Do all species age?

Bill Hamilton (1936-2000)

 

“…no life schedule, even under the most benign ecology possible, could escape my spectrum of forces of senescence..in the farthest reaches of almost any bizarre universe.”

– Hamilton (1966)

 

 

Ageing (or “senescence”) is defined as an increase in mortality (you have a greater chance of dying) or a decline in function (you produce fewer offspring, can’t run as fast, etc) with age.  There are a lot of theories for why animals age, but many of them focus on the idea that evolution simply doesn’t care about older organisms.  Once an organism has reached reproductive age and has passed on its genes, any effect that natural selection can have is significantly diminished.  The difference between producing zero offspring and one offspring is huge, but the difference between six and seven is less significant.  Since evolution is all about the babies, it tends to act to enhance survival at younger ages, but problems at older ages tend to go unchecked.  The result of this is that individuals start to fall apart, either through activation of genes that only act in later life or through simple wear-and-tear.

It has surprised some scientists, then, to find that there are some animals and plants that show negligible senescence.  These “Methuselah species”, named after the oldest person in the Bible, have been studied and have been found to not exhibit this age-related decline in survival or function.  It is worth mentioning that there are some clonal species (including many plants) and colonial species (such as jellyfish) where the notion of an “individual” is a little bit unclear.  In these cases there are a great many examples that could be argued to be “immortal”, but we will focus here on some clearer-cut examples.  Here is the list, as provided by the AnAge Database:

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Eastern box turtle (Terrapene carolina), maximum age 138

A turtle shell is made up of two parts: the “carapace” which sites on the back of the animal, and the “plastron”, which is attached to the belly.  Contrary to what children’s cartoons suggest, turtles cannot leave their shells.  However, in the box turtles the plastron has a “hinge” which allows the animal to not only retract its legs and head inside the carapace but to pull the plastron closed behind it.  This is an important defense against animals like raccoons and foxes that might try to make a meal of these terrestrial turtles.  You can see an interesting little video of this behaviour here:

Unfortunately, because the eastern box turtle is so slow moving and likes to bask on roads, it tends to suffer from high rates of mortality due to collisions with cars.  Due to its long life and the long time that it takes to reach reproductive maturity, this mortality is enough to render the species “vulnerable” according to the International Union for the Conservation of Nature (IUCN) Red List.

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Blanding’s turtle (Emydoidea blandingii), maximum age 77

The Blanding’s turtle is closely related to the eastern box turtle.  However, Blanding’s turtles are considered “semi-box” turtles because the hinge on the plastron does not allow the shell to be closed quite as tightly, and they are semi-aquatic rather than terrestrial.  As with the eastern box turtle, the Blanding’s turtle is endangered, but mostly due to habitat loss and nest predation.

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Painted turtle (Chrysemys picta), maximum age 61

Based on fossils, we think that this species has remained largely unchanged over the last 15 million years.  Where the eastern box is terrestrial, and the Blanding’s is semi-aquatic, the painted turtle is almost entirely aquatic.  Revered through Native American folk legends, this species is the state reptile of Vermont, Illinois, Michigan, and Colorado.

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Olm (Proteus anguinus), maximum age 102

The olm is an amphibian, like frogs and salamanders, but has a number of fascinating adaptations.  The animal lives in caves of southern Europe, is entirely aquatic, and is entirely blind, but its hearing and smell are highly developed.  Olms exhibit what is called “neoteny”, where they retain some features that are characteristic of younger forms in the adult (the red external gills that you can see in this picture are a good example.  In this sense, the olm is like the American axolotl.

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Rougheye rockfish (Sebastes aleutianus), maximum age 205

This is a deepwater fish that lives in mid- to high-latitudes in the Pacific Ocean.  This relatively constant environment may contribute to its longevity, as the cold waters reduce metabolic rates and the relatively low biodiversity limits the number of potential predators.  The species has a strong claim to the title of “longest-lived fish”.

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Red sea urchin (Strongylocentrotus franciscanus), maximum age 200

Sea urchins are part of a group of animals called the “echinoderms” (literally “spiny skin”). Despite its name, the red sea urchin can vary in colour from orange to nearly black. They really are fascinating organisms, with bilaterally symmetrical (i.e. they have one line of symmetry separating the “left” from the “right” sides of the body, like a worm) larvae but a radially symmetrical (lots of lines of symmetry, like a starfish) adult. They also have a fascinating feeding apparatus known as “Aristotle’s lantern”, found on the underside of the animal and with which they graze material from rocks and coral. I couldn’t find any reason why this species in particular, of all the urchins, survives for a particularly long time, so it may be that we simply haven’t looked for other examples.

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Ocean quahog (Arctica islandica), maximum age 400

This is another example of a species with a remarkably long lifespan that inhabits stable ocean environments.  The longest-lived specimen ever recorded was 374 years old in 1868.  To put this into context, this specimen was “born” within two years of Colombus arriving in the Americas.  The specimen also provided a very detailed climatological record from its growth rings in the shell (you can see these clearly in the image to the left), within which were visible the impacts of volcanic eruptions that affected ocean temperatures in the area.

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Great Basin bristlecone pine (Pinus longaeva), maximum age 4,713

But the species that potentially puts all the other to shame is the bristlecone pine.  The  location of the oldest living tree (called “Methusaleh“) is a closely-guarded secret, because the previous record-holder (called “Prometheus“) was cut down under controversial circumstances.  Allegedly, a graduate student was using a very expensive drill to sample Prometheus and, when the drill bit snapped off in the truck, decided to fell the tree to recover the bit.  They only discovered that this was the oldest living organism known when the rings were counted after the tree was cut down.  Methusaleh isn’t quite so old, but still pre-dates the Egyptian pyramids at Giza by around 200 years and actually lived five times longer than the biblical character after which it was named (Methusaleh himself only managed to live to 969 years (allegedly)).  A common factor with the bristlecones and the other specie son this list is that they inhabit very stable but extreme environments.  Rather than being deep sea and cold, the bristlecones are high altitude and very dry.  This means that they have low competition for space because other species cannot persist in the area, and the adaptations to low water availability reduce growth rates to a crawl.  The combination of low growth rates, low competition and low rates of disease mean that these bristlecones have extreme longevity.

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I’ll leave you with a fascinating little video about some of the science behind the study of ageing.

References

Hamilton, W.D. (1966) The moulding of senescence by natural selection, Journal of Theoretical Biology, 12: 12-45.

Image credits:

  • Eastern box turtle: Martin Reinbold
  • Blanding’s turtle: Public domain
  • Painted turtle: Andre Karwath
  • Olm: Arne Hodalic
  • Rougheye rockfish: NOAA
  • Red sea urchin: Kirt L. Onthank
  • Ocean quahog: G.-U. Tolkhiehn
  • Great Basin bristlecone pine: Oke
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