Why do some turtles outlive humans?

(⇒ write for LongeCity )


The oldest human recorded in modernity was Jeanne Louise Calment, she died in the age of 122 years and 164 days [1] .

There are rumors that the oldest tortoise called Adwaita (Aldabra giant tortoise) died in the age of about 250 years [2] or that it was 188-year-old radiated tortoise named Tui Malila [3] , or that the highest verified age of 177 years had Galapagos giant tortoise Harriet [4] . The oldest currently living turtle is considered to be Jonathan (Seychelles giant tortoise), estimated to be over 180 years old these days [5] . Although all aforementioned numbers are estimations, it seems these turtles were older than human supercentenarians.

All previously mentioned species are terrestrial tortoises, a group with longest lifespans among turtles. The most famous of them, well-researched Galapagos giant tortoise, was observed by Charles Darwin when he was forming his well-known theory of evolution by natural selection [6] . There is only one freshwater turtle known to be able to outlive human, it is the common snapping turtle estimated to live up to more than hundred years [7] . While being considerably less researched, recorded maximal lifespan of sea turtles is usually shorter, not exceeding 80 years, however, it is believed that the green sea turtle can live up to 100 years. [8]

It is a difficult question to answer why these reptiles can outlive us because even to determine the actual age of animals with a long lifespan is complicated – partially due to the fact that it takes such a long time to study. Furthermore, many turtles are endangered species [9] so there may not be as many organisms to hand as needed for proper statistics. Nonetheless, we can still claim that turtles are among the most long-living vertebrates on earth [10] . Why?

Firstly, turtles, like all reptiles, benefit from being ectothermic organisms. They do not maintain body temperature and thus save a lot of energy. But that also means they are less flexible: it is crucial for their lifespan to be in natural temperature environment of daily cycles with night-time temperature drop [11] . If they do not live under these conditions in captivity, metabolic pathways change and turtles die much sooner. [12]

Turtles are well-adapted in other ways: their famous shell – the carapace –is good protection against natural predators. Most of hatchling turtles with a soft shell do not survive the first year [13] . A research of natural populations of freshwater turtles showed that only one per cent of them can celebrate the twentieth birthdays, but once the adulthood is reached, mortality rate drops and remains constant throughout the rest of life [14] .

Some turtles can survive under extreme environmental conditions, such as freezing [15] or lack of oxygen for months [16] . They can even undergo hibernation and anaerobic metabolism and therefore deal with hypoxia and anoxia, it was also proposed that the same genes can play a role in longevity itself [17] and also in oxidative stress resistance [18] that further promotes longer life [19] .

Turtle’s bones and shell are used as lactate buffer lowering metabolic acidosis caused by anaerobic glycolysis during the period of lack of oxygen [20] ; [21] Their organism is protected by strong innate immunity compensating slow acquired immune reactions [22] .

Because turtles have very slow metabolism as well as growth, their bodies do not need to deal with excessive metabolic heat and byproducts as mammals [23] . Their natural diet is very simple but also necessary for their longevity. [24]

According to the evolutionary theories, staying alive is less important after menopause. Galapagos giant tortoises achieve sexual maturity late (around the age of up to forty years in the wild, and between twenty and twenty-five years of life in captivity [25] ), then staying fertile until death [26] .

The Hayflick limit is said to determine how many times a cell can divide [27] . The Hayflick limit of Galapagos giant tortoise was said to be about 110 divisions [28] , approximately twice as many as 50 of human cells [29] . Studies in this context have highlighted the importance of telomeres, the protective end sequences of chromosomes, that get shorter with each cell division [30] , can play at least a partially role in life expectancy. It was observed that telomeres in European freshwater turtle’s cells are of the same length in both embryo and adult organism [31] .

Thus, it was believed that turtles are negligibly senescent organisms [32] . In other words, the cells do not age and no age-related diseases appear, which is very different cell behavior than in human bodies [33] and probably the key to any natural longevity. However, evidence now suggests that turtles may not be really negligibly senescent because of observations of survival and reproductive senescence in late age in the painted turtle population [34]

As we can see, turtles have some advantages in the lifespan field. Some of these might inspire researchers to increase lifespans in humans.