comparing lions in the zoo to lions in the wild is ... a survey, an experiment, an observational study, a null hypothesis

The word comparing indicates observational.

especially for life expectancy.

Step-by-step explanation:

In inferential statistics, the null hypothesis is a general statement or default position that there is no relationship between two measured phenomena or no association among groups.

The relationship is associated, for example lions are fed meat and data shows they survive longer because of this if survival is matched or close range, or longer survival rates in zoo. I don't have the data but this sort of data is observational.

Another reasoning is how a single male lion are introduced in compatibility with female litter and how they use strategy of violence to demonstrate respect and how this back fires on both females and male lions at the start of introduction to young lionesses.

This could be argued experimental for lion management but stays observational for comparing.