Guppies and water fleas live only days or 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 standardised 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 standardised the trajectories—dividing mortality rates at each point in the lifespan by the average mortality rate.
They 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 life spans and 13 had relatively short life spans. A similar split in life span occurred in the species that had a less abrupt increase in mortality.
When the researchers organised the species along a senescence continuum, mammals were clustered at one end of the spectrum, among the organisms that have an abrupt shift in mortality, and plants, which boast vastly lower relative mortality, populated the other end. Birds and invertebrates were scattered throughout.
The authors suggest that the diversity of ageing strategies across the spectrum should challenge theoreticians. “The (evolutionary) theories we have are applicable in lots of situations, but they can’t explain some cases,” says Jones. “It’s not about throwing out old theories; it’s about modifying theories to work on all species.”
This is the first attempt to standardise cross-species comparisons of mortality and survivorship, but it has several scientists scratching their heads because, they say, the diversity of life strategies is well-established. More importantly, they question the biological basis of the comparison.
At issue is the validity of comparing lab-raised, sometimes inbred, populations to field studies, some of which go on for decades. “This approach is like making a fruit salad and