Humans have a pretty straightforward view of aging: They're born, they mature, they gradually become weaker and lose fertility, and eventually, they die.
But in nature, aging is far more diverse, new research finds. In fact, some animals are actually less likely to die the older they get — at least up to a point.
"Evolution has come up with a huge diversity of different ways of arranging one's demographic schedule," said study researcher Owen Jones, a biologist at the University of Southern Denmark.
The findings are intriguing, Jones told LiveScience, because classical evolutionary theory explains only one of these ways of aging.
Evolutionary theorists working in the 1950s through 1970s explained the familiar pattern of increasing mortality with age as a trade-off between reproduction and survival. If an organism only has a certain number of resources, it has to decide whether to allocate them to creating offspring (searching for mates, wooing them and mating) or to surviving for another year.
"The question is how do you balance that," Jones sad. "If you put everything into survival, you don't reproduce very much or at all. If you put all your energy into reproduction, then you will have a low survival." [5 Mysterious Animal Die-Offs]
No matter how healthy and resource-rich you are, there is always a slight chance that you'll die. You could be hit by a bus, struck by lightning, or end up in the path of a rockslide. For this reason, Jones said, evolution favors those who reproduce early, before anything bad can befall them. Thus, genetic mutations that favor early reproduction, even at the expense of an organism's later life, will be preserved.
Still, a few studies had revealed that some species don't age as classical theory suggests, Jones said. Jellylike animals called hydras (Hydra magnipapillata) have low mortality rates that are constant throughout their lives. Hydra die so infrequently in laboratory conditions that researchers estimate it would take 1,400 years for 95 percent of a population to die of natural causes.
The desert tortoise (Gopherus agassizii) actually becomes less likely to die with age. The tortoises aren't immortal, of course — they do still die. But their mortality rate in youth is actually higher than their mortality rate in old age. If they make it past their younger years, they're likely to keep trucking until as old as 80 years of age.
The diversity of aging
Most people who study aging focus on just a few species. Jones and his colleagues wanted a broader view, so they drew from across the tree of life, comparing aging patterns in 11 mammals, 12 other vertebrates (animals with backbones), 10 invertebrates, 12 plants and a green alga. They picked species for which there was good quality data on the life trajectory. [The Science of Death: 10 Tales from the Crypt]
"We were restricting ourselves to the datasets which followed enough individuals that you had good pictures over the whole life course, which we defined as following 95 percent of the individuals until death," Jones said.
The results highlighted the diversity between organisms, Jones said. "Mortality can go up [with age], it can stay constant, or it could go down," he said. "And the same for fertility."
At 102, the age at which 95 percent of humans are dead, a Japanese woman has 20 times the risk of mortality than the average for adult humans over the life span. In comparison, a white mangrove tree at the same so-called "terminal age" (123 years, for mangroves) is less than half as likely to die than the average adult of its species.
The Southern fulmar (Fulmarus glacialoides), a seabird, becomes more likely to die with age. But it also becomes more fertile as it grows older. Hydras have constant fertility rates their entire lives. And many animals other than humans have life spans that continue past their reproductive years, including killer whales (Orcinus orca), mynah birds (Leucopsar rothschildi) and nematode worms (Caenorhabditis elegans).
The diversity of mortality and aging is independent of life span, Jones added. It's not only long-lived creatures like the desert tortoise that show declining or constant mortality with age. The collared flycatcher (Ficedula albicollis), a migratory black-and-white bird, lives only about five years, maximum — at that age, 95 percent of collared flycatchers are dead. But the flycatcher's mortality is fairly constant throughout adulthood, not rising with age.
The findings challenge the assumptions of classical theory, suggesting the old ideas need a tweak, Jones said.
"In order to make sense of what we're seeing, theoreticians need to figure out why it is that we're seeing these patterns and make sense of it," he said.
It's likely that body size plays a role, he said. Organisms that grow with age without stopping at a certain size, like some trees, may be less vulnerable in old age to environmental fluctuations or other threats. Fish that outgrow all of their predators are likely to make it to a ripe old age, for example.
Jones and his colleagues plan to study wider populations of species and to get a sense of the reasons behind the varying life spans. For example, does it matter whether a plant is a tree or a shrub? Do certain environments promote longevity?
"There's good evidence that a lot of these plants that live a very long time tend to live in arid regions," Jones said. "Aridity might have some kind of effect."
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Stephanie Pappas is a contributing writer for Live Science, covering topics ranging from geoscience to archaeology to the human brain and behavior. She was previously a senior writer for Live Science but is now a freelancer based in Denver, Colorado, and regularly contributes to Scientific American and The Monitor, the monthly magazine of the American Psychological Association. Stephanie received a bachelor's degree in psychology from the University of South Carolina and a graduate certificate in science communication from the University of California, Santa Cruz.