Not all species age the same, US researchers say, and humans may be outside the norm because adult humans get weaker as they age and then die.
That's not the typical pattern across species, scientists at the University of Central Florida said, and some organisms don't appear to show signs of aging at all.
In a study published in the journal Nature, the researchers described their work comparing the aging patterns of humans and 45 other species.
"We all have preconceived notions about aging and what it should be like," university biologist Pedro F. Quintana-Ascencio, who participated in the international study, said. "But this study shows we really need to look at the aging process in more depth. All is not what it appears across species. Humans, especially modern humans, appear to be outliers."
However, the researchers noted, there is no strong correlation between the patterns of aging and the typical life spans of the species. Species can have increasing mortality and still live a long time, or have declining mortality and still live a short time, they said.
"It makes no sense to consider aging to be based on how old a species can become," evolutionary biologist Owen Jones of the University of Southern Denmark said. "Instead, it is more interesting to define aging as being based on the shape of mortality trajectories: whether rates increase, decrease or remain constant with age."