{"id":654,"date":"2017-12-06T13:18:28","date_gmt":"2017-12-06T13:18:28","guid":{"rendered":"http:\/\/joshmitteldorf.peachpuff-wolverine-566518.hostingersite.com\/?p=654"},"modified":"2017-12-06T13:18:28","modified_gmt":"2017-12-06T13:18:28","slug":"the-varieties-of-aging-in-nature","status":"publish","type":"post","link":"https:\/\/scienceblog.com\/joshmitteldorf\/2017\/12\/06\/the-varieties-of-aging-in-nature\/","title":{"rendered":"The Varieties of Aging in Nature"},"content":{"rendered":"

In 1999, I met <\/span>Cynthia Kenyon<\/span><\/a> for the first time, and she told me her one-line proof that aging is an evolved trait. \u00a0Lifespans in nature range from hours to thousands of years. This shows that natural selection is not constrained, but can implement aging on whatever time scale is appropriate.<\/span><\/em><\/p>\n

A few years ago, <\/span>Annette Baudisch<\/span><\/a> added another dimension to this proof: It\u2019s not only the duration of life, but the shape of the aging curve that takes on so many various forms. \u00a0Misguided theories of aging are based on the human life cycle (and others like it) with Gompertz mortality. \u00a0(In the 19th Century, Benjamin Gompertz first noted that risk of death increases exponentially with age.)\u00a0 Several smart theorists have been seduced into attempting proofs\u2014either from thermodynamics or from evolution\u2014that gradual aging is a necessary consequence of the conditions of life. \u00a0\u00a0<\/span><\/em><\/p>\n

But Baudisch gathered data on hundreds of animals and plants, demonstrating that the exponential shape of the human mortality curve is just one among many possible<\/a>. \u00a0Furthermore, every conceivable shape is paired with every time scale. \u00a0Any theory of aging must account for all these ways to age. \u00a0Or<\/span> not<\/b> to age: Baudisch got her start in research collecting examples of <\/span>negative senescence<\/span><\/a>. \u00a0Given this variety, the only viable theory is, \u201cnature can do whatever she wants\u201d. \u00a0More formally, natural selection can mold aging as appropriate to fit every possible niche in every ecology. \u00a0<\/span><\/em><\/p>\n

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Aging is ancient, but it is not universal. \u00a0We are accustomed to think that animals age gradually beginning at maturity, ending with inevitable death, but life is stranger than this. \u00a0Some animals and many plants have escaped from aging entirely. \u00a0Many more pass through long periods of their lives without aging. \u00a0<\/span>Cicada<\/span><\/a> nymphs mature underground for seventeen years, while not being subject to increasing death rates or aging in any other sense. \u00a0Then the cicada emerges, mates, ages and dies all in a single day. \u00a0This is a dramatic example of semelparity<\/a>, in which aging occurs all in a rush after a single burst of reproduction. \u00a0In many such cases, the aging can be experimentally decoupled from the reproduction, demonstrating once again that the aging is a separate adaptation. \u00a0The simplest example of this is the pansies in your garden. \u00a0As long as you snip off the flowers before they go to seed, you can keep the plant blooming all summer. \u00a0<\/span><\/p>\n

Snipping off flowers before they go to seed will keep the plant alive all summer.<\/figcaption><\/figure>\n

Many plants and animals \u00a0die when they are done reproducing, as evolutionary theory predicts; but among those that long outlive their fertility, there are some (like <\/span>C. elegans<\/span><\/i> worms) that don\u2019t tend to their children or grandchildren. \u00a0What evolutionary force has provided for their continued life?<\/a><\/span><\/p>\n

A few animals and many plants don\u2019t age at all, but grow larger and stronger and more fertile through their entire lifespans. \u00a0Some have been observed to regress from mature states, and start life anew as larvae, with a full life expectancy ahead of them.<\/span><\/p>\n

 <\/p>\n

What does life without aging look like?<\/b><\/p>\n

Sanicula is a shrub growing in the meadows of Sweden, and one plot in particular has been studied continuously for seventy years. Sanicula has a life expectancy comparable to a human, but sanicula does not age. \u00a0For people, the probability of dying gets higher with each passing year, whereas for sanicula, about one shrub in 75 dies each year, irrespective of age. \u00a0A 75-year-old plant has no more mortality risk than a 10-year-old plant. \u00a0For a person, the life expectancy at birth might be 75 years; the life expectancy for someone 60 years of age might be 18 more years, and for someone 80 years old, perhaps the life expectancy is 7 more years. \u00a0For a sanicula, the life expectancy of a seedling is 75 years, and the life expectancy of a 60-year-old shrub is 75 <\/span>more<\/i><\/b> years. \u00a0There are, in fact, a few 200-year-old saniculas, and they have a life expectancy of 75 more years. At this rate, about one plant in a million should live a thousand years. \u00a0A thousand-year-old sanicula is no closer to death than a sapling.<\/span><\/p>\n

It is unknown today whether lobsters age or not. \u00a0Lobsters are fished so heavily that they rarely grow larger than a pound, but lobsters weighing more than 5 lbs are still caught occasionally (and usually released). The largest lobster on record was 44 lbs. The reason that the large lobsters are released back into the ocean is not just that they won\u2019t fit on a dinner plate. Lobsters become more fertile as they grow larger, and their young are more viable. A few large lobsters can be the breeding stock for a large area. We don\u2019t have an age record for the oldest lobster ever caught because lobsters don\u2019t have annual rings or layers that broadcast their age. The 44-lb animal was said to be more than one hundred years old, but no one knows for sure.<\/span><\/p>\n

…and not the largest on record, either.<\/figcaption><\/figure>\n

Clams also can grow larger and more fertile indefinitely. But clams have growth rings that count the years for us. The oldest clam on record (an ocean quahog of the species <\/span>Arctica islandica<\/span><\/i>) has been tagged at 507 years. Small clams have natural predators, including starfish that latch onto their shells and pull them apart by brute force. But once a clam outgrows the arms of a starfish, it can keep growing indefinitely. Clams have one foot, one mouth, no eyes or ears or stomach, no brain. Giant clams, up to 800 lbs, live the same lifestyle as their smaller relatives, sucking in the seawater, taking in thirty thousand times their weight in water every day, and filtering out plankton and algae, which continue to grow and reproduce inside them. Like giant lobsters, the giant clams provide eggs for a whole community. They have been known to release half a billion eggs in a day.<\/span><\/p>\n

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Giant clams can live hundreds of years.<\/figcaption><\/figure>\n

All of the longest-living species in the world are trees. \u00a0There are several reasons for this. \u00a0Trees invest a great deal in growth, always trying to project their leaves upward, out of the shade of other trees, to compete for the best light. \u00a0The oldest trees tower above the forest, and get first dibs at the sun\u2019s energy. \u00a0So there is a powerful evolutionary incentive for trees to live a long time so they can grow taller than their competitors, and the sky is the limit.<\/span><\/p>\n

As opposed to plants, animals\u2019 life spans are limited by a requirement of ecological stability. \u00a0Most plants produce their own food, but all animals depend on other species (either animals or plants) for their food. \u00a0Hence it is natural for a plant to live as long as possible and make as many seeds as it can make. \u00a0Trees are the best examples of Darwin\u2019s dictate that life is about reproduction. \u00a0(Sequoia trees can produce more than a billion seeds.) \u00a0But animals can\u2019t get away with reproducing faster than the plants at the base of the food chain. \u00a0Animals are evolved to guard the species lower down on the food chain, and they must never reproduces faster than the animals they eat\u2014otherwise, in a very few generations, they will wipe out their food source and their children will starve.<\/span><\/p>\n

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