Turtles are icons of longevity. Not only do some live for well over a century, they also seem to remain robust as they age. But according to findings published in Proceedings of the National Academy of Sciences this week, certain wild female turtles experience declines in both reproduction and survival as they get older.
Lifespan varies a lot across the animal kingdom. While some show little to no signs of aging, others deteriorate substantially and exhibit what’s called senescence – or declining reproduction and mortality with advancing age. It’s thought that senescence doesn’t occur in many reptiles, especially in turtles. They continue to grow throughout their lives, and reproductive activity increases with age. Researchers think that senescence should evolve when natural selection is weaker on deleterious traits that are expressed at an old age, compared to those expressed at a young age.
To test this, Iowa State University’s Fredric Janzen and colleagues gathered birth, death, and reproductive data on more than 1,000 painted turtles (Chrysemys picta) that live in the backwaters of the Mississippi River in northwest Illinois. This population was marked back in 1988, and researchers have been collecting capture-mark-recapture data on them for 24 years.
After quantifying the survival and physical data on 2,234 nests built by 600 painted turtles, the team found that while total egg output tended to increase with age, older females produced eggs with reduced hatching success. Furthermore, the age-related capture-mark-recapture histories of 1,031 females revealed that mortality accelerated with advancing age. Their mortality rate was estimated to double every 13.8 years.
These findings – which challenge our traditional view of long-lived reptiles – suggest that senescence occurs in turtles, albeit at a slower rate than in most mammals. Although, this population of turtles does have a history of human-related deaths, so it’s possible that an external source of mortality helped drive their selection for peak reproductive fitness at an early age.
Image in the text: Adult female painted turtle returning to the water after nesting. David Delaney