The findings of the study, published in the journal Nature, suggest that aging does not increase the chances of dying or growing infertile in certain animals.
The research looked at aging patterns and life histories of 46 species, which included 11 mammals, 12 other vertebrates, 10 invertebrates, 12 vascular plants and a green alga, using long-term data collected by Max Planck Institute for Demographic Research in Germany, and at the Max-Planck Odense Center on the Biodemography of Aging in Denmark.
“By taking a grand view and doing a survey across species, we found plenty of violations of this underpinning theory,” said Owen Jones, a biologist at the University of Southern Denmark in Odense, who led the study.
When the researchers placed all the studied species along a timeline based on their aging processes, all the mammals were grouped at one end, those that showed an abrupt shift in their mortality as they age, and plants, known for their vastly lower relative mortality, were on the other end. Birds and invertebrates were scattered between.
Animals like the desert tortoise, which had a high mortality rate early on in life, showed a decrease in this rate as they grew older. Meanwhile the hydra magnipapillata, a microscopic freshwater animal that can live centuries, showed a constant mortality rate unaffected by the aging process.
But critics of the study say this variation doesn't counter evolutionary theory.
“This study is a useful reminder that the patterns of ageing are diverse, but it is not a refutation of existing theory,” says Stephen Stearns, an evolutionary biologist at Yale University. “That would require difficult empirical measures of the trade-offs between reproduction and mortality, which haven't yet been done.”