> I should be able to state that two species have a different lifespan based on a difference in mean age of death - without needing to provide any specific biological mechanism in my study.

You need to control for confounders. For example, if I told you:

- Mean lifespan of species 1 is: 3 years - Mean lifespan of species 2 is: 10 years

Does that mean that species 2 has a longer lifespan than species 1?

What happens when you find out that species 1 is domesticated cattle, which has (almost) all of its males slaughtered at age 2?

You could then say "I meant for two species that are in controlled conditions", but now you need to define what conditions you're controlling, which implies what casual mechanisms you're controlling for.