Captive turtles grow old slower than wild ones, and some don’t even age at all

(Wikimedia Commons CC BY-SA 3.0 https://bit.ly/3bCwlLT)

The aging rate of many turtle species in captivity is zero, found the zoologists of the conservation organization Species360.

Also, in the case of some species, the risk of death even decreases with age.

Turtles are famous for living very long lives: Jonathan, a male giant tortoise, is around 190 years old; and Harriet, a Galapagos tortoise, lived till 176 years old.

Some scientists even suggest that at least two species of turtles age negligibly slowly.

That means that their aging rates are difficult to statistically distinguish from zero, and the correlation between age and the probability of death is zero, too.

Now a team of zoologists led by Fernando Colchero decided to learn more about how turtles live so long.

The researchers collected information about 52 species of turtles in captivity:

• differing in body weight
• life expectancy
• features of life cycles

The team estimated the mortality, life expectancy, and the rate of aging from males of 39 species and females of 47 species.

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No aging
The authors calculated the rate of aging at the age when the survival function reaches 0.2 (that is, by the age when 80% of adults have time to die).

Colchero’s team found out that for females of 35 species (74.5%) and males of 31 species of turtles (79.5%), the aging rate is zero.

Even more, females of 2 species and males of 1 species showed a negative aging rate. In several more species, the aging rate turned out to be close to 0.

The authors believe that zero, negative, and close to zero aging rates may indicate negligible aging in most turtle species.

However, the rate of aging was positive in females of 10 species and males of 7 species of tortoises.

This indicates that their risk of death increases with age even in captivity, where they are protected from predators and receive food and treatment.

At the same time, the aging rate in females of 37 species and males of 34 species was lower than in modern humans.

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The reason
The Species360’s team hypothesized that the adult life expectancy for both sexes of tortoises is positively correlated with body weight.

Males of species with reverse sexual dimorphism (when females are larger than males), tend to live longer than males of species in which males are larger than females.

This is probably because males reverse sexual dimorphism are less aggressive towards each other, which reduces the risk of death during conflicts over partners.

Calculations also showed that male turtles live on average 20% longer than females. However, the authors could not find differences in the rate of aging between the sexes.

Colchero also wanted to see if the same results happen with wild populations of these reptiles.

The scientists compared the lifespan and rate of aging in captivity and in the wild for three species of tortoise:

• painted (Chrysemys picta)
• red-eared (Trachemys scripta)
• forest kinix (Kinixys homeana)

The authors found that the last two species in nature age several times faster than in captivity.

On the other hand, painted turtles from wild populations age slightly slower than their relatives from captivity.

Studies like this would help refine the existing evolutionary theories of aging, which have mostly been tested in mammals and birds; and maybe one day… reverse it

Sources:

• Science: https://www.science.org/doi/10.1126/science.abl7811 and https://www.science.org/doi/10.1126/science.abm0151
• New Scientist: https://www.newscientist.com/article/2325563-some-turtles-that-live-longer-have-a-lower-chance-of-dying-each-year/

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