Editing Senescence (section)

==Variation among species==
{{Further|Longevity#Non-human biological longevity}}
Different speeds with which mortality increases with age correspond to different [[maximum life span]] among [[species]]. For example, a [[mouse]] is elderly at 3 years, a [[human]] is elderly at 80 years,<ref>{{cite journal | vauthors = Austad SN | title = Comparative biology of aging | journal = The Journals of Gerontology. Series A, Biological Sciences and Medical Sciences | volume = 64 | issue = 2 | pages = 199–201 | date = February 2009 | pmid = 19223603 | pmc = 2655036 | doi = 10.1093/gerona/gln060 }}</ref> and [[ginkgo]] trees show little effect of age even at 667 years.<ref name="Wang">{{cite journal | vauthors = Wang L, Cui J, Jin B, Zhao J, Xu H, Lu Z, Li W, Li X, Li L, Liang E, Rao X, Wang S, Fu C, Cao F, Dixon RA, Lin J | title = Multifeature analyses of vascular cambial cells reveal longevity mechanisms in old ''Ginkgo biloba'' trees | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 117 | issue = 4 | pages = 2201–10 | date = January 2020 | pmid = 31932448 | pmc = 6995005 | doi = 10.1073/pnas.1916548117 | bibcode = 2020PNAS..117.2201W | doi-access = free }}</ref>

Almost all organisms senesce, including [[bacterial senescence|bacteria]] which have asymmetries between "mother" and "daughter" cells upon [[cell division]], with the mother cell experiencing aging, while the daughter is rejuvenated.<ref>{{cite journal | vauthors = Ackermann M, Stearns SC, Jenal U | title = Senescence in a bacterium with asymmetric division | journal = Science | volume = 300 | issue = 5627 | pages = 1920 | date = June 2003 | pmid = 12817142 | doi = 10.1126/science.1083532 | s2cid = 34770745 }}</ref><ref>{{cite journal | vauthors = Stewart EJ, Madden R, Paul G, Taddei F | title = Aging and death in an organism that reproduces by morphologically symmetric division | journal = PLOS Biology | volume = 3 | issue = 2 | pages = e45 | date = February 2005 | pmid = 15685293 | pmc = 546039 | doi = 10.1371/journal.pbio.0030045 | doi-access = free }}</ref> There is [[negligible senescence]] in some groups, such as the genus ''[[Hydra (genus)|Hydra]]''.<ref name="Dańko Kozłowski Schaible 2015 pp. 137–149">{{cite journal | vauthors = Dańko MJ, Kozłowski J, Schaible R | title = Unraveling the non-senescence phenomenon in Hydra | journal = Journal of Theoretical Biology | volume = 382 | pages = 137–49 | date = October 2015 | pmid = 26163368 | doi = 10.1016/j.jtbi.2015.06.043 | bibcode = 2015JThBi.382..137D | doi-access = free }}</ref> [[Planarian]] [[flatworm]]s have "apparently limitless [[telomere]] regenerative capacity fueled by a population of highly proliferative adult [[stem cell]]s."<ref>{{cite journal | vauthors = Tan TC, Rahman R, Jaber-Hijazi F, Felix DA, Chen C, Louis EJ, Aboobaker A | title = Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 109 | issue = 11 | pages = 4209–14 | date = March 2012 | pmid = 22371573 | pmc = 3306686 | doi = 10.1073/pnas.1118885109 | bibcode = 2012PNAS..109.4209T | doi-access = free }}</ref> These planarians are not [[Biological immortality|biologically immortal]], but rather their death rate slowly increases with age. Organisms that are thought to be biologically immortal would, in one instance, be [[Turritopsis dohrnii|''Turritopsis'' ''dohrnii'']], also known as the "immortal jellyfish", due to its ability to revert to its youth when it undergoes stress during adulthood.<ref>{{cite journal | vauthors = Lisenkova AA, Grigorenko AP, Tyazhelova TV, Andreeva TV, Gusev FE, Manakhov AD, Goltsov AY, Piraino S, Miglietta MP, Rogaev EI | title = Complete mitochondrial genome and evolutionary analysis of Turritopsis dohrnii, the "immortal" jellyfish with a reversible life-cycle | journal = Molecular Phylogenetics and Evolution | volume = 107 | pages = 232–8 | date = February 2017 | pmid = 27845203 | doi = 10.1016/j.ympev.2016.11.007 | doi-access = free | bibcode = 2017MolPE.107..232L }}</ref> The [[reproductive system]] is observed to remain intact, and even the gonads of ''Turritopsis'' ''dohrnii'' are existing.<ref>{{cite journal | vauthors = Piraino S, Boero F, Aeschbach B, Schmid V | title = Reversing the Life Cycle: Medusae Transforming into Polyps and Cell Transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa) | journal = The Biological Bulletin | volume = 190 | issue = 3 | pages = 302–312 | date = June 1996 | pmid = 29227703 | doi = 10.2307/1543022 | jstor = 1543022 }}</ref>

Some species exhibit "negative senescence", in which reproduction capability increases or is stable, and mortality falls with age, resulting from the advantages of increased body size during aging.<ref>{{cite journal | vauthors = Vaupel JW, Baudisch A, Dölling M, Roach DA, Gampe J | title = The case for negative senescence | journal = Theoretical Population Biology | volume = 65 | issue = 4 | pages = 339–51 | date = June 2004 | pmid = 15136009 | doi = 10.1016/j.tpb.2003.12.003 | bibcode = 2004TPBio..65..339W }}</ref>