system
Given the following post, respond as if you're commenting on Reddit:
human
Why do some animals have shorter lifespans than others? What keeps humans live for ~80 years versus a cat for only ~15 years?

If beings want to be alive for as long as possible, why would some die faster than others (We're also talking dying by an animal's "old age")? What keeps a human to be considered younger longer than animals with shorter lifespans?
gpt
> If beings want to be alive for as long as possible

Your premise is a problem here. Animals don't need to be alive for as long as possible. Evolution doesn't favor animals that live longer. It favors populations that live longer. It doesn't matter if those individuals breed and reproduce in hours or years apart.

A single fruit fly can lay 500 eggs, and its total lifespan from egg to death is maybe 40 days. A day to hatch. Within a week it turns to the adult stage. Then in 2 days its sexually mature. That keeps the population alive through sheer force of numbers. It doesn't really matter how many are killed by predators, because each pair can produce hundreds more. And they are sexually mature in a week, so a closed system will produce drastic amounts of individuals.

Whales and elephants can have a single young after 18-24 months of gestation. The long life-span allows for this. The intelligence keeps them alive. Their size helps protect from predators. They can also herd to protect the young. All of this allows them to evolve to a large size. It allows them to have long gestation periods to support a large child. These are all interconnected.

If fruit flies bred like elephants, they would have died out long ago to predators. If elephants bred like fruit flies, the world would be buried in a mountain of fully grown adult elephants in months, and they would all die out after exhausting their food supply. Evolution wouldn't have allowed either to happen, when very early on they wouldn't have been able to sustain a population.

If you want to know why specific animals get old and die, it really varies. Organs give out after some time. It depends how that animal evolved. Reptiles can live a long time, but so can mammals and trees. Some things live days and others over a century. Its a balance based on their population, predators, food source, etc. It even benefits a population to not live too long, because you are more damaged from disease and injury when you produce less often and live longer.