Why do all living creatures have such wildly different life expectancies?
In: Biology
Under ideal conditions, natural selection would favor animals that live forever and keep reproducing forever. Selection favors animals (or plants) that produce the most successful offspring.
So why don’t things live forever? What we have here is a trade-off. The world is a dangerous place, and keeping yourself alive costs resources. Resources that could be spent on making more offspring. If spending resources to keep yourself alive longer results in more offspring (because you live to breed another day) then great. But if you get eaten by a predator the next day all those saved up resources never get used. Ideally, an animal that is likely to die from environmental causes will want to spend all its resources early on reproduction, causing it to wear out faster and have a lower life expectancy.
In other words, innate life expectancy should be related to the amount of time an animal would expect to survive in the wild before being killed by something. You can see this with many living things. Turtles for example are hard to kill, they carry around strong protection. And turtles have a long innate genetic life expectancy. They reproduce more slowly and move more slowly but make up for it by living longer and having a longer lifetime reproductive output. In contrast, mice are easy to kill. Lots of things eat them, and the average mouse is likely to be snapped up by a bird or snake or cat before it reaches its first birthday. But even if you protect a mouse from all predators, it still has a short life expectancy. It’s entire body is tuned to live fast and breed fast. It just wears out fast, it’s dumping a lot of energy into reproducing and less into maintenance, because there’s no point in trying to take it slow like a turtle…it would just get eaten.
There are other things going on here too, the above explanation is _a bit_ simplified. For example, some animals have structures that are useful but just don’t last forever…mammals can’t really replace or repair adult teeth, for example, and as a result grazing mammal lifespans are limited by how long their teeth last. And some animals just don’t seem to age at all. But it’s a good framework for understanding the question.
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