According to Birgitte Svennevig in “Surprising diversity in aging revealed in nature“
There is no strong correlation between the patterns of ageing 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.
“It makes no sense to consider ageing to be based on how old a species can become. Instead, it is more interesting to define ageing as being based on the shape of mortality trajectories: whether rates increase, decrease or remain constant with age”, says Owen Jones.
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. Increasing weakness with age is not a law of nature.
Researchers from the University of Southern Denmark have studied ageing in 46 very different species including mammals, plants, fungi and algae, and they surprisingly find that there is a huge diversity in how different organisms age. Some become weaker with age – this applies to e.g. humans, other mammals, and birds; others become stronger with age – this applies to e.g. tortoises and certain trees, and others become neither weaker nor stronger – this applies to e.g. Hydra, a freshwater polyp.
In lab conditions, [a hydra] has such a low risk of dying at any time in its life that it is effectively immortal.
“Extrapolation from laboratory data show that even after 1400 years five per cent of a hydra population kept in these conditions would still be alive”, says Owen Jones.
Several animal and plant species show remarkably little change in mortality throughout their life course. For example, these include rhododendron (Rhododendron maximum), great tit (Parus major), hermit crab (Pagurus longicarpus), common lizard (Lacerta vivapara), collared flycatcher (Ficedula albicollis), viburnum plants (Viburnum furcatum ), oarweed (Laminaria digitata), red abalone (Haliotis rufescens), the plant armed saltbush (Atriplex acanthocarpa), red-legged frog (Rana aurora) and the coral red gorgonian (Paramuricea clavata).