Water fleas live only days or under optimal conditions weeks, but their mortality increases sharply with age, as is the case in longer-lived animals such as humans. But other animals — such as the hermit crab, the red abalone and the hydra, a microscopic freshwater animal that can live centuries — buck that trend, enjoying near constant levels of fertility and mortality.
A comparison of standardized demographic patterns across 46 species, published in Nature, suggests that the vast diversity of ‘ageing strategies’ among them challenges the notion that evolution inevitably leads to senescence, or deterioration of mortality and fertility, with age, says Owen Jones, a biologist at the University of Southern Denmark in Odense, who led the study.
“By taking a grand view and doing a survey across species, we found plenty of violations of this underpinning theory,” says Jones.
To compare fertility and mortality patterns, the authors assembled published life-history data sets for 11 mammals, 12 other vertebrates, 10 invertebrates, 12 vascular plants and a green alga, and standardized the trajectories — dividing mortality rates at each point in the lifespan by the average mortality rate.
The researchers found no association between the length of life and the degree of senescence. Of the 24 species showing the most abrupt increase in mortality with age, 11 had relatively long lifespans and 13 had relatively short lifespans. A similar split in lifespan occurred in the species that had a less abrupt increase in mortality.
Laurence Mueller, an evolutionary biologist at the University of California at Irvine, agrees. “Organisms in the field die from a lot of causes — for example, predation or disease — other than ageing,” he says. “Unfortunately, the unknown source of mortality in field-data sets confounds the age-related patterns of senescence, which is what we’re all interested in,” he adds.