest-lived vertebrate by a massive cold-water fish, the Greenland Shark. These mild-mannered sharks are so slow and strange-looking their Latin name – Somniosus microcephalus – translates to “sleepy and small headed”. They are cold-blooded and live in frigid arctic waters year round. They grow exceptionally slowly – less than 1 cm per year, yet can grow to over 15 feet in length – and have an extremely slow metabolic rate; this allows them to live a very long time. But even the scientists that study them were shocked to find out just how old these gentle giants can be.
To determine the age of these sharks, the researchers used a technique called bomb radiocarbon dating. The process works as follows: most carbon atoms – nearly 99% of them – have 6 neutrons, but one-in-a-trillion carbon atoms has 8 neutrons. These different forms of carbon are known as the isotopes carbon-12 and carbon-14. As a result of the detonation of hundreds of atomic weapons in the 1950s, carbon-14 in the atmosphere doubled in the late 1950s and 1960s, and has been tapering off ever since. Because living organisms assimilate carbon from the environment, that doubling of carbon-14 in the atmosphere is reflected in the tissues and cells of all organisms that lived during the period of nuclear testing.
Scientists studying the Greenland Shark used differing quantities of “bomb pulse” carbon-14 in each individual to obtain an age estimate. There is a negative relationship between percentage of carbon-14 and age. In other words, the less carbon-14, the older the animal, because more of its tissues would have formed prior to nuclear testing and thus have incorporated more pre-bomb carbon-12. Using these methods, the authors calculated that the oldest shark in their sample was 392 years old, with a potential error of 120 years. This means that even the lowest estimate of its age (272) still beats out Bowhead Whale 95WW5 as the oldest vertebrate by over 60 years! Most likely this shark was around 400 years old, born during the time of Shakespeare and Galileo. At the upper end of the estimate, this individual could have been 500 years old, four times as old as the maximum human lifespan.
These findings were so groundbreaking it was covered by nearly 200 news outlets worldwide and it landed the Greenland Shark on the cover of Science, one of the most prestigious scientific publications in the world. Another remarkable finding was that Greenland Sharks don’t become sexually mature until at least 150 years old. This is by far the most delayed sexual maturation of any animal known. Their extremely long lifespan and extraordinarily slow reproduction also means they are vulnerable to exploitation and disturbance. If their population were reduced by some anthropogenic or natural perturbation, it would take them centuries to recover, if they recover at all. For this reason, the longest-lived organisms on the planet are also some of the most imperiled by human activities. The lesson is that we are still learning about the world around us, especially in the ocean’s depths. As a result, we need to be especially cautious when taking actions that affect the natural world. If we’re not careful, we could wipe out the most fascinating species before we can unlock the mysteries they’re holding.
Matthew Savoca holds a PhD in Ecology from the University of California, Davis. His research interests include sensory behavioral ecology, marine conservation biology, and seabird ecology.