Icons of longevity still decline in old age.
For a long time scientists have marveled at the unusually long lifespans of turtles. Even more puzzling than their longevity was the widely held belief that turtles do not go through senescence — the gradual deterioration that most organisms experience as they age.
Senescence does not appear to make much evolutionary sense. One might expect the genes that underlie deterioration with age to be weeded out by natural selection, as these genes can greatly reduce an individual’s fitness. But as organisms age, natural selection becomes weak, and that’s when senescence starts to kick in.
Turtles were thought to sidestep senescence by remaining fertile right to the very end. Having offspring into old age enables natural selection to keep operating, so this was believed to be the reason why turtles don’t decline with age.
In a study published in Proceedings of the National Academy of Sciences, researchers from Iowa State University have turned this idea on its head. Nearly 30 years of data collected on painted turtles in the Mississippi River near Clinton, Iowa, revealed that females suffer a steep drop in fertility near the end of their lives.
"Turtles are these icons of longevity," said Fredric Janzen, one of the study’s authors, in a press release. "People assumed there was never a cost to reproduction right up to the end of life."
The researchers found that when females got old, they kept producing lots of large eggs, as had previously been observed. The problem was that many of the eggs just weren’t hatching.
"Similar to the finding that reproduction declines, we also found that survival rates decline as the turtles age but at rates slower than mammals – including humans and non-human primates," said study co-author Anne Bronikowski.
It appears that, contrary to conventional wisdom, turtles do decline as they age. However, they still manage to age more gracefully than most mammals, delaying senescence until just before death.
According to Bronikowski, turtle lifespans are similar to those of humans, who also take years to mature and tend to have long lives. That means studying turtle senescence may have implications for human health and aging.
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