ageing is based on the shape of mortality trajectories

Not all species weaken and become more likely to die as they age

Some species get stronger and less likely to die with age, while others are not affected by age at all.
Some species become stronger with age: tortoises and certain trees,
Some species become neither weaker nor stronger: Hydra, a freshwater polyp
desert tortoise (Gopherus agassizii) experiences the highest mortality early on in life and a steadily declining mortality as it ages.
Many plant species, e.g. the white mangrove tree (Avicennia marina) follow the same pattern
The freshwater polyp Hydra magnipapillata has constant low mortality. In fact, in lab conditions, it has such a low risk of dying at any time in its life that it is effectively immortal.
even after 1400 years five per cent of a hydra population kept in these conditions would still be alive
Human fertility is characterized by being concentrated in a relatively short period of life, and by the fact that humans live for a rather long time both before and after the fertile period.
Some species become more and more fertile with age. this pattern is especially common in plants such as the agave (Agave marmorata) and the rare mountain plants hypericum (Hypericum cumulicola) and borderea (Borderea pyrenaica).
On the contrary fertility occurs very early in the nematode worm Caenorhabditis elegans. Actually this species starts its life with being fertile, then it quite quickly and quite suddenly loses the ability to produce offspring
Species can have increasing mortality and still live a long time, or have declining mortality and still live a short time
ageing is not how old a species can become

ageing is based on the shape of mortality trajectories
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