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From my understanding of animals, lifespan is all based on the speed of its metabolic rate. As endothermic animals, mammals live a high-octane life - growing up fast, being quick on activity regardless of the weather and demanding a lot of food in a short period of time. As a result, they, if you will, "live fast and die young".

Ectotherms such as crocodiles, live life more slowly and leisurely and don't require as much food. However, since they can't create their own body heat, their personalities and environmental choices are limited. Regardless, this leisurely metabolism means that their lifespans get expanded.

Then there's this recent popularity of great whites, tunas and probably dinosaurs being mesothermic, neither warm- nor cold-blooded, but bearing the advantages of both. You can still lead a high-octane life, but your body won't require as much food.

One of the most popular characteristics of fictional hominids - like elves and dwarves - is a lifespan longer than humans. Which metabolism would be better suited for these primates, ectothermy or mesothermy?

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  • $\begingroup$ Beyond the biological question of immortality, which is well addressed by James, consider that Elves are historically very in balance with nature, suggesting their temperature may be whatever temperature suits nature at the time. $\endgroup$
    – Cort Ammon
    Commented Sep 17, 2015 at 16:21
  • $\begingroup$ Certain species of animal can alter their genetic makeup to suit the environment, meaning your elf can become either cold or warm blooded. You may consider removing the specific genes responsible for capping the cell divisions limits and aging :) $\endgroup$
    – user6760
    Commented Sep 18, 2015 at 4:39

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While I agree with James' assertion that immortality isn't simple, I think it's worth mentioning that there most animals repair their own DNA with telomerase to remove errors -- and creatures with more telomerase (such as lobsters, which express telomerase in most of their tissues) repair their DNA more efficiently, potentially living forever. While lobsters can't be immortal due to the high cost of moulting (which increases with the size of the lobster and the lobster is constantly growing throughout its life), there's no reason this couldn't be applied to a humanoid race -- your elves won't be growing too big for their skin.

On the other hand, you could always have elves that "reincarnate" into their next generation, like the transdifferentiation in some jellyfish.

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    $\begingroup$ I'm not familiar with jellyfish avatars. $\endgroup$
    – JohnWDailey
    Commented Sep 17, 2015 at 19:57
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    $\begingroup$ @JohnWDailey en.wikipedia.org/wiki/Turritopsis_dohrnii , they have a way of looping through their lives over and over again :). $\endgroup$
    – CandiedMango
    Commented Sep 18, 2015 at 5:40
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Aging is a lot more complex than simply metabolism or hot blood vs cold. In fact there are few vertebrates that live longer than humans and most of those are sea based, giant tortoises and some whales from a quick glance at the web.

Its not metabolic rate that increases aging. We actually influence that rate with diet and exercise. People in great shape tend to be healthier, have higher metabolic rate and live longer.

Aging comes down to our body being able to repair itself or not...in the case of aging. At a high level the process is this:

Every time your cells replicate themselves there is a chance that an error will be made in the replication process at the sub-cellular DNA level. Its like the concept of making a photo copy of a photo copy until the original looks nothing like the current version. While your body does its best to clean up these errors, over time they accumulate and the body is less efficient, and less accurate about replacing old cells, thus we age and eventually die. This is a decent high level article on the biology of aging.

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  • $\begingroup$ Among many different mechanisms to cope with these issues, one of my favorite has always been Radiodurans' method of protection. $\endgroup$
    – Black
    Commented Sep 11, 2018 at 20:50
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To echo the other answers, neither. It's entirely possible for warm or cold blooded creatures to have biological immortality. The thing is though that there is no evolutionary advantage to it.

That may seem counter intuitive but consider that you need reproduction anyway to continue the species as otherwise you have no ways to increase your numbers or recover from disasters.

That reproduction already works very effectively to reset the biological age and at the same time undo most environmental damage. If you lose an arm to accident or injury then your children are not born missing an arm.

On the other hand building repair systems into an adult that are capable of regrowing an arm would be complicated (and potentially increase risks of cancer or infection and would certainly increase calorie requirements at a time where you are already going to struggle to acquire more calories due to the injury). Similarly healing without scarring would be possible, but is slower. It is not safe to take your time about sealing up a wound "in the wild".

All of these factors mean that as far as our genetics are concerned we are already biologically immortal - through having children. There is no adaptive advantage to increasing our lifespans further than they already are.

So creating an immortal is simple enough, you just need to be able to repair damage done to the body. Repair DNA, regrow limbs, heal scars, etc. The much harder thing to explain though is the evolutionary process that would make it worthwhile to develop that capability.

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  • $\begingroup$ Tolkien Elves do not have the ability to regrow limbs. Maedhros lost a hand and never grew it back in the Silmarillion. $\endgroup$
    – Oldcat
    Commented Mar 10, 2016 at 18:04
  • $\begingroup$ To add to your list of problems with immortality; in an immortal species adults will easily outcompete children (they have thousands of years of experience) for limited resources. So evolution effectively stops for the immortal species, children making it to adulthood becomes very rare. Over time a mortal species evolves to out compete the immortal species (which was only able to compete due to its incredible experience so can never "improve the design") $\endgroup$
    – Richard Tingle
    Commented Mar 10, 2016 at 22:00
  • $\begingroup$ Interesting to think about is that the biggest evolutionary advantage you'd think is wisdom. But the things with actual immortality in nature have none (I don't think I'd call a Hydra wise). And even if wisdom was an advantage, immortality would only help in non-Alzheimer, "correct-thinking" people (for some evolutionary advantageous definition of correct) which is several large hurdles. Interesting ball of wax when you think of it from an evolutionary pressure perspective. $\endgroup$
    – Black
    Commented Sep 11, 2018 at 20:58

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