Adult humans get weaker as they age and then die, but that's not the typical pattern across species. Some organisms don't appear to show signs of aging at all.

These are among the findings in a first-of-its-kind study published in the journal Nature this week. The study compares the aging patterns of humans and 45 other species.

"We all have preconceived notions about aging and what it should be like," said Pedro F. Quintana-Ascencio, a biologist at the University of Central Florida and one of the contributors to the study led by evolutionary biologist Owen Jones at the Max-Planck Odense Center at the University of Southern Denmark.

"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."

The team contrasted how vertebrates, invertebrates, plants, and a green alga age. Modern day people, frogs, lions, lice and the Hypericum cumulicola, a native Florida plant, were among the species compared.

The study found that mortality of some species, like humans and birds, increased with age. For some, such as Florida's hypericum the increase is slower. And for others, like the desert tortoise and certain trees, mortality declines with age.

Other researchers on the project were from the Max Planck Institute for Demographic Research in Rostock, Germany, the University of Queensland in Australia, and the University of Amsterdam in Holland. Several American universities also contributed to the study.

The researchers point out 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, according to the Max-Planck Odense Center.

"It makes no sense to consider aging to be based on how old a species can become," Jones 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."