Some cold-blooded animals can slow their ageing so effectively that they're essentially not ageing at all. By studying these animals, researchers hope to gain insights that could inform treatments and medicines for human age-related conditions.
The most comprehensive study of ageing to date was completed by scientists at Penn State and Northeastern Illinois University and featured 77 different species of reptiles and amphibians. The team found evidence that turtles, crocodiles and their relations, and salamanders all aged so slowly that their lifespans were much longer than would be expected for animals of their size.
In fact, some turtles had such slow ageing processes that the team concluded they experienced 'negligible ageing’, where their bodies didn't age at all as they got older. This doesn't mean they are immortal, only that their chance of dying isn't related to their age – unlike in humans, where our likelihood of dying increases as we get older.
“Negligible ageing means that if an animal’s chance of dying in a year is 1 per cent at age 10, if it is alive at 100 years, it’s chance of dying is still 1 per cent," said Professor David Miller, a corresponding author of the new paper. "By contrast, in adult females in the US, the risk of dying in a year is about 1 in 2,500 at age 10 and 1 in 24 at age 80."
Though there were no mammals with negligible ageing in Miller and colleagues' study, previous research has shown that naked mole-rats do age slowly enough to be essentially un-ageing.
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The team also found that animals with in-built physical or chemical protections – like a hard shell, a spiky spine, or a bite full of venom – had slower ageing and lived longer than animals that didn't. As these features affect an animal's chance of dying, they likely influenced how the species evolved.
"For the species we looked at, having protective adaptations, being larger, and taking longer to mature are all characteristics of species that age slower," said Miller. "These [traits] all affect mortality, and likely shape how evolution selects for physiological adaptations [in the animal] that limits ageing."
While these characteristics offer animals protection from their predators, they can't always safeguard against threats like climate change and habitat loss. Their slow pace of life can become easily unbalanced.
"Long-lived species that take a long time to mature are especially dependent on maintaining that low mortality... Many turtles need to survive a decade or more just to reach the age where they can first reproduce.Small changes in survival become amplified over many years to reduce the likelihood of animals becoming adults."
However, an animals' longevity can have advantages, helping them endure and overcome challenges. For example, one species in the new study was shown to grow more slowly in times of trouble. The western terrestrial garter snake, Thamnophis elegans,was able to slow its growth rate in areas where food supplies were unreliable, and changed how it used resources to prioritise living.
"This allows them to weather droughts that reduce the availability of their prey by taking a break from reproducing and hunkering down until conditions get better," said Miller.
Understanding the evolution and impact of protective traits in animals could bring us closer to understand our own ageing, and point towards new treatments and medicines for ailments related to human ageing.
"We believe that turtles and other slow-ageing reptiles can be a model for learning about the physiological and genetic processes that underlie ageing [across all animals]," said Miller. "Turtles and some of our longer-lived reptiles we studied, like the tuatara and crocodiles, share some characteristics with humans.
"Like humans, they take a long time to mature. Like humans, their external sources of mortality are lower than most species, and, like humans, they are long lived.
"The difference in the 'shape' of ageing between humans and turtles is that when we compare modern humans to the long-lived turtles in our studies, mortality is much lower at younger ages – humans have extremely low mortality rates up to say age 60 or 70 as compared to most any other species – however, in later life, human mortality rates increase with age relatively more quickly than what we saw with the turtles."
