Turtles are considered to be particularly long-lived. However, reliable figures on this have so far been scarce. A large research group led by Beth Reinke from Northeastern Illinois University in Chicago and a smaller team led by Rita da Silva and Fernando Colchero from the University of Southern Denmark in Odense are now providing data in two independent studies in the journal “Science” that support this idea .
For example, the Danish group of 52 turtle species examined data on the lives of individual animals in zoos, animal parks and aquariums. Three quarters of these species aged extremely slowly or almost imperceptibly. About 80 percent of these species had a slower aging rate than humans, which are considered long-lived compared to many other mammals. “In science, aging means that the death rate continues to rise over the course of life,” explains biologist and age researcher Alexander Scheuerlein from the University of Greifswald, who was not involved in either of the Science studies.
In humans, mortality initially decreases after birth; it reaches its lowest value in eight to ten-year-old children. In this age group, an average of twenty out of every 100,000 people die within a year, almost half of whom die in accidents. Thereafter, mortality continues to rise, with disease becoming the cause of death more and more over time. For example, for every 100,000 people in their 50s and 60s, an average of 800 die each year from heart attacks, strokes, cancer and other diseases.
But why do tortoise species age on average twenty times more slowly than animals with a relatively high and even body temperature, such as birds and mammals?
The answer may lie in the turtle’s shell, which provides good protection against external dangers such as falling rocks or enemies. A similar connection has been known for a long time in mammals: “Small animals like mice are much more at risk from such natural hazards or enemies like the not too big cats than large elephants, which hardly any other animal attacks,” explains age researcher Alexander Scheuerlein. “Mice therefore have to flee from enemies and other dangers much more frequently than elephants.” Since this also produces more substances that can harm the body, mortality increases.
So if a shell protects a turtle, the animal can run its metabolism at a much slower pace. The resting heart rate of an adult is around 70 beats per minute, while that of a Galapagos giant tortoise is six beats. And indeed, one of these giant tortoises holds the current age record: “Harriet” died of cardiac death in 2006 at the age of 170 in a zoo in Australia.
However, animals in the zoo live in an artificial environment in which many life risks are excluded or at least reduced. The study by Beth Reinke’s team looked at the age rates of animals that live in nature. In addition to 14 species of turtles, amphibians, snakes, crocodiles and the primeval tuataras were also examined, of which only one species survived in New Zealand, the tuataras. In nature, too, the mortality rate of turtles, but also of some species of salamanders and tuataras, was particularly low.
“This result is not too surprising,” explains Alexander Scheuerlein. After all, these are cold-blooded animals whose organism runs at significantly lower temperatures than mammals and birds. These have higher body temperatures and accordingly accumulate more damage than cold-blooded animals. Species that live in warmer regions age faster. However: In the case of frogs, salamanders and the like, the species living in the warm tropics live much older than the animals in cooler regions.
The Tuatara tuataras were surprised. While humans age twice as fast as tortoises, the aging rate of tuataras is 90 percent lower than that of tortoises. Their life expectancy is around 137 years. The reptiles are 50 to 75 centimeters long and weigh only around one kilogram. Possible reasons for their low mortality: The tuataras not only have a very low metabolic rate, they also move very slowly and are still active at a body temperature of eleven degrees Celsius.
However, all of these species have one thing in common with the animals with a higher mortality rate: eventually they will die.