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Animals behave in this way for the same reason that, in other circumstances, they fight or drive each other away: because they or their young gain some advantage.
Is the 'advantage' above a countable-singular or uncountable noun?
Comments  
I'm quite sure it can be treated either way in this context, but I'd prefer to read it as singular countable.

- A.
AvangiI'm quite sure it can be treated either way in this context, but I'd prefer to read it as singular countable.

So would I. After all, it wouldn't be an outrageous mistake to use the plural: gain some advantages.
CB
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It can be either way...hmm...But in combination with 'some', the meaning might change a bit.
Some advantage (advantage:countable singular)→some kind of advantage/an advantage
Some advantage (advantage:uncountable)→some amount of advantage
Right now I'm trying to translate the sentence into Japanese, and when it comes to translation, the difference kind of matters...
What movie did you see last night? (reply) Oh, it was some movie about a guy who always wanted to be a jet pilot.

Why are you spending all this money to go to college?
(reply) Oh, I don't know. There must be some advantage to walking around with all that knowledge.

It would be nice if you could at least do some small thing to show your appreciation.

These are all intended as countables. The second one could go either way.

I'm trying to gain some advantage by eating only healthy food the day before the exam. I'd say only uncountable for this one. But just by changing "some" to "an," you can switch it to countable.

If the noun is one like "advantage," which may be taken either way, than context can often help pin it down. But not always. Unfortunately, your example presents a case in which either interpretation works.

- A.
For your information, here is the entire text. Still no clue?
In order to survive and produce young, most animals dependon their relationship with other members of their own species. They can be aggressive towards one another, as in the fights which occurover food or living space, but much of the time they work closely together and give each other help.
It would be a mistake to think that this co-operation is due to kind-heartedness or because their attitude to their neighbours is simply unselfish and generous. They behave in this way for the same reason that, in other circumstances, they fight or drive each otheraway: because they or their young gain some advantage. This explanation may at first be hard to accept, considering the behaviourof an animal, for example, when it defends its young against anenemy even at the risk of its own life, with no advantage to itself. Taking a wider view, however, it becomes clear that animals which protect their young, even at the cost of suffering themselves, have the chance of increasing their numbers and thus strengthening their position in a highly competitive world.
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some advantage: more abstract [this is what I'd use in the context
some advantages: more specific, so it's like asking for a list of these advantages [could work here, but I prefer the first