Negligible Senescence: Why Do Some Animals Age Differently?

Theoreticians in the 1960s who were trying to wrap their heads around the principles of why and how animals on this planet age argued that senescence is "inevitable." As time goes by, organisms grow old, and their probability of dying increases, they said.

But research into a wide range of organisms suggests this almost definitely isn't the case: there's a growing variety in how creatures grow old. On all branches of the evolutionary tree, some animals live fast and die young and those that are so old we don't even know how to measure their age.

Female sand-burrowing mayflies have about five minutes to two hours to mate before they die. Giant Sunda rats live about half a year, while the Rougheye rockfish can live over 200.

While many evolutionary principles behind this baffling discrepancy have started to surface, the molecular and biological reasons for aging — and why different animals do it at different rates — still baffles scientists daily.

Animal senescence is the natural process of aging in which an organism experiences physical and functional changes as it grows older, eventually leading to a decline in its overall health and vitality.

Research published in Science in 2022 shows that, out of 52 species of turtles studied, about 75 percent of them showed slow or even "negligible" senescence. They don't age. Crocodilians, tuatara, and salamanders were similarly slowly aging — some salamanders also have regenerative properties, though, so that could help. 

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Different animals tend to age according to a fast-slow continuum. On the one hand, some species develop quickly, reach reproductive maturity quickly, and die quickly — like mice, who in the wild live on average till they're one year old. On the other hand, some organisms develop slowly, take years to reach reproductive maturity, and die much later in life.

From this vantage point, it's possible to retroactively notice some patterns about what factors might have put these animals in their position on the spectrum.

"I think from an evolutionary perspective, it's now largely understood. It mostly has to do with the particular ecological niche and environment of organisms," says João Pedro de Magalhães, the chair of Molecular Biogerontology at the University of Birmingham in the U.K. He researches aging and how to stop it, and recently made headlines for saying that humans could live till they're 20,000.

"You have a lot of evolutionary circumstances in which you can have the evolution of a long lifespan or the evolution of a short lifespan," says Magalhães.

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In general, the bigger you are, the harder it is for another animal to kill and eat you. Here are some of the animals with the longest lifespans.

This is one of the hypotheses for why animals like the bowhead whale — the longest-lived mammal on the planet, sometimes reaching ages of up to 200 years — lives for so long: they don't have many predators out to get them.

Elephants also live long for similar reasons, and the size factor would also explain why animals like mice, rats, and voles are so short-lived, according to Magalhães. They're an easier snack. The typical life expectancy of elephants is approximately 65 years or even longer.

Bats, though, are tiny, yet one of the longest-lived mammals, too — reaching 40 years old. From an evolutionary perspective, it doesn't matter that they're so tiny, according to Magalhães, because they're still really good at escaping predators. 

There's some evidence that Greenland sharks can live up to 500 years, as can the ocean quahog. They live in extremely icy Arctic ocean environments, which is often associated with slow metabolism and maturation and correlates with living longer, according to Magalhães. 

Animals living and evolving in environments that allow them to escape mortality also easily preserve their longevity. Take the Galapagos tortoise. They live on an island and don't have natural predators, so they can take longer to reproduce and grow older — even past 150. "We don't actually understand very well even how long Galapagos tortoises live," says Magalhães. 

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Animals with the longest lifespans are often characterized by unique adaptations, habitat, and evolutionary factors that allow them to live for extended periods.

Turtles and tortoises have hard shells that easily protect them from predators. One could argue that animals, like humans and monkeys, evolved intelligence to escape predation — and that's what's been keeping us alive for longer, according to Magalhães.

Over the past 200 years, the average lifespan of a person living in a developed country has more than doubled. 

The habitat in which an animal lives can have a significant impact on its lifespan. Cold environments can slow down metabolism and contribute to longevity.

"The same animal at 20 degrees or 25 degrees [will] have a short lifespan at 25 [degrees]," says Magalhães, citing a review paper he helped pen in 2015. Maybe temperature factors into why bowhead whales live so long, too.

Something about being coldblooded also plays a part here, according to Magalhães. While all mammals seem to age, other vertebrates, like reptiles, amphibians, and fish, have species that don't seem to age at all.

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Animal longevity is not always consistent because it is influenced by a wide range of factors, both internal and external, and these vary among different species.

"These organisms that senescence in different ways, or don't senescence at all […] These tend to have life courses where something other than age is important," says Owen Jones, a population dynamics and biology professor at Syddansk Universitet, in Denmark. He penned a seminal paper plotting lifespans for 46 species and comparing them side-by-side.

"I think a lot of it comes down to external risk factors. So the more protected you can be, the more it's worth holding out to your body for tomorrow," says Jones.

Also, just because there are anti-aging factors that make some sense evolutionarily, it doesn't mean there's a formula that can explain exactly what combination of factors are needed to live longer than other animals.

"We know how to describe it, but we don't know the fundamental basis that allows species that have the same kinds of cells and heart and lungs and breathe air, how they can have differences that range as short as a year, or as long as 200 years," says Caleb Finch, a professor at the USC Davis School of Gerontology who researches the basic mechanisms in human aging. He also introduced the concept of "negligible senescence" in his 1990 book Longevity, senescence, and the genome.

"We know lots of things happen, but we don't have anything like a global understanding of aging in terms of mechanisms." 

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In fact, one of the reasons it's so hard to make sense of aging is that scientists still don't know what happens inside an animal's body to make it age — at a mechanistic, molecular, or genetic level — and why it happens.

"Why do mice live much faster than human beings even though they have a relatively similar biology?" says Magalhães. "Even if you treat your mouse well, you make it watch television, it's still not going to live as long as humans."

Some species possess unique genetic traits that contribute to their longevity. Magalhães's lab has been trying to unpack whether Peto's Paradox (how animals regulate cell growth and avoid cancers) has something to do with it.

"I hypothesize that a mouse develops and ages 20 to 30 times faster than a human being because its developmental software program runs 20 to 30 faster than in a human being," Magalhães says, citing his recent paper on the topic.

Similar research is helping to etch at the question.

Read More: Aging is Still One of Biology's Greatest Mysteries

Research has also identified specific genes in animals that are associated with longevity.

Scientists at the University of Rochester published a study in Nature describing how they implanted some genes from naked mole rats — mammals notorious for living up to their 30s, reproducing throughout their entire lifetime, and being exceptionally good at resistance to age-related diseases — into mice.

As a result, the mice lived slightly longer, 4.4 percent more to be precise, shining a light on how helpful these specific genes might be in determining aging. But the cell's ability to make a difference in the animal's longevity was still too tiny to explain why naked mole rats live so much longer than other rodents.

On the other hand, an analysis of bowhead whales has pinpointed the gene duplication of a gene called CDKN2C as a strategy that allows the body to halt cell division and prevent programmed cell death. This could be a key to their longevity and avoidance of tumors.

That's not too far from research Joshua Schiffman's lab at the University of Utah has found after studying the reason elephants don't develop cancers. Because of a specific gene called TP53, elephants have a longer life span and a lower risk of developing cancer. We have two copies of this tumor-suppressing gene, and the researchers encode 20 copies.

The Arctic University of Norway researchers have been analyzing the Greenland's shark immune systems. They have no bone marrow or white cells, yet they seem pretty good at fending off disease. Is that their trick for not aging?

"We're getting some clues," says Magalhães. "But this is really just the tip of the iceberg."

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These animals have developed various strategies to achieve their long lifespans, including adaptations to their environments, reduced predation risks, and unique physiological characteristics.

Bowhead whales are known for their exceptionally long lifespans, sometimes reaching ages of up to 200 years.

The typical life expectancy of elephants is approximately 65 years, or even longer. They are known for living relatively long lives, and their large size contributes to their ability to avoid many natural predators, which allows them to thrive for several decades.

Bats, despite their small size, are among the longest-lived mammals, with some species reaching up to 40 years of age.

There's some evidence that Greenland sharks can live up to 500 years. Living in extremely icy Arctic ocean environments, they are known for their long lifespans, which are often associated with slow metabolism and maturation.

The exact lifespan of tortoises can vary by species and environmental factors, but some tortoises, like the Galapagos tortoise, can live for more than 150 years. As with other species, the specific lifespan of a tortoise depends on the species, habitat, and individual factors.

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