Here's the backstory, just skip down to The fifth rebuttal: (Underneath the dashed line) if you don't care for the backstory.
The sentence: Ferrets like to live their lives irresponsibly. It's ferrets, after all.
The correction: They're* ferrets after all.
The rebuttal: It's ferrets, after all. It's sounds correct to me. Using the word It conveys a broader sense of the topic at hand (which is ferrets) instead of referring directly to the animal.
The second rebuttal: It (is) implies only one subject, while they (are) implies many. Since the direct object (ferrets) is plural, the latter would be correct.
The third rebuttal: Ferrets like to live their lives irresponsibly. It's ferrets, after all.
could be read as:
Ferrets like to live their lives irresponsibly. It's their type of lifestyle, after all.
Irresponsibility is the antecedent and noun that it is referring to. Irresponsibility is not directly in the first sentence, but can be derived from it. Irresponsibility is a singular noun. Therefore, "It's there type of lifestyle, after all." is correct and so are the sentences Ferrets like to live their lives irresponsibly. It's ferrets, after all.
The sentence could read: Ferrets like to live their lives irresponsibly. Irresponsibility is their type of lifestyle, after all. Which avoids ambiguity from It's.
So It's ferrets, after all. is either correct or slang. But even as slang it's a common occurrence form native speakers.
The fourth rebuttal: That's not really how I gathered it, very much at all. It seemed to me more like "It's their nature being ferrets, after all." "their* type of lifestyle" would be more of the case that they're more dependent on lifestyle and not on them being ferrets, which likely is not the case. They have a choice to change their lifestyle, but they can't make themselves not be ferrets. I suppose you could look at it as irresponsibility being tempting, but in direct correlation with ferrets it doesn't make all that much sense to me. So, in conclusion, I see that the sentence must come out one of two ways:
Ferrets like to live their lives irresponsibly. They're ferrets, after all. (Referring directly to ferrets, and their tendency to be irresponsible. Kind of a redundant statement put together, but oh well.)
Ferrets like to live their lives irresponsibly. It's irresponsibility, after all. (Referring directly to irresponsibility, because one could see the temptation to live your life irresponsibly. The less redundant statement of the two.)
--------------------------------------------------------------------------------- The fifth rebuttal: They have a choice to change their lifestyle, but they can't make themselves not be ferrets. This whole thing about whether or not they can change their lifestyle. That has no bearing, no influence, nothing to do with the sentence structure and grammar. Both sentences are a joke. Are ferrets irresponsible? Probably not, but it could be possible or at the least possible in the creative/cartoon realm. So, just because irresponsibility and ferrets doesn't work well for you it has nothing to do with the sentence and the grammar at all. You're not arguing actual grammar rules here, but the essence of ferrets.
Ferrets like to live their lives irresponsibly. They're ferrets, after all. (Referring directly to ferrets, and their tendency to be irresponsible. Kind of a redundant statement put together, but oh well.)
This seems redundant to you because you're breaking down the sentence and examining for an extended period. I know because the same thing crossed my mind. But, this is not redundant. The second sentence reinforces the first. It also lets the reader know that irresponsibility is common to ferrets in general (although irresponsible ferrets may not be true, as a joke it works and this is a comedic sentence).
Ferrets like to live their lives irresponsibly. It's irresponsibility, after all. (Referring directly to irresponsibility, because one could see the temptation to live your life irresponsibly. The less redundant statement of the two.)
This doesn't work with the situation at hand. it would not make sense to use this sentence in this situation. The whole topic prior to this and within the sentence is ferrets. The two sentences are comments on ferrets and there lifestyles. Using the sentence you have would work if the topic was irresponsibility and we were highlighting that even animals like to live irresponsibly.
Here is the simple way to rephrase the sentence and avoid ambiguity:
Ferrets like to live their lives irresponsibly. Irresponsibility is ferrets, after all.
The second sentence tells the reader that the ferrets don't just enjoy irresponsibly but it encompasses their lifestyle and behavior. Don't make the mistake of arguing whether or not the ferrets can actually be irresponsible. The whole point of the two sentences was a joke, they're supposed to far fetched.
