Going back to Daphnia: there is a lineage of Daphnia pulicaria in cold, fishless Lake Czarny (in the Tatra Mountains of Poland) that lives *two years.* Same species that in most lakes lives 3 months at most. https://sci-hub.tw/10.1007/s10750-012-1420-6 …
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That is, under conditions of low extrinsic mortality (no predators, no seasonal drying-up), Daphnia were able to evolve the capacity to octuple their lifespan.
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In fact, there are *two* lineages in Lake Czarny: a short-lived, exclusively asexual strain (asexual reproduction accumulates more deleterious mutations than sexual reproduction; it seems to be another r-selected adaptation) and the long-lived sexually-reproducing strain.
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The long-lived Daphnia are larger, and have more fat reserves to help them survive the winter. They are orange, possibly due to beta-carotene. And they stay in deeper, colder water, lowering their metabolic rates, which makes them live longer.
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Small aquatic animals are governed by the Arrhenius equation: if you cool them down, *all* their chemical reactions go slower, including the ones that make their bodies break down with age. This obviously doesn't apply directly to warm-blooded mammals like us.
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The connection between metabolic rate and lifespan does extend to mammals though. Smaller, short-lived, more r-selected mammals (like mice) have higher metabolic rates than long-lived K-selected mammals.
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Vertebrates with exceptional longevity (tortoises and sharks can live centuries) are all cold-blooded; the naked mole rat (a mammal that appears not to senesce with age) is a rare example of a cold-blooded mammal; and mammals that hibernate age slower than mammals that don't.
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Also, inducing hypothyroidism in rats (which lowers their metabolic rate) makes them live 30% longer. http://sci-hub.tw/https://doi.org/10.1016/0047-6374(86)90052-7 …
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Genetic differences between short- and long-lived strains of similar species could point the way towards mechanisms to delay aging in humans. I don't expect simply lowering metabolism to be a good trade-off, but there may be other resilience mechanisms that we could "borrow."
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The "bad" news is that humans are already pretty K-selected; we may already have most of the long-life adaptations we see in other animals. (Though it's worth doing a lot more comparative genomics to find out!)
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The "good" news is that evidence is accumulating all over the place that aging rates respond to evolutionary incentives. Any knob that Evolution can turn, Man can in principle learn to turn.
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