You made some astute observations about categorizing specific lions vs. lions in general (including in your comments). You give two pairs of examples at the end of your question. Both sets are similar and in both sets, either sentence can be correct, depending on what you mean. I'll stick with lions and add some other examples to clarify. The key point is that the general context or "setting" of the sentence gives clues as to which version to use, but it is your intended meaning that determines it.
Leave out "the" when you refer to lions in a general sense: as a class of animal, or to lions in general, or to quantities of lions that are so large and contain so much diversity that the aggregate does not seem to be a meaningful subset. "Lions in zoos", or your example of "lions in India", would be such cases. I'll cover an exception, later.
It is more obvious when you are referring to large, diverse collections of lions, but you can mean "generic" lions even when referring to a relatively small quantity, or a collection of lions that would commonly be viewed as a specific group, like the collection of lions at one zoo. Some examples:
- You could say "Lions at the zoo are well fed", referring to the class of animal.
- If you were, say, a nature photographer and were assigned to get pictures of lions, you would say, "I'm going to the zoo to photograph lions." In that case you are talking about lions in the general sense, and they happen to be at the zoo.
If you mean to refer to a specific subset of lions, include the article. For example, if you are planning a trip to the zoo to observe the collection of lions there, you would say, "I'm going to the zoo to see the lions", referring to that specific collection. It's not so much whether it's a specific collection of lions, it's the meaning in which you want to refer to them.
I mentioned an exception earlier. You define a subset if you make a comparison. For example, if you were comparing lions in zoos to lions not in captivity, that distinction creates specific lions that you are referring to. In that case, you would say "the lions in zoos" to differentiate them. if you were comparing Indian lions to lions in America, you are defining each of those groups as a specific subset, so you would use "the lions in India" in that context to differentiate them.
The specificity of "the" can actually introduce ambiguity if you use it in a way that isn't consistent with what seems like the obvious context of the sentence, or to match apparent specificity when that is contrary to your meaning. A few examples:
When you refer to huge aggregates of lions, like all lions in India, we tend not to think of them as a specific group, so we don't expect to see an article. If you include an article, people will infer a reason. If you say, "the lions in India", people will assume that you mean to differentiate those lions from some other lions, even if it was not your intention to make a comparison.
Take the earlier example of "lions at the zoo are well fed." That would unambiguously refer to lions as a class of animal. The sentence also qualifies that statement as talking about the ones at that zoo, so you don't really need "the" to establish specificity.
If instead, you use, "the lions at the zoo are well fed", it isn't clear whether "the" is an unnecessary, redundant reference to the specific collection of lions at that zoo. "The" can also be added to make a comparison. So perhaps "the" differentiates how well the lions are fed compared to some other types of animals at the zoo. "The lions at the zoo are well fed; the monkeys aren't so lucky."
Lions in the zoo are very aggressive.
The lions in the zoo are very aggressive.
The first example refers to the case in your comment of general lions at a specific zoo. It refers generically to the class of animal. It could have a number of meanings, though, depending on how you interpret the rest of the sentence. For example:
Lions in the [or "a"] zoo are very aggressive, just like lions in the wild. or
Lions in the [or "a"] zoo are very aggressive, unlike lions in the wild. or
Lions in that particular zoo are very aggressive, unlike lions at other zoos.
Note that in the first two examples, "zoo" is being used to draw a comparison to non-zoos. So "the" can mean differentiation rather than reference to that specific zoo, in which case, "a" would have the same meaning of referring to any zoo.
The second example refers to the specific lions at that zoo. It could similarly have a number of meanings, though, depending on how you interpret the rest of the sentence. For example:
Those particular lions are very aggressive compared to lions at other zoos, or lions in the wild.
The lions at that zoo are very aggressive compared to some other animals at that zoo.